Solved: A bicyclist is coasting straight down a hill at a

Which of the following statements can be explained by Newtons first law? (A): When your car suddenly comes to a halt, you lunge forward. (B): When your car rapidly accelerates, you are pressed backward against the seat. (a) Neither A nor B (b) Both A and B (c) A but not B (d) B but not A

Two people are pushing a stalled car, as Figure 4.5a indicates. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other applies a force of 395 N. Both forces act in the same direction. A third force of 560 N also acts on the car, but in a direction opposite to that in which the people are pushing. This force arises because of friction and the extent to which the pavement opposes the motion of the tires. Find the acceleration of the car.

The net external force acting on an object is zero. Which one of the following statements is true? (a) The object can only be stationary. (b) The object can only be traveling with a constant velocity. (c) The object can be either stationary or traveling with a constant velocity. (d) The object can only be traveling with a velocity that is changing.

In Case A an object is moving straight downward with a constant speed of 9.80 m/s, while in Case B an object is moving straight downward with a constant acceleration of magnitude 9.80 m/s2 . Which one of the following is true? (a) A nonzero net external force acts on the object in both cases. (b) A nonzero net external force acts on the object in neither case. (c) A nonzero net external force acts on the object in Case A only. (d) A nonzero net external force acts on the object in Case B only.

A man is stranded on a raft (mass of man and raft 1300 kg), as shown in Figure 4.6a. By paddling, he causes an average force of 17 N to be applied to the raft in a direction due east (the x direction). The wind also exerts a force on the raft. This force has a magnitude of 15 N and points 67 north of east. Ignoring any resistance from the water, find the x and y components of the rafts acceleration.

At the moment that the forces and begin acting on the raft in Example 2, the velocity of the raft is 0.15 m/s, in a direction due east (the x direction). Assuming that the forces are maintained for 65 s, find the x and y components of the rafts displacement during this time interval.

Newtons second law indicates that when a net force acts on an object, it must accelerate. Does this mean that when two or more forces are applied to an object simultaneously, it must accelerate?

All of the following, except one, cause the acceleration of an object to double. Which one is the exception? (a) All forces acting on the object double. (b) The net force acting on the object doubles. (c) Both the net force acting on the object and the mass of the object double. (d) The net force acting on the object remains the same, while the mass of the object is reduced by a factor of two.

Suppose that the mass of the spacecraft in Figure 4.7 is mS  11 000 kg and that the mass of the astronaut is mA  92 kg. In addition, assume that the astronaut pushes with a force of  36 N on the spacecraft. Find the accelerations of the spacecraft and the astronaut.

A father and his seven-year-old daughter are facing each other on ice skates. With their hands, they push off against one another. Which one or more of the following statements is (are) true? (a) Each experiences an acceleration that has a different magnitude. (b) Each experiences an acceleration of the same magnitude. (c) Each experiences a pushing force that has a different magnitude. (d) Each experiences a pushing force that has the same magnitude.

What is the magnitude of the gravitational force that acts on each particle in Figure 4.9, assuming m1 12 kg (approximately the mass of a bicycle), m2 25 kg, and r 1.2 m? Re

The mass of the Hubble Space Telescope is 11 600 kg. Determine the weight of the telescope (a) when it was resting on the earth and (b) as it is in its orbit 598 km above the earths surface.

A vehicle designed for exploring the moons surface is being tested on earth, where it weighs roughly six times more than it will on the moon. The acceleration of the vehicle along the ground is measured. To achieve the same acceleration on the moon, will the required net force be (a) the same as, (b) greater than, or (c) less than that on earth?

When a body is moved from sea level to the top of a mountain, what changes: (a) the bodys mass, (b) its weight, or (c) both its mass and its weight?

Object A weighs twice as much as object B at the same spot on the earth. Would the same be true at a given spot on Mars?

Three particles have identical masses. Each particle experiences only the gravitational forces due to the other two particles. How should the particles be arranged so each one experiences a net gravitational force that has the same magnitude? (a) On the corners of an equilateral triangle (b) On three of the four corners of a square (c) On the corners of a right triangle

Two objects with masses m1 and m2 are separated by a distance 2d. Mass m2 is greater than mass m1. A third object has a mass m3. All three objects are located on the same straight line. The net gravitational force acting on the third object is zero. Which one of the drawings correctly represents the locations of the objects?

In a circus balancing act, a woman performs a headstand on top of a standing performers head, as Figure 4.15a illustrates. The woman weighs 490 N, and the standing performers head and neck weigh 50 N. It is primarily the seventh cervical vertebra in the spine that supports all the weight above the shoulders. What is the normal force that this vertebra exerts on the neck and head of the standing performer (a) before the act and (b) during the act?

A stack of books whose true weight is 165 N is placed on a scale in an elevator. The scale reads 165 N. From this information alone, can you tell whether the elevator is moving with a constant velocity of 2 m/s upward, is moving with a constant velocity of 2 m/s downward, or is at rest?

A 10-kg suitcase is placed on a scale that is in an elevator. In which direction is the elevator accelerating when the scale reads 75 N and when it reads 120 N? (a) Downward when it reads 75 N and upward when it reads 120 N (b) Upward when it reads 75 N and downward when it reads 120 N (c) Downward in both cases (d) Upward in both cases

You are standing on a scale in an elevator that is moving upward with a constant velocity. The scale reads 600 N. The following table shows five options for what the scale reads when the elevator slows down as it comes to a stop, when it is stopped, and when it picks up speed on its way back down. Which one of the five options correctly describes the scales readings? Note that the symbol means less than and means greater than.

A skier is standing motionless on a horizontal patch of snow. She is holding onto a horizontal tow rope, which is about to pull her forward (see Figure 4.22a). The skiers mass is 59 kg, and the coefficient of static friction between the skis and snow is 0.14. What is the magnitude of the maximum force that the tow rope can apply to the skier without causing her to move?

A sled and its rider are moving at a speed of 4.0 m/s along a horizontal stretch of snow, as Figure 4.24a illustrates. The snow exerts a kinetic frictional force on the runners of the sled, so the sled slows down and eventually comes to a stop. The coefficient of kinetic friction is 0.050. What is the displacement x of the sled?

Suppose that the coefficients of static and kinetic friction have values such that s  1.4 k for a crate in contact with a cement floor. Which one of the following statements is true? (a) The magnitude of the static frictional force is always 1.4 times the magnitude of the kinetic frictional force. (b) The magnitude of the kinetic frictional force is always 1.4 times the magnitude of the static frictional force. (c) The magnitude of the maximum static frictional force is 1.4 times the magnitude of the kinetic frictional force.

A person has a choice of either pushing or pulling a sled at a constant velocity, as the drawing illustrates. Friction is present. If the angle is the same in both cases, does it require less force to push or to pull the sled?

