Two football players are pushing a 60 kg blocking sled acrossthe fie ld at a constant

An object is subject to two forces that do not point in opposite directions. ls it possible lo choose their magnitudes so that the object is in equilibri um? Explain.

Are the objects described here in static equilibrium, dynamic equilibrium, or not in equilibri um at all? a. A girder is lifted at constant speed by a crane. b. A girder is lowered by a crane. Tt is slowing down. c. You' re straining to hold a 200 lb barbell ovec your head. d. A jet plane has reached its cruising speed and altitude. e. A rock is falling into the Grand Canyon. f. A box in the back of a truck doesn' t slide as the truck stops.

What forces are acting on you right now? What net force is acting on you right now?

Decide whether each of the following is true or false. Give a reason! a. The mass of an object depends on its location. b. The weight of an object depends on its location. c. Mass and weight describe the same thing in different units.

An astronaut takes his bathroom scale to the moon and then stands on it. Is the reading of the scale his true weight? Explain.

A light block of mass m and a heavy block of mass M are attached to the ends of a rope. A student holds the heavier block and lets the lighter block hang below it, as shown in Figure M Q5.6. Then she lets go. Air resistance can be neglected. a. What is the tension in the rope while the blocks are falli ng, before either hits the ground? b. Would your answer be different if she had been holding the lighter block initially?

a. Can the normal force on an object be directed FIGURE 05.6 hori zontally? If not, why not? If so, provide an example. b. Can the normal force on an object be directed downward? If not, why not? If so, provide an example.

A ball is thrown straight up. Taking the drag force of air into account, does it take longer for the ball to travel to the top of its motion or for it to fall back down agai n?

You are going sledding with your friends, sliding down a snowy hill. Friction can't be ignored. Riding solo on your sled, you have a certain acceleration. Would the acceleration change if you let a friend ride with you, increasing the mass? Explain.

Suppose you are holding a box in front of you and away from your body by squeezing the sides, as shown in Figure Q5. l 0. Draw a free-body diagram showing all of the forces on the box. What is the force that is holding the box up, the force that is opposite the weight force?

You are walking up an icy slope. Suddenly your feet slip, and you start to slide backward. Will you slide at a constant speed, or will you accelerate?

Three objects move through the air as shown in Figure Q5. l 2. Rank in order, from largest to smallest, the three drag forces D1, D2 , and D3 Some may be equal. Give your answer in the form A < B = C and state your reasoning.

A skydi ver is falling at her terminal speed. Right after she opens her parachute, which has a very large area, what is the direction of the net force on her?

Raindrops caa fall al different speeds; some fall quite quickly, others quite slowly. Why might this be true?

An ai rplane moves through the air at a constant speed. The engines' thrust aiJpl ies a force in the direction of motion, and this force is equal in magnitude and opposite in direction to the drag force. Reducing thrust will cause the plane to fly at a slower- but still constant-speed. Explain why this is so.

Is it possible for an object to travel in air (aster than its terminal speed? If not, why not? If so, explain how this might happen.

For Questions 17 through 20, determine the tension in the rope at the point indicated with a dot. All objects are at rest. The strings and pulleys are massless, and the pulleys are fri ctionless.

"For Questions 17 through 20, determine the tension in the rope at the point indicated with a dot. All objects are at rest. The strings and pulleys are massless, and the pulleys are fri ctionless."

"For Questions 17 through 20, determine the tension in the rope at the point indicated with a dot. All objects are at rest. The strings and pulleys are massless, and the pulleys are fri ctionless."

"For Questions 17 through 20, determine the tension in the rope at the point indicated with a dot. All objects are at rest. The strings and pulleys are massless, and the pulleys are fri ctionless."

In Figure QS.2 1, block2 ismoving to the right. There is no friction between the floor and block 2, but there is a friction force between blocks I and 2. In which di rection is the kinetic fri ction force on block I? On block 2? Explain

The wood block in Figure QS.22 is at rest on a wood ramp. fn which direction is the static friction force on block I? A. Up the slope. B. Down the slope. C. The fri ction force is zero.D. There's not enough information to tell.