A box has a weight of 150 N and is being pulled across a horizontal floor by a force that has a magnitude of 110 N. The pulling force can point horizontally, or it can point above the horizontal at an angle . When the pulling force points horizontally, the kinetic frictional force acting on the box is twice as large as when the pulling force points at the angle . Find . 17

A box rests on the floor of an elevator. Because of static friction, a force is required to start the box sliding across the floor when the elevator is (a) stationary, (b) accelerating upward, and (c) accelerating downward. Rank the forces required in these three situations in ascending orderthat is, smallest first.

A rope is used in a tug-of-war between two teams of five people each. Both teams are equally strong, so neither team wins. An identical rope is tied to a tree, and the same ten people pull just as hard on the loose end as they did in the contest. In both cases, the people pull steadily with no jerking. Which rope sustains the greater tension, (a) the rope tied to the tree or (b) the rope in the tug-of-war, or (c) do the ropes sustain the same tension?

Select the object (often called the system) to which Equations 4.9a and 4.9b are to be applied. It may be that two or more objects are connected by means of a rope or a cable. If so, it may be necessary to treat each object separately according to the following steps.

Draw a free-body diagram for each object chosen above. Be sure to include only forces that act on the object. Do not include forces that the object exerts on its environment

Choose a set of x, y axes for each object and resolve all forces in the free-body diagram into components that point along these axes. Select the axes so that as many forces as possible point along one or the other of the two axes. Such a choice minimizes the calculations needed to determine the force components.

Apply Equations 4.9a and 4.9b by setting the sum of the x components and the sum of the y components of the forces each equal to zero.

Solve the two equations obtained in Step 4 for the desired unknown quantities, remembering that two equations can yield answers for only two unknowns at most.

Figure 4.27a shows a traction device used with a foot injury. The weight of the 2.2-kg object creates a tension in the rope that passes around the pulleys. Therefore, tension forces and are applied to the pulley on the foot. (It may seem surprising that the rope applies a force to either side of the foot pulley. A similar effect occurs when you place a finger inside a rubber band and push downward. You can feel each side of the rubber band pulling upward on the finger.) The foot pulley is kept in equilibrium because the foot also applies a force to it. This force arises in reaction (Newtons third law) to the pulling effect of the forces and . Ignoring the weight of the foot, find the magnitude of .

An automobile engine has a weight , whose magnitude is W  3150 N. This engine is being positioned above an engine compartment, as Figure 4.28a illustrates. To position the engine, a worker is using a rope. Find the tension in the supporting cable and the tension in the positioning rope.

A jet plane is flying with a constant speed along a straight line, at an angle of 30.0 above the horizontal, as Figure 4.29a indicates. The plane has a weight whose magnitude is W 86 500 N, and its engines provide a forward thrust of magnitude T 103 000 N. In addition, the lift force (directed perpendicular to the wings) and the force of air resistance (directed opposite to the motion) act on the plane. Find and .R

In which one of the following situations could an object possibly be in equilibrium? (a) Three forces act on the object; the forces all point along the same line but may have different directions. (b) Two perpendicular forces act on the object. (c) A single force acts on the object. (d) In none of the situations described in (a), (b), and (c) could the object possibly be in equilibrium.

A stone is thrown from the top of a cliff. Air resistance is negligible. As the stone falls, is it (a) in equilibrium or (b) not in equilibrium?

During the final stages of descent, a sky diver with an open parachute approaches the ground with a constant velocity. There is no wind to blow him from side to side. Which one of the following statements is true? (a) The sky diver is not in equilibrium. (b) The force of gravity is the only force acting on the sky diver, so that he is in equilibrium. (c) The sky diver is in equilibrium because no forces are acting on him. (d) The sky diver is in equilibrium because two forces act on him, the downward-acting force of gravity and the upward-acting force of the parachute.

A crate hangs from a ring at the middle of a rope, as the drawing illustrates. A person is pulling on the right end of the rope to keep the crate in equilibrium. Can the rope ever be made to be perfectly horizontal?

A supertanker of mass m 1.50  108 kg is being towed by two tugboats, as in Figure 4.30a. The tensions in the towing cables apply the forces and at equal angles of 30.0 with respect to the tankers axis. In addition, the tankers engines produce a forward drive force , whose magnitude is D 75.0  103 N. Moreover, the water applies an opposing force , whose magnitude is R 40.0  103 N. The tanker moves forward with an acceleration that points along the tankers axis and has a magnitude of 2.00  103 m/s2 . Find the magnitudes of the tensions and . Re

A truck is hauling a trailer along a level road, as Figure 4.32a illustrates. The mass of the truck is m1 8500 kg and that of the trailer is m2 27 000 kg. The two move along the x axis with an acceleration of ax 0.78 m/s2 . Ignoring the retarding forces of friction and air resistance, determine (a) the tension in the horizontal drawbar between the trailer and the truck and (b) the force that propels the truck forward. Rea

Figure 4.33 shows a water skier at four different moments: (a) The skier is floating motionless in the water. (b) The skier is being pulled out of the water and up onto the skis. (c) The skier is moving at a constant speed along a straight line. (d) The skier has let go of the tow rope and is slowing down. For each moment, explain whether the net force acting on the skier is zero.

A flatbed truck is carrying a crate up a 10.0 hill, as Figure 4.34a illustrates. The coefficient of static friction between the truck bed and the crate is 0.350. Find the maximum acceleration that the truck can attain before the crate begins to slip backward relative to the truck.

Block 1 (mass m1 8.00 kg) is moving on a frictionless 30.0 incline. This block is connected to block 2 (mass m2 22.0 kg) by a massless cord that passes over a massless and frictionless pulley (see Figure 4.35a). Find the acceleration of each block and the tension in the cord. R

A window washer on a scaffold is hoisting the scaffold up the side of a building by pulling downward on a rope, as in Figure 4.36a. The magnitude of the pulling force is 540 N, and the combined mass of the worker and the scaffold is 155 kg. Find the upward acceleration of the unit.

A circus performer hangs stationary from a rope. She then begins to climb upward by pulling herself up, hand over hand. When she starts climbing, is the tension in the rope (a) less than, (b) equal to, or (c) greater than it is when she hangs stationary?

A freight train is accelerating on a level track. Other things being equal, would the tension in the coupling between the engine and the first car change if some of the cargo in the last car were transferred to any one of the other cars?

Two boxes have masses m1 and m2, and m2 is greater than m1. The boxes are being pushed across a frictionless horizontal surface. As the drawing shows, there are two possible arrangements, and the pushing force is the same in each. In which arrangement, (a) or (b), does the force that the left box applies to the right box have a greater magnitude, or (c) is the magnitude the same in both cases?

Suppose the spacecraft were stationary. What would be the direction of ? F B 3

When the spacecraft is moving at a constant velocity of 850 m/s, what is the direction of ?