A 2.0 kg ball is suspended by two light strings as shown in Figure QS.23. What is the tens ion T in the angled string? A. 9.5 N C. 20 N B. 15 N D. 26N E. 30 N

Whi le standing in a low tunne l, you raise your arms and push against the ceil ing with a force of 100 N. Your mass is 70 kg. a. What force does the cei ling exert on you? A. ION C. 690 N D. 790 N B. IOO N E. 980 N

A 5.0 kg dog s its on the floor of an e levator that is accelerating downward at 1.20 mls2 . a. What is the magnitude of the normal force of the e levator floor on the dog? A. 34 N B. 43N C. 49N D. SS N E. 74 N b. What is the magnitude of the force of the dog on the e levator floor? A. 4.2N B. 49N C. SSN D. 43N E. 74 N

A 3.0 kg puck slides due east on a horizontal frictionless surface at a constant speed of 4 .5 m/s. Then a force of magnitude 6 .0 N, d irected due north, is appl ied for 1.5 s. Afterward, a. What is the northward component of the puck's velocity? A. 0.50 mis B. 2.0 mis C. 3.0 mis D. 4 .0 mis E. 4 .5 mis b. What is the speed of the puck? A. 4.9 mis B. 5.4 mis C. 6.2 mis D. 7.5 mis E. 11 m/s

Eric has a mass of 60 kg. He is standi ng on a scale in an elevator that is accelerating downward at I. 7 m/s2 . What is the approximate reading on the scale? A. 0 N B. 400 N C. 500 N D. 600 N

The two blocks in Figure QS.28 are at rest on frictionless surfaces. What must be the mass of the right block in order that the two blocks remain stationary? A. 4.9 kg B. 6. 1 kg C. 7.9 kg D. 9.8 kg E. 12 kg

A football player al practice pushes a 60 kg b locking sled across the fie ld at a constant speed. The coeffi cient of kinetic friction between the grass and the sled is 0 .30. How much force must he apply to the sled? A. 18N B. 60N C. l 80 N D. 600 N

Two football players are pushing a 60 kg blocking sled across the fie ld at a constant speed of 2.0 mis. The coefficient of kinetic fri ction between the grass and the sled is 0.30 . Once they stop pushing, how far will the sled slide before coming to rest? A. 0.20 m B. 0.68 m C. 1.0 m D. 6.6 m

Land Rover ads used to claim that their vehicles could climb a slope of 45. For this to be possible, what must be the minimum coefficient of static friction between the vehicle's tires and the road? A. 0.5 B. 0.7 C. 0.9 D. 1.0

A truck is traveling at 30 111/s on a slippe1y road. The driver slams on the brakes and the truck starts to skid. If the coeffi cient of kinetic friction between the tires and the road is 0.20, how far will the truck skid before stopping? A. 230 m B. 300 m C. 450 m D. 680 m

What is the minimum downward force on the box in Figure P5.33 that will keep it from slipping? The coefficients of static and kinetic friction between the box and the floor are 0.35 and 0.25, respectively.

What is the drag force on a 1.6-m-wide, 1.4-m-high car traveling at a. IO mis ( = 22 mph)? b. 30 mis ( = 65 mph)?

A 22-cm-diameter bowling ball has a terminal speed of77 mis. What is the ball's mass?

Running on a treadmill is slightly easier than mnning outside BIO because there is no drag force to work against. Suppose a 60 kg runner completes a 5.0 km race in 18 minutes. Use the cross-section area estimate of Example 5.14 to determine the drag force on the runner during the race. What is this force as a fraction of the runner's weight?

A 75 kg skydiver can be modeled as a rectangular "box" with dimensions 20 cm X 40 cm X 1.8 m. What is his terminal speed if he falls feet first?

The air is less dense at higher elevations, so skydivers reach a high terminal speed. The highest recorded speed for a skydiver was achieved in a jump from a height of 39,000 m. At this elevation, the density of the air is only 4.3% of the surface density. Use the data from Example 5. 14 to estimate the terminal speed of a skydiver at this elevation.