On earth a block has a weight of 88 N. This block is sliding on a horizontal surface on the moon, where the acceleration due to gravity is 1.60 m/s2 . As Figure 4.38a shows, the block is being pulled by a horizontal rope in which the tension is T  24 N. The coefficient of kinetic friction between the block and the surface is k  0.20. Determine the acceleration of the block.

Which of Newtons laws of motion provides a way to determine the acceleration of the block?

This problem deals with a situation on the moon, but the blocks mass on the moon is not given. Instead, the blocks earth-weight is given. Why can the earth-weight be used to obtain a value for the blocks mass that applies on the moon?

Does the net force Fx equal the tension T?

An object is moving at a constant velocity. All but one of the following statements could be true. Which one cannot be true? (a) No forces act on the object. (b) A single force acts on the object. (c) Two forces act simultaneously on the object. (d) Three forces act simultaneously on the object.

A cup of coffee is sitting on a table in a recreational vehicle (RV). The cup slides toward the rear of the RV. According to Newtons first law, which one or more of the following statements could describe the motion of the RV? (A) The RV is at rest, and the driver suddenly accelerates. (B) The RV is moving forward, and the driver suddenly accelerates. (C) The RV is moving backward, and the driver suddenly hits the brakes. (a) A (b) B (c) C (d) A and B (e) A, B, and C

Two forces act on a moving object that has a mass of 27 kg. One force has a magnitude of 12 N and points due south, while the other force has a magnitude of 17 N and points due west. What is the acceleration of the object? (a) 0.63 m/s2 directed 55 south of west (b) 0.44 m/s2 directed 24 south of west (c) 0.77 m/s2 directed 35 south of west (d) 0.77 m/s2 directed 55 south of west (e) 1.1 m/s2 directed 35 south of west

Which one of the following is true, according to Newtons laws of motion? Ignore friction. (a) A sports utility vehicle (SUV) hits a stationary motorcycle. Since it is stationary, the motorcycle sustains a greater force than the SUV does. (b) A semitrailer truck crashes all the way through a wall. Since the wall collapses, the wall sustains a greater force than the truck does. (c) Sam (18 years old) and his sister (9 years old) go ice skating. They push off against each other and fly apart. Sam flies off with the greater acceleration. (d) Two astronauts on a space walk are throwing a ball back and forth between each other. In this game of catch the distance between them remains constant. (e) None of the above is true, according to the third law.

wo ice skaters, Paul and Tom, are each holding on to opposite ends of the same rope. Each pulls the other toward him. The magnitude of Pauls acceleration is 1.25 times greater than the magnitude of Toms acceleration. What is the ratio of Pauls mass to Toms mass? (a) 0.67 (b) 0.80 (c) 0.25 (d) 1.25 (e) 0.50

In another solar system a planet has twice the earths mass and three times the earths radius. Your weight on this planet is ________ times your earth-weight. Assume that the masses of the earth and of the other planet are uniformly distributed. (a) 0.667 (b) 2.000 (c) 0.111 (d) 0.444 (e) 0.222

What is the mass on Mercury of an object that weighs 784 N on the earths surface? (a) 80.0 kg (b) 48.0 kg (c) 118 kg (d) 26.0 kg (e) There is not enough information to calculate the mass.

The apparent weight of a passenger in an elevator is greater than his true weight. Which one of the following is true? (a) The elevator is either moving upward with an increasing speed or moving upward with a decreasing speed. (b) The elevator is either moving upward with an increasing speed or moving downward with an increasing speed. (c) The elevator is either moving upward with a decreasing speed or moving downward with a decreasing speed. (d) The elevator is either moving upward with an increasing speed or moving downward with a decreasing speed. (e) The elevator is either moving upward with a decreasing speed or moving downward with an increasing speed.

The drawings show three examples of the force with which someone pushes against a vertical wall. In each case the magnitude of the pushing force is the same. Rank the normal forces that the wall applies to the pusher in ascending order (smallest first). (a) C, B, A (b) B, A, C (c) A, C, B (d) B, C, A (e) C, A, B

The drawing shows three blocks, each with the same mass, stacked one upon the other. The bottom block rests on a frictionless horizontal surface and is being pulled by a force that is parallel to this surface. The surfaces where the blocks touch each other have identical coefficients of static friction. Which one of the following correctly describes the magnitude of the net force of static friction that acts on each block? (a) fs,A fs,B fs,C (b) (c) (d) (e) fs,A fs,C 1 2 fs,B fs,C 0 and fs,A 1 2 f fs,A 0 and fs,B s,B 1 2 fs,C fs,A fs,B 1 2 fs,C fs F B

Three identical blocks are being pulled or pushed across a horizontal surface by a force , as shown in the drawings. The force in each case F has the same magnitude. Rank the kinetic frictional forces that act on the blocks in ascending order (smallest first). (a) B, C, A (b) C, A, B (c) B, A, C (d) C, B, A (e) A, C, B

A heavy block is suspended from a ceiling using pulleys in three different ways, as shown in the drawings. Rank the tension in the rope that passes over the pulleys in ascending order (smallest first). (a) B, A, C (b) C, B, A (c) A, B, C (d) C, A, B (e) B, C, A

A certain object is in equilibrium. Which one of the following statements is not true? (a) The object must be at rest. (b) The object has a constant velocity. (c) The object has no acceleration. (d) No net force acts on the object.

Two identical boxes are being pulled across a horizontal floor at a constant velocity by a horizontal pulling force of 176 N that is applied to one of the boxes, as the drawing shows. There is kinetic friction between each box and the floor. Find the tension in the rope between the boxes. (a) 176 N (b) 88.0 N (c) 132 N (d) 44.0 N (e) There is not enough information to calculate the tension.

A man is standing on a platform that is connected to a pulley arrangement, as the drawing shows. By pulling upward on the rope with a force the man can raise the platform and himself. The total mass of the man plus the platform is 94.0 kg. What pulling force should the man apply to create an upward acceleration of 1.20 m/s2 ?

An airplane has a mass of 3.1  104 kg and takes off under the influence of a constant net force of 3.7  104 N. What is the net force that acts on the planes 78-kg pilot?

A boat has a mass of 6800 kg. Its engines generate a drive force of 4100 N due west, while the wind exerts a force of 800 N due east and the water exerts a resistive force of 1200 N due east. What are the magnitude and direction of the boats acceleration?

Two horizontal forces, and , are acting on a box, but only F B F 1 is shown in the drawing. can point either to the right or to the left. The box moves only along the x axis. There is no friction between the box and the surface. Suppose that and the mass of the box is 3.0 kg. Find the magnitude and direction of when the acceleration of the box is (a) 5.0 m/s2 , (b) 5.0 m/s2 , and (c) 0 m/s2 .

In the amusement park ride known as Magic Mountain Superman, powerful magnets accelerate a car and its riders from rest to 45 m/s (about 100 mi/h) in a time of 7.0 s. The combined mass of the car and riders is 5.5  103 kg. Find the average net force exerted on the car and riders by the magnets.

A person in a kayak starts paddling, and it accelerates from 0 to 0.60 m/s in a distance of 0.41 m. If the combined mass of the person and the kayak is 73 kg, what is the magnitude of the net force acting on the kayak?

Scientists are experimenting with a kind of gun that may eventually be used to fire payloads directly into orbit. In one test, this gun accelerates a 5.0-kg projectile from rest to a speed of 4.0  103 m/s. The net force accelerating the projectile is 4.9  105 N. How much time is required for the projectile to come up to speed?

A 1580-kg car is traveling with a speed of 15.0 m/s. What is the magnitude of the horizontal net force that is required to bring the car to a halt in a distance of 50.0 m?

The space probe Deep Space 1 was launched on October 24, 1998. Its mass was 474 kg. The goal of the mission was to test a new kind of engine called an ion propulsion drive. This engine generated only a weak thrust, but it could do so over long periods of time with the consumption of only small amounts of fuel. The mission was spectacularly successful. At a thrust of 56 mN how many days were required for the probe to attain a velocity of 805 m/s (1800 mi/h), assuming that the probe started from rest and that the mass remained nearly constant?

Two forces and are applied to an object whose mass is 8.0 kg. The larger force is . When both forces point due east, the objects acceleration has a magnitude of 0.50 m/s2 . However, when points due east and points due west, the acceleration is 0.40 m/s2 , due east. Find (a) the magnitude of and (b) the magnitude of .

An electron is a subatomic particle (m 9.11  1031 kg) that F B F B B A F B B F B A F B A F B F B B A is subject to electric forces. An electron moving in the x direction accelerates from an initial velocity of 5.40  105 m/s to a final velocity of 2.10  106 m/s while traveling a distance of 0.038 m. The electrons acceleration is due to two electric forces parallel to the x axis: , and , which points in the x direction. Find the magnitudes of (a) the net force acting on the electron and (b) the electric force .

Only two forces act on an object (mass 3.00 kg), as in the drawing. Find the magnitude and direction (relative to the x axis) of the acceleration of the object.

At an instant when a soccer ball is in contact with the foot of a player kicking it, the horizontal or x component of the balls acceleration is 810 m/s2 and the vertical or y component of its acceleration is 1100 m/s2 . The balls mass is 0.43 kg. What is the magnitude of the net force acting on the soccer ball at this instant?

A rocket of mass 4.50  105 kg is in flight. Its thrust is directed at an angle of 55.0 above the horizontal and has a magnitude of 7.50  106 N. Find the magnitude and direction of the rockets acceleration. Give the direction as an angle above the horizontal.

A billiard ball strikes and rebounds from the cushion of a pool table perpendicularly. The mass of the ball is 0.38 kg. The ball approaches the cushion with a velocity of 2.1 m/s and rebounds with a velocity of 2.0 m/s. The ball remains in contact with the cushion for a time of 3.3  103 s. What is the average net force (magnitude and direction) exerted on the ball by the cushion?

When a parachute opens, the air exerts a large drag force on it. This upward force is initially greater than the weight of the sky diver and, thus, slows him down. Suppose the weight of the sky diver is 915 N and the drag force has a magnitude of 1027 N. The mass of the sky diver is 93.4 kg. What are the magnitude and direction of his acceleration?

Two skaters, a man and a woman, are standing on ice. Neglect any friction between the skate blades and the ice. The mass of the man is 82 kg, and the mass of the woman is 48 kg. The woman pushes on the man with a force of 45 N due east. Determine the acceleration (magnitude and direction) of (a) the man and (b) the woman.

A space probe has two engines. Each generates the same amount of force when fired, and the directions of these forces can be independently adjusted. When the engines are fired simultaneously and each applies its force in the same direction, the probe, starting from rest, takes 28 s to travel a certain distance. How long does it take to travel the same distance, again starting from rest, if the engines are fired simultaneously and the forces that they apply to the probe are perpendicular?

At a time when mining asteroids has become feasible, astronauts have connected a line between their 3500-kg space tug and a 6200-kg asteroid. Using their tugs engine, they pull on the asteroid with a force of 490 N. Initially the tug and the asteroid are at rest, 450 m apart. How much time does it take for the tug and the asteroid to meet?

A 325-kg boat is sailing 15.0 north of east at a speed of 2.00 m/s. Thirty seconds later, it is sailing 35.0 north of east at a speed of 4.00 m/s. During this time, three forces act on the boat: a 31.0-N force directed 15.0 north of east (due to an auxiliary engine), a 23.0-N force directed 15.0 south of west (resistance due to the water), and (due to the wind). Find the magnitude and direction of the force . Express the direction as an angle with respect to due east.

A 5.0-kg rock and a 3.0  104 -kg pebble are held near the surface of the earth. (a) Determine the magnitude of the gravitational force exerted on each by the earth. (b) Calculate the magnitude of the acceleration of each object when released.

Mars has a mass of 6.46  1023 kg and a radius of 3.39  106 m. (a) What is the acceleration due to gravity on Mars? (b) How much would a 65-kg person weigh on this planet?

On earth, two parts of a space probe weigh 11 000 N and 3400 N. These parts are separated by a center-to-center distance of 12 m and may be treated as uniform spherical objects. Find the magnitude of the gravitational force that each part exerts on the other out in space, far from any other objects.

A raindrop has a mass of 5.2  107 kg and is falling near the astronaut has a mass of 57 kg. What is the astronauts apparent weight during takeoff? surface of the earth. Calculate the magnitude of the gravitational force exerted (a) on the raindrop by the earth and (b) on the earth by the raindrop.

The weight of an object is the same on two different planets. The mass of planet A is only sixty percent that of planet B. Find the ratio rA/rB of the radii of the planets.

Review Conceptual Example 7 in preparation for this problem. In tests on earth a lunar surface exploration vehicle (mass 5.90  103 kg) achieves a forward acceleration of 0.220 m/s2 . To achieve this same acceleration on the moon, the vehicles engines must produce a drive force of 1.43  103 N. What is the magnitude of the frictional force that acts on the vehicle on the moon?

Synchronous communications satellites are placed in a circular orbit that is 3.59  107 m above the surface of the earth. What is the magnitude of the acceleration due to gravity at this distance?

The drawing (not to scale) shows one alignment of the sun, earth, and moon. The gravitational force that the sun exerts on the moon is perpendicular to the force that the earth exerts on the moon. The masses are: mass of sun 1.99  1030 kg, mass of earth 5.98  1024 kg, mass of moon 7.35  1022 kg. The distances shown in the drawing are rSM 1.50  1011 m and rEM 3.85  108 m. Determine the magnitude of the net gravitational force on the moon. Sun

The drawing shows three particles far away from any other objects and located on a straight line. The masses of these particles are mA 363 kg, mB 517 kg, and mC 154 kg. Find the magnitude and direction of the net gravitational force acting on (a) particle A, (b) particle B, and (c) particle C. 3

A space traveler weighs 540.0 N on earth. What will the traveler weigh on another planet whose radius is twice that of earth and whose mass is three times that of earth?