A 1000 kg car pushes a 2000 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push backward against the ground with a force of 4500 N. a. What is the magnitude of the force of the car on the truck? b. What is the magnitude of the force of the tmck on the car?

A 2200 kg tmck has put its front bumper against the rear bumper of a 2400 kg SUV to give it a push. With the engine at full power and good tires on good pavement, the maximum forward force on the tmck is 18,000 N. a. What is the maximum possible acceleration the tmck can give the SUV? b. At this acceleration, what is the force of the SUV's bumper on the truck's bumper?

Blocks with masses of 1.0 kg, 2.0 kg, and 3.0 kg are lined up in a row on a frictionless table. All three are pushed forward by a 12 N force applied to the 1.0 kg block. How much force does the 2.0 kg block exert on (a) the 3.0 kg block and (b) the 1.0 kg block?

What is the tension in the rope of Figure P5.42?

A 2.0-m-long, 500 g rope pulls a IO kg block of ice across a horizontal, frictionless su1face. The block accelerates at 2.0 mls2 . How much force pulls frnward on (a) the block of ice, (b) the rope?

Figure P5.44 shows two 1.00 kg blocks connected by a rope. A second rope hangs beneath FIGURE P5.42 the lower block. Both ropes have a mass of 250 g. The entire asse)llbly is accelerated upward at 3.00 mls2 by force F. a. What is F? b. What is the tension at the top end of rope I? Rope I c. What is the tension at the bottom end of rope I? d. What is the tension at the top end of rope 2?

Each of 100 identical blocks sitting on a FIGURE P5.44 frictionless surface is connected to the next block by a massless string. The first block is pulled with a force of IOON. a. What is the tension in the string connecting block 100 to block 99? b. What is the tension in the string connecting block 50 to block 51?

Two blocks on a frictionless table, A and B, are connected by a massless string. When block A is pulled with a certain force, dragging block B, the tension in the stri ng is 24 N. When block B is pulled by the same force, dragging block A, the tension is 18 N. What is the ratio mAlm8 of the blocks' masses?

A 500 kg piano is being lowered into position by a crane while two people steady it with ropes pulling to the sides. Bob's rope pulls to the left, 15 below horizontal, with 500 N of tension. Ellen's rope pulls toward the right, 25 below hotizontal. a. What tension must Ellen maintain in her rope to keep the piano descending vertically at constant speed? b. What is the tension in the vertical main cable supporting the piano?

Dana has a sports medal suspended by a long tibbon from her rearview min-or. As she accelerates onto the highway, she notices that the medal is hanging at an angle of I 0 from the vertical. a. Does the medal lean toward or away from the windshield? Explain. b. What is her acceleration?

Figure P5.49 shows the INT velocity graph of a 2.0 kg object as it moves along the x-axis. What is the net force acting on this object at t = I s? At 4 s? At 7 s?

Your forehead can withstand a force of about 6.0 kN before fracturing, while your cheekbone can only withstand about 1.3 kN. a. Tf a 140 g baseball strikes your head at 30 mis and stops in 0.0015 s, what is the magnitude of the ball's acceleration? b. What is the magnitude of the force that stops the baseball? c. What force does the baseball apply to your head? Explain. d. Are you in danger of a fracture if the ball hits you in the forehead? In the cheek?

A 50 kg box hangs from a rope. What is the tension in the rope if a. The box is at rest? b. The box has v_v = 5.0 mis and is speeding up at 5.0 m/s2?

A fisherman has caught a very large, 5.0 kg fi sh from a dock that is 2.0 m above the water. He is using lightweight fi shing line that will break under a tension of 54 Nor more. He is eager to get the fi sh to the dock in the shortest possible time. If the fish is at rest at the water's surface, what's the least amount of time in which the fi sherman can raise the fi sh to the dock without losing it?

A 50 kg box hangs from a rope. What is the tension in the rope if a. The box moves up at a steady 5.0 mis? b. The box has vy = 5.0 mis and is slowing down at 5.0 mls2?