The mass of a robot is 5450 kg. This robot weighs 3620 N more on planet A than it does on planet B. Both planets have the same radius of 1.33  107 m. What is the difference MA  MB in the masses of these planets?

A spacecraft is on a journey to the moon. At what point, as measured from the center of the earth, does the gravitational force exerted on the spacecraft by the earth balance that exerted by the moon? This point lies on a line between the centers of the earth and the moon. The distance between the earth and the moon is 3.85  108 m, and the mass of the earth is 81.4 times as great as that of the moon.

As a moon follows its orbit around a planet, the maximum gravitational force exerted on the moon by the planet exceeds the minimum gravitational force by 11%. Find the ratio rmax/rmin, where rmax is the moons maximum distance from the center of the planet and rmin is the minimum distance.

A neutron star has a mass of 2.0  1030 kg (about the mass of 0.500 m A BC 0.250 m 124 Chapter 4 Forces and Newtons Laws of Motion our sun) and a radius of 5.0  103 m (about the height of a good-sized mountain). Suppose an object falls from rest near the surface of such a star. How fast would this object be moving after it had fallen a distance of 0.010 m? (Assume that the gravitational force is constant over the distance of the fall and that the star is not rotating.)

The sun is more massive than the moon, but the sun is farther from the earth. Which one exerts a greater gravitational force on a person standing on the earth? Give your answer by determining the ratio Fsun /Fmoon of the magnitudes of the gravitational forces. Use the data on the inside of the front cover.

At a distance H above the surface of a planet, the true weight of a remote probe is one percent less than its true weight on the surface. The radius of the planet is R. Find the ratio H/R.

Two particles are located on the x axis. Particle 1 has a mass m and is at the origin. Particle 2 has a mass 2m and is at x L. A third particle is placed between particles 1 and 2. Where on the x axis should the third particle be located so that the magnitude of the gravitational force on both particle 1 and particle 2 doubles? Express your answer in terms of L .

A 35-kg crate rests on a horizontal floor, and a 65-kg person is standing on the crate. Determine the magnitude of the normal force that (a) the floor exerts on the crate and (b) the crate exerts on the person.

A 60.0-kg crate rests on a level floor at a shipping dock. The coefficients of static and kinetic friction are 0.760 and 0.410, respectively. What horizontal pushing force is required to (a) just start the crate moving and (b) slide the crate across the dock at a constant speed?

A rocket blasts off from rest and attains a speed of 45 m/s in 15 s. An face of the earth. (a) Determine the magnitude of the gravitational force exerted on each by the earth. (b) Calculate the magnitude of the acceleration of each object when released. 21. Mars has a mass of 6.46  1023 kg and a radius of 3.39  106 m. (a) What is the acceleration due to gravity on Mars? (b) How much would a 65-kg person weigh on this planet? 22. On earth, two parts of a space probe weigh 11 000 N and 3400 N. These parts are separated by a center-to-center distance of 12 m and may be treated as uniform spherical objects. Find the magnitude of the gravitational force that each part exerts on the other out in space, far from any other objects. 23. A raindrop has a mass of 5.2  107 kg and is falling near the astronaut has a mass of 57 kg. What is the astronauts apparent weight during takeoff?

A car is traveling up a hill that is inclined at an angle above the horizontal. Determine the ratio of the magnitude of the normal force to the weight of the car when (a) 15 and (b) 35. 42.

A woman stands on a scale in a moving elevator. Her mass is 60.0 kg, and the combined mass of the elevator and scale is an additional 815 kg. Starting from rest, the elevator accelerates upward. During the acceleration, the hoisting cable applies a force of 9410 N. What does the scale read during the acceleration?

A Mercedes-Benz 300SL (m 1700 kg) is parked on a road that rises 15 above the horizontal. What are the magnitudes of (a) the normal force and (b) the static frictional force that the ground exerts on the tires? 4

Consult Multiple-Concept Example 9 to explore a model for solving this problem. A person pushes on a 57-kg refrigerator with a horizontal force of 267 N; the minus sign indicates that the force points in the x direction. The coefficient of static friction is 0.65. (a) If the refrigerator does not move, what are the magnitude and direction of the static frictional force that the floor exerts on the refrigerator? (b) What is the magnitude of the largest pushing force that can be applied to the refrigerator before it just begins to move?

A 6.00-kg box is sliding across the horizontal floor of an elevator. The coefficient of kinetic friction between the box and the floor is 0.360. Determine the kinetic frictional force that acts on the box when the elevator is (a) stationary, (b) accelerating upward with an acceleration whose magnitude is 1.20 m/s2 , and (c) accelerating downward with an acceleration whose magnitude is 1.20 m/s2

A cup of coffee is on a table in an airplane flying at a constant altitude and a constant velocity. The coefficient of static friction between the cup and the table is 0.30. Suddenly, the plane accelerates forward, its altitude remaining constant. What is the maximum acceleration that the plane can have without the cup sliding backward on the table?

An 81-kg baseball player slides into second base. The coefficient of kinetic friction between the player and the ground is 0.49. (a) What is the magnitude of the frictional force? (b) If the player comes to rest after 1.6 s, what was his initial velocity?

Consult Multiple-Concept Example 10 in preparation for this problem. Traveling at a speed of 16.1 m/s, the driver of an automobile suddenly locks the wheels by slamming on the brakes. The coefficient of kinetic friction between the tires and the road is 0.720. What is the speed of the automobile after 1.30 s have elapsed? Ignore the effects of air resistance.

A person is trying to judge whether a picture (mass 1.10 kg) is properly positioned by temporarily pressing it against a wall. The pressing force is perpendicular to the wall. The coefficient of static friction between the picture and the wall is 0.660. What is the minimum amount of pressing force that must be used?

Multiple-Concept Example 17 reviews the basic concepts involved in this problem. Air rushing over the wings of high-performance race cars generates unwanted horizontal air resistance but also causes a vertical downforce, which helps the cars hug the track more securely. The coefficient of static friction between the track and the tires of a 690-kg race car is 0.87. What is the magnitude of the maximum acceleration at which the car can speed up without its tires slipping when a 4060-N downforce and an 1190-N horizontal-air-resistance force act on it?

While moving in, a new homeowner is pushing a box across the floor at a constant velocity. The coefficient of kinetic friction between the box and the floor is 0.41. The pushing force is directed downward at an angle below the horizontal. When is greater than a certain value, it is not possible to move the box, no matter how large the pushing force is. Find that value of . s

The helicopter in the drawing is moving horizontally to the right at a constant velocity . The weight of the helicopter is W 53 800 N. The lift force generated by the rotating blade makes an angle of 21.0 with respect to the vertical. (a) What is the magnitude of the lift force? (b) Determine the magnitude of the air resistance that opposes the motion. 5

Three forces act on a moving object. One force has a magnitude of 80.0 N and is directed due north. Another has a magnitude of 60.0 N and is directed due west. What must be the magnitude and direction of the third force, such that the object continues to move with a constant velocity?

The steel I-beam in the drawing has a weight of 8.00 kN and is being lifted at a constant velocity. What is the tension in each cable attached to its ends?

The drawing shows a wire tooth brace used by orthodontists. The topmost tooth is protruding slightly, and the tension in the wire exerts two forces and on this tooth in order to bring it back into alignment. If the forces have the same magnitude of 21.0 N, what is the magnitude of the net force exerted on the tooth by these forces?

Part a of the drawing shows a bucket of water suspended from the pulley of a well; the tension in the rope is 92.0 N. Part b shows the same bucket of water being pulled up from the well at a constant velocity. What is the tension in the rope in part b?

A worker stands still on a roof sloped at an angle of 36 above the horizontal. He is prevented from slipping by a static frictional force of 390 N. Find the mass of the worker.

A stuntman is being pulled along a rough road at a constant velocity by a cable attached to a moving truck. The cable is parallel to the ground. The mass of the stuntman is 109 kg, and the coefficient of kinetic friction between the road and him is 0.870. Find the tension in the cable

A 1.40-kg bottle of vintage wine is lying horizontally in a rack, as shown in the drawing. The two surfaces on which the bottle rests are 90.0 apart, and the right surface makes an angle of 45.0 with respect to the horizontal. Each surface exerts a force on the bottle that is perpendicular to the surface. Both forces have the same magnitude F. Find the value of F.

The drawing shows a circus clown who weighs 890 N. The coefficient of static friction between the clowns feet and the ground is 0.53. He pulls vertically downward on a rope that passes around three pulleys and is tied around his feet. What is the minimum pulling force that the clown must exert to yank his feet out from under himself?

The drawing shows box 1 resting on a table, with box 2 resting on top of box 1. A massless rope passes over a massless, frictionless pulley. One end of the rope is connected to box 2, and the other end is connected to box 3. The weights of the three boxes are W1 55 N, W2 35 N, and W3 28 N. Determine the magnitude of the normal force that the table exerts on box 1. *

During a storm, a tree limb breaks off and comes to rest across a barbed wire fence at a point that is not in the middle between two fence posts. The limb exerts a downward force of 151 N on the wire. The left section of the wire makes an angle of 14.0 relative to the horizontal and sustains a tension of 447 N. Find the magnitude and direction of the tension that the right section of the wire sustains.

A 44-kg chandelier is suspended 1.5 m below a ceiling by three wires, each of which has the same tension and the same length of 2.0 m (see the drawing). Find the tension in each wire.

A block is pressed against a vertical 2.0 m 1.5 m lying horizontally in a rack, as shown in the drawing. The two surfaces on which the bottle rests are 90.0 apart, and the right surface makes an angle of 45.0 with respect to the horizontal. Each surface exerts a force on the bottle that is perpendicular to the surface. Both forces have the same magnitude F. Find the value of F. 60. The drawing shows a circus clown who Problem 61 90.0 2 3 1 wall by a force , as the drawing shows. This force can either push the block upward at a constant velocity or allow it to slide downward at a constant velocity. The magnitude of the force is different in the two cases, while the directional angle is the same. Kinetic friction exists between the block and the wall, and the coefficient of kinetic friction is 0.250. The weight of the block is 39.0 N, and the directional angle for the force is 30.0. Determine the magnitude of when the block slides (a) up the wall and (b) down the wall. P

A toboggan slides down a hill and has a constant velocity. The angle of the hill is 8.00 with respect to the horizontal. What is the coefficient of kinetic friction between the surface of the hill and the toboggan?

The person in the drawing is standing on crutches. Assume that the force exerted on each crutch by the ground is directed along the crutch, as the force vectors in the drawing indicate. If the coefficient of static friction between a crutch and the ground is 0.90, determine the largest angle MAX that the crutch can have just before it begins to slip on the floor.

A bicyclist is coasting straight down a hill at a constant speed. The combined mass of the rider and bicycle is 80.0 kg, and the hill is inclined at 15.0 with respect to the horizontal. Air resistance opposes the motion of the cyclist. Later, the bicyclist climbs the same hill at the same constant speed. How much force (directed parallel to the hill) must be applied to the bicycle in order for the bicyclist to climb the hill?

A kite is hovering over the ground at the end of a straight 43-m line. The tension in the line has a magnitude of 16 N. Wind blowing on the kite exerts a force of 19 N, directed 56 above the horizontal. Note that the line attached to the kite is not oriented at an angle of 56 above the horizontal. Find the height of the kite, relative to the person holding the line.

A damp washcloth is hung over the edge of a table to dry. Thus, part (mass mon) of the washcloth rests on the table and part (mass moff) does not. The coefficient of static friction between the table and the washcloth is 0.40. Determine the maximum fraction [moff /(mon  moff)] that can hang over the edge without causing the whole washcloth to slide off the table. S

A 1450-kg submarine rises straight up toward the surface. Seawater exerts both an upward buoyant force of 16 140 N on the submarine and a downward resistive force of 1030 N. What is the submarines acceleration?

A 15-g bullet is fired from a rifle. It takes 2.50  103 s for the bullet to travel the length of the barrel, and it exits the barrel with a speed of 715 m/s. Assuming that the acceleration of the bullet is constant, find the average net force exerted on the bullet.

A fisherman is fishing from a bridge and is using a 45-N test line. In other words, the line will sustain a maximum force of 45 N without breaking. What is the weight of the heaviest fish that can be pulled up vertically when the line is reeled in (a) at a constant speed and (b) with an acceleration whose magnitude is 2.0 m/s2 ?

Only two forces act on an object (mass 4.00 kg), as in the drawing. Find the magnitude and direction (relative to the x axis) of the acceleration of the object.

A helicopter flies over the arctic ice pack at a constant altitude, towing an airborne 129-kg laser sensor that measures the thickness of the ice (see the drawing). The helicopter and the sensor both move only in the horizontal direction and have a horizontal acceleration of magnitude 2.84 m/s2 . Ignoring air resistance, find the tension in the cable towing the sensor.

Review Conceptual Example 16 as background for this problem. The water skier there has a mass of 73 kg. Find the magnitude of the net force acting on the skier when (a)she is accelerated from rest to a speed of 11 m/s in 8.0 s and (b) she lets go of the tow rope and glides to a halt in 21 s.

A rescue helicopter is lifting a man (weight 822 N) from a capsized boat by means of a cable and harness. (a) What is the tension in the cable when the man is given an initial upward acceleration of 1.10 m/s2 ? (b) What is the tension during the remainder of the rescue when he is pulled upward at a constant velocity?