Riders on the Tower of Doom, an amusement park ride, experience 2.0 s of free fall, after which they are slowed to a stop in 0.50 s. What is a 65 kg rider's apparent weight as the ride is coming to rest? By what factor does this exceed her actual weight?

Seat belts and air bags save lives by reducing the forces exerted on the driver and passengers in an automobile collision. Cars are designed with a "crumple zone" in the front of the car. In the event of an impact, the passenger compa1tment decelerates over a distance of about I m as the front of the car crumples. An occupant restrained by seat belts and air bags decelerates with the car. By contrast, an unrestrained occupant keeps moving forward with no loss of speed (Newton's first law!) until hitting the dashboard or windshield, as we saw in Figure 4.2. These are unyielding surfaces, and the unfortunate occupaat thel) decelerates over a distance of only about 5 mm. a. A 60 kg person is in a head-on coll isjon. The car's speed at impact is 15 mis. Estimate the net force on the person if he or she is wearing a seat belt and if the air bag deploys. b. Estimate the net force that ultimately stops the person if he or she is not restrained by a seat belt or air bag. c. How do these two forces compare to the person's weight?

Elite quarterbacks can throw a football 70 m. To achieve such a throw, a quarterback accelerates the 0.42 kg ball over a distance of approximately 1.0 m. a. If you assume that the quarterback throws the ball at the optimal angle, at what speed does he throw the ball? b. What force must he apply to the ball to throw it at this speed?

A 20,000 kg rocket has a rocket motor that generates 3.0 X 105 N of thrust. a. What is the rocket's initial upward acceleration? b. At an altitude of 5.0 km the rocket's acceleration has increased to 6.0 mls2 What mass of fuel has it burned?

You've always wondered about the acceleration of the elevators in the IOI -story-tall Empire State Building. One day, while visiting New York, you take your bathroom scale into the elevator and stand on it. The scale reads 150 lb as the door closes. The reading varies between 120 lb and 170 lb as the elevator travels I 0 I floors. a. What is the magnitude of the acceleration as the elevator starts upward? b. What is the magnitude of the acceleration as the elevator brakes to a stop?

A 23 kg child goes down a straight slide inclined 38 above ho1izontal. The child is acted on by his weight, the normal force from the slide, kinetic friction, and a horizontal rope exe1ting a 30 N force as shown in Figure P5.59. How large is the normal force of the slide on the child?

An impa la is an African antelope capable of a remarkable vertical leap. In one recorded leap, a 45 kg impala went into a deep crouch, pushed straight up for 0.21 s, and reached a height of 2.5 m above the ground. To achieve th is vertical leap, with what force did the impala push down on the ground? What is the ratio of this force to the antelope's weight?

Josh starts his sled at the top of a 3.0-m-high hill that has a constant slope of 25. After reaching the bottom, he slides across a horizontal patch of snow. Ignore friction on the hill, but assume that the coefficient of kinetic friction between his sled and the horizontal patch of snow is 0.050. How far from the base of the hill does he end up?

The drag force is an i mportant fact of li fe for the small marine crustaceans called copepods. The drag force for small objects in water is very different from the drag force in air, so the drag equations of this chapter don ' t apply. However, we can estimate the drag force from data. The velocity graph of Figure P5.62 shows two phases of the swimming motion of a 1.8 mg copepod. First it swims vigorously, speeding up; then it slows down under the influence of drag. What is the magnitude of the drag force? What is the ratio of the drag force to the copepod's weight?

A wood block, after bei ng given a starti ng push, slides down a wood ramp at a constant speed. What is the angle of the ramp above horizontal?

Researchers often use force plates to measure the forces that people exert against the floor during movement. A force plate works like a bathroom scale, but it keeps a record of how the reading changes with time. Figure P5.64 shows the data from a force plate as a woman jumps straight up and then lands. a. What was the vertical component of her acceleration during push-off? b. What was the vertical component of her acceleration while in the air? c. What was the vertical component of her acceleration during the landing? d. What was her speed as her feet left the force plate? e. How high did she jump?