A car is towing a boat on a trailer. The driver starts from rest and accelerates to a velocity of 11 m/s in a time of 28 s. The combined mass of the boat and trailer is 410 kg. The frictional force acting on the trailer can be ignored. What is the tension in the hitch that connects the trailer to the car?

A 292-kg motorcycle is accelerating up along a ramp that is inclined 30.0 above the horizontal. The propulsion force pushing the motorcycle up the ramp is 3150 N, and air resistance produces a force of 250 N that opposes the motion. Find the magnitude of the motorcycles acceleration.

A student is skateboarding down a ramp that is 6.0 m long and inclined at 18 with respect to the horizontal. The initial speed of the skateboarder at the top of the ramp is 2.6 m/s. Neglect friction and find the speed at the bottom of the ramp.

A man seeking to set a world record wants to tow a 109 000-kg airplane along a runway by pulling horizontally on a cable attached to the airplane. The mass of the man is 85 kg, and the coefficient of static friction between his shoes and the runway is 0.77. What is the greatest acceleration the man can give the airplane? Assume that the airplane is on wheels that turn without any frictional resistance.

The principles used to solve this problem are similar to those in Multiple-Concept Example 17. A 205-kg log is pulled up a ramp by means of a rope that is parallel to the surface of the ramp. The ramp is inclined at 30.0 with respect to the horizontal. The coefficient of kinetic friction between the log and the ramp is 0.900, and the log has an acceleration of magnitude 0.800 m/s2 . Find the tension in the rope.

To hoist himself into a tree, a 72.0-kg man ties one end of a nylon rope around his waist and throws the other end over a branch of the tree. He then pulls downward on the free end of the rope with a force of 358 N. Neglect any friction between the rope and the branch, and determine the mans upward acceleration.

Two objects (45.0 and 21.0 kg) are connected by a massless string that passes over a massless, frictionless pulley. The pulley hangs from the ceiling. Find (a) the acceleration of the objects and (b) the tension in the string.

A train consists of 50 cars, each of which has a mass of Problems 127 6.8  103 kg. The train has an acceleration of 8.0  102 m/s2 . Ignore friction and determine the tension in the coupling (a) between the 30th and 31st cars and (b) between the 49th and 50th cars.

This problem uses the same concepts as Multiple-Concept Example 17. In Problem 80, an 85-kg man plans to tow a 109 000-kg airplane along a runway by pulling horizontally on a cable attached to it. Suppose that he instead attempts the feat by pulling the cable at an angle of 9.0 above the horizontal. The coefficient of static friction between his shoes and the runway is 0.77. What is the greatest acceleration the man can give the airplane? Assume that the airplane is on wheels that turn without any frictional resistance.

The drawing shows a large cube (mass 25 kg) being accelerated across a horizontal frictionless surface by a horizontal force . A small cube (mass 4.0 kg) is in contact with the front surface of the large cube and will slide downward unless is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that can have in order to keep the small cube from sliding downward? *

The alarm at a fire station rings and an 86-kg fireman, starting from rest, slides down a pole to the floor below (a distance of 4.0 m). Just before landing, his speed is 1.4 m/s. What is the magnitude of the kinetic frictional force exerted on the fireman as he slides down the pole?

Two blocks are sliding to the right across a horizontal surface, as Frictionless P s = 0.71 B P B P B P B the drawing shows. In Case A the mass of each block is 3.0 kg. In Case B the mass of block 1 (the block behind) is 6.0 kg, and the mass of block 2 is 3.0 kg. No frictional force acts on block 1 in either Case A or Case B. However, a kinetic frictional force of 5.8 N does act on block 2 in both cases and opposes the motion. For both Case A and Case B determine (a) the magnitude of the forces with which the blocks push against each other and (b) the magnitude of the acceleration of the blocks.

A person whose weight is 5.20  102 N is being pulled up vertically by a rope from the bottom of a cave that is 35.1 m deep. The maximum tension that the rope can withstand without breaking is 569 N. What is the shortest time, starting from rest, in which the person can be brought out of the cave?

A girl is sledding down a slope that is inclined at 30.0 with respect to the horizontal. The wind is aiding the motion by providing a steady force of 105 N that is parallel to the motion of the sled. The combined mass of the girl and the sled is 65.0 kg, and the coefficient of kinetic friction between the snow and the runners of the sled is 0.150. How much time is required for the sled to travel down a 175-m slope, starting from rest?

In the drawing, the rope and the pulleys are massless, and there is no friction. Find (a) the tension in the rope and (b) the acceleration of the 10.0-kg block. (Hint: The larger mass moves twice as far as the smaller mass.)

A small sphere is hung by a string from the ceiling of a van. When the van is stationary, the sphere hangs vertically. However, when the van accelerates, the sphere swings backward so that the string makes an angle of with respect to the vertical. (a) Derive an expression for the magnitude a of the acceleration of the van in terms of the angle and the magnitude g of the acceleration due to gravity. (b) Find the acceleration of the van when 10.0. (c) What is the angle when the van moves with a constant velocity? **

A penguin slides at a constant velocity of 1.4 m/s down an icy incline. The incline slopes above the horizontal at an angle of 6.9. At the bottom of the incline, the penguin slides onto a horizontal patch of ice. The coefficient of kinetic friction between the penguin and the ice is the same for the incline as for the horizontal patch. How much time is required for the penguin to slide to a halt after entering the horizontal patch of ice?

A 5.00-kg block is placed on top of a 12.0-kg block that rests on a frictionless table. The coefficient of static friction between the two blocks is 0.600. What is the maximum horizontal force that can be applied before the 5.00-kg block begins to slip relative to the 12.0-kg block, if the force is applied to (a) the more massive block and (b) the less massive block?

A 55-kg bungee jumper has fallen far enough that her bungee cord is beginning to stretch and resist her downward motion. Find the force (magnitude and direction) exerted on her by the bungee cord at an instant when her downward acceleration has a magnitude of 7.6 m/s2 . Ignore the effects of air resistance.

A person with a black belt in karate has a fist that has a mass of 0.70 kg. Starting from rest, this fist attains a velocity of 8.0 m/s in 0.15 s. What is the magnitude of the average net force applied to the fist to achieve this level of performance?

A 95.0-kg person stands on a scale in an elevator. What is the apparent weight when the elevator is (a) accelerating upward with an acceleration of 1.80 m/s2 , (b) moving upward at a constant speed, and (c) accelerating downward with an acceleration of 1.30 m/s2 ?

Two forces, and , act on the 7.00-kg block shown in the drawing. The magnitudes of the forces are F1 59.0 N and F2 33.0 N. What is the horizontal acceleration (magnitude and direction) of the block? 9

A student presses a book between his hands, as the drawing indicates. The forces that he exerts on the front and back covers of the book are perpendicular to the book and are horizontal. The book weighs 31 N. The coefficient of static friction between his hands and the book is 0.40. To keep the book from falling, what is the magnitude of the minimum pressing force that each hand must exert?

The speed of a bobsled is increasing because it has an acceleration of 2.4 m/s2 . At a given instant in time, the forces resisting the motion, including kinetic friction and air resistance, total 450 N. The combined mass of the bobsled and its riders is 270 kg. (a) What is the magnitude of the force propelling the bobsled forward? (b) What is the magnitude of the net force that acts on the bobsled?

A 1.14  104 -kg lunar landing craft is about to touch down on the surface of the moon, where the acceleration due to gravity is 1.60 m/s2 . At an altitude of 165 m the crafts downward velocity is 18.0 m/s. To slow down the craft, a retrorocket is firing to provide an upward thrust. Assuming the descent is vertical, find the magnitude of the thrust needed to reduce the velocity to zero at the instant when the craft touches the lunar surface.

In a European country a bathroom scale displays its reading in kilograms. When a man stands on this scale, it reads 92.6 kg. When he pulls down on a chin-up bar installed over the scale, the reading decreases to 75.1 kg. What is the magnitude of the force he exerts on the chin-up bar?

A 1380-kg car is moving due east with an initial speed of 27.0 m/s. After 8.00 s the car has slowed down to 17.0 m/s. Find the magnitude and direction of the net force that produces the deceleration.

When a 58-g tennis ball is served, it accelerates from rest to a speed of 45 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 44 cm. What is the magnitude of the net force acting on the ball?

In preparation for this problem, review Conceptual Example 7. A space traveler whose mass is 115 kg leaves earth. What are his weight and mass (a) on earth and (b) in interplanetary space where there are no nearby planetary objects?

(a) Calculate the magnitude of the gravitational force exerted on a 425-kg satellite that is a distance of two earth radii from the center of the earth. (b) What is the magnitude of the gravitational force exerted on the earth by the satellite? (c) Determine the magnitude of the satellites acceleration. (d) What is the magnitude of the earths acceleration?

The drawing shows Robin Hood (mass 77.0 kg) about to escape from a dangerous situation. With one hand, he is gripping the rope that holds up a chandelier (mass 195 kg). When he cuts the rope where it is tied to the floor, the chandelier will fall, and he will be pulled up toward a balcony above. Ignore the friction between the rope and A the beams over which it slides, and find (a) the acceleration with which Robin is pulled upward and (b) the tension in the rope while Robin escapes.

A skater with an initial speed of 7.60 m/s stops propelling himself and begins to coast across the ice, eventually coming to rest. Air resistance is negligible. (a) The coefficient of kinetic friction between the ice and the skate blades is 0.100. Find the deceleration caused by kinetic friction. (b) How far will the skater travel before coming to rest?

The central ideas in this problem are reviewed in Multiple-Concept Example 9. One block rests upon a horizontal surface. A second identical block rests upon the first one. The coefficient of static friction between the blocks is the same as the coefficient of static friction between the lower block and the horizontal surface. A horizontal force is applied to the upper block, and the magnitude of the force is slowly increased. When the force reaches 47.0 N, the upper block just begins to slide. The force is then removed from the upper block, and the blocks are returned to their original configuration. What is the magnitude of the horizontal force that should be applied to the lower block so that it just begins to slide out from under the upper block?

A mountain climber, in the process of crossing between two +x +y Top view 55.0 54.0 N 88.0 N F1 F2 B B 25.0 kg 80.0 kg 10.0 kg cliffs by a rope, pauses to rest. She weighs 535 N. As the drawing shows, she is closer to the left cliff than to the right cliff, with the result that the tensions in the left and right sides of the rope are not the same. Find the tensions in the rope to the left and to the right of the mountain climber.

At an airport, luggage is unloaded from a plane into the three cars of a luggage carrier, as the drawing shows. The acceleration of the carrier is 0.12 m/s2 , and friction is negligible. The coupling bars have negligible mass. By how much would the tension in each of the coupling bars A, B, and C change if 39 kg of luggage were removed from car 2 and placed in (a) car 1 and (b) car 3? If the tension changes, specify whether it increases or decreases.

Consult Multiple-Concept Example 10 for insight into solving this type of problem. A box is sliding up an incline that makes an angle of 15.0 with respect to the horizontal. The coefficient of kinetic friction between the box and the surface of the incline is 0.180. The initial speed of the box at the bottom of the incline is 1.50 m/s. How far does the box travel along the incline before coming to rest?

A duck has a mass of 2.5 kg. As the duck paddles, a force of 0.10 N acts on it in a direction due east. In addition, the current of the water exerts a force of 0.20 N in a direction of 52 south of east. When these forces begin to act, the velocity of the duck is 0.11 m/s in a direction due east. Find the magnitude and direction (relative to due east) of the displacement that the duck undergoes in 3.0 s while the forces are acting.

Three uniform spheres are located at the corners of an equilateral triangle. Each side of the triangle has a length of 1.20 m. Two of the spheres have a mass of 2.80 kg each. The third sphere (mass unknown) is released from rest. Considering only the gravitational forces that the spheres exert on each other, what is the magnitude of the initial acceleration of the third sphere?

Refer to Multiple-Concept Example 10 for help in solving problems like this one. An ice skater is gliding horizontally across the ice with an initial velocity of 6.3 m/s. The coefficient of kinetic friction between the ice and the skate blades is 0.081, and air resistance is negligible. How much time elapses before her velocity is reduced to 2.8 m/s?

As part a of the drawing shows, two blocks are connected by a rope that passes over a set of pulleys. One block has a weight of 412 N, and the other has a weight of 908 N. The rope and the pulleys are massless and there is no friction. (a) What is the acceleration of the lighter block? (b) Suppose that the heavier block is removed, and a downward force of 908 N is provided by someone pulling on the rope, as part b of the drawing shows. Find the acceleration of the remaining block. (c) Explain why the answers in (a) and (b) are different.

The three objects in the drawing are connected by strings that pass over massless and friction-free pulleys. The objects move, and the coefficient of kinetic friction between the middle object and the surface of the table is 0.100. (a) What is the acceleration of the three objects? (b) Find the tension in each of the two strings.

The basic concepts in this problem are presented in MultipleConcept Example 9. A 225-kg crate rests on a surface that is inclined above the horizontal at an angle of 20.0. A horizontal force (magnitude  535 N and parallel to the ground, not the incline) is required to start the crate moving down the incline. What is the coefficient of static friction between the crate and the incline?

The drawing shows a 25.0-kg crate that is initially at rest. Note that the view is one looking down on the top of the crate. Two forces, and , are applied to the crate, and it begins to move. The coefficient of kinetic friction between the crate and the floor is k  0.350. Determine the magnitude and direction (relative to the x axis) of the acceleration of the crate.

A bowling ball (mass 7.2 kg, radius 0.11 m) and a billiard ball (mass 0.38 kg, radius 0.028 m) may each be treated as uniform spheres. What is the magnitude of the maximum gravitational force that each can exert on the other? 26.