A person with compromised pinch strength in his fingers can only exert a normal force of 6.0 N to either side of a pinch-held object, such as the book shown in Figure P5.65. What is the greatest mass book he can hold onto ve1tically before it slips out of his fingers? The coeffi cient of static friction of the surface between the fingers and the book cover is 0.80.

lt'spossible foradetermined INT group of people to pull an aircraft. Drag is negligible at low speeds, and the only force impeding motion is the rolling friction of the rubber tires on the concrete runway. Tn 2000, a team of 60 British police officers set a world record by pulling a Boeing 747, with a mass of 200,000 kg, a distance of I 00 m in 53 s. The plane started at rest. Estimate the force with which each officer pulled on the plane, assuming constant pulling force and constant acceleration.

A 1.0 kg wood block is pressed against a vertical wood wall by a 12 N force as shown in Figure P5.67. If the block is initially at rest, will it move upward, move downward, or stay at rest?

A 2.0 kg wood block is launched up a FIGURE P5.67 wooden ramp that is inclined at a 35 angle. The block's initial speed is 1 O m/s. a. What vertical height does the block reach above its starting point? b. What speed does it have when it slides back down to its starting point?

Two blocks are at rest on a flictionless incline, as shown in Figure P5.69. What are the tensions in the two strings?

Running indoors on a treadmill is sl ightly easier than running outside because you aren't moving through the air and there is no drag force to oppose your motion. A 60 kg man is running at 4.5 mis on an indoor treadmill. To experience the same intensity workout as he'd get outdoors, he tilts the treadmill at a slight angle, choosing the angle so that the component of his weight force down the ramp is the same as the missi ng drag force. What is the necessary angle? Use the cross-section area e stimate of Example 5. 14 to compute the drag force.

Two identical 2.0 kg blocks are stacked as shown in Figure P5.7 1. The bottom block is free to slide on a frictionless surface. The coefficient of static friction between the blocks is 0.35. What is the maximum horizontal force that can be applied to the lower block without the upper block slipping?

A wood block is sliding up a wood ramp. If the angle of the ramp is very steep, the block will reverse direction at some point and slide back down. If the angle of the ramp is shallow, the block will stop when it reaches the highest point of its motion. What is the smallest ramp angle, measured from the hmizontal, for which the block will slide back down after reaching its highest point?

A 2. 7 g Ping-Pong ball has a diameter of 4.0 cm. a. The ball is shot straight up at twice its terminal speed. What is its acceleration immediately after launch? b. The ball is shot straight down at twice its terminal speed. Whalis its acceleration immediately after launch?

Two blocks are connected by a string as in Figure PS.74. What is the upper block's acceleration if the coefficient of kinetic friction between the block and the table is 0.20?

The ramp in Figure PS.75 is frictionless. If the blocks are released from rest, which way does the I 0 kg block slide, and what is the magnitude of its acceleration?

The 100 kg block in Figure PS.76 takes 6.0 s to reach the floor after being released from rest. What is the mass of the block on the left?

A curler pushes a stone to a speed of 3.0 m/s over a time of 2.0 s. Ignoring the force of friction, how much force must the curler apply to the stone to bring it up to speed? A. 3.0N B. ISN C. 30N D. ISO N

The sweepers in a curling competition adjust the trajectory of the stone by A. Decreasing the coefficient of friction between the stone and the ice. B. Increasing the coefficient of friction between the stone and the ice. C. Changing friction from kinetic to static. D. Changing fri ction from static to kinetic.

Suppose the stone is launched with a speed of 3 mis and travels 40 m before coming to rest. What is the approximate magnitude of the friction force on the stone? A. ON B. 2N C. 20N D. 200N

Suppose the stone's mass is increased to 40 kg, but it is launched at the same 3 mis. Which one of the following is true? A. The stone would now travel a longer distance before coming to rest. B. The stone would now travel a shorter distance before coming to rest. C. The coefficient of fri ction would now be greater. D. The force of friction would now be greater.