?Energy and WorkWhy do you run out of breath when running up the stairs but not when

Momentum and Impulse Which has greater momentum: an automobile at rest or a moving skateboard?

Momentum and Impulse When a ball is hit with a given force, why does contact over a long time impart more speed to the ball than contact over a short time?

Impulse Changes Momentum Why is it a good idea to extend your bare hand forward when you are getting ready to catch a fast-moving baseball?

Impulse Changes Momentum Why is it poor judgment to have the back of your hand up against the outfield wall when you catch a long fly ball?

Impulse Changes Momentum Why is it advantageous, in karate, to apply a force for a short time?

Impulse Changes Momentum In boxing, why is it advantageous to roll with the punch?

Impulse Changes Momentum If a ball has the same speed just before being caught and just after being thrown, in which case does the ball undergo the greatest change in momentum: (1) When it is caught, (2) when it is thrown, or (3) when it is caught and then thrown back?

Impulse Changes Momentum In which of cases (1), (2), and (3) in Question 7 is the greatest impulse required?

Conservation of Momentum What does it mean to say that momentum (or any quantity) is conserved?

Conservation of Momentum When a cannonball is fired, momentum is conserved for the system of cannon + cannonball. Would momentum be conserved for the system if momentum were not a vector quantity? Explain.

Conservation of Momentum Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?

Conservation of Momentum If the equally massive railroad cars of Question 11 couple together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?

Energy and Work When is energy most evident?

Energy and Work Cite an example in which a force is exerted on an object without doing work on the object.

Energy and Work Which requires more work: lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance of 4 m?

Energy and Work A car is raised a certain distance in a service station lift and therefore has potential energy relative to the floor. If it were raised twice as high, how much potential energy would it have compared with what it had in the first case?

Energy and Work Two cars are raised to the same elevation on service station lifts. If one car is twice as massive as the other, how do their potential energies compare?

Energy and Work If a moving car speeds up until it is going three times as fast as it was, how much kinetic energy does it have compared with its initial kinetic energy?

Energy and Work What is the relationship between work and power?

Work–Energy Theorem What is the relationship between the gain in kinetic energy and the work when work is done?

Work–Energy Theorem Compared with the work that the brakes must supply to stop a car moving at some original speed, how much more work must the brakes supply to stop that car if it is moving four times as fast as the original speed? How do the respective stopping distances compare?

Conservation of Energy What will be the kinetic energy of the ram of a pile driver when it suddenly undergoes a 10-kJ decrease in potential energy?

Conservation of Energy An apple hanging from a limb has potential energy because of its height. If the apple falls, what becomes of this energy just before the apple hits the ground? When it hits the ground?

Machines Can a machine multiply input force? Input distance? Input energy? (If your three answers are the same, seek help, because this is an important question.)

Machines If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?

Efficiency What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?

Efficiency What becomes of energy when efficiency is lowered in a machine?

Sources of Energy What is the ultimate source of the energy supplied by fossil fuels, dams, and windmills?

Sources of Energy What is the ultimate source of geothermal energy?

Sources of Energy Can we correctly say that hydrogen is a relatively new source of energy? Why or why not?

If your instructor has an air table or air track, play around with carts or air pucks. Most important, predict what will happen before you initiate collisions.

When you get a bit ahead in your studies, cut classes some afternoon and visit your local pool or billiards parlor to bone up on momentum conservation. Note that, no matter how complicated the collision of balls, the momentum along the line of action of the cue ball before impact is the same as the combined momentum of all the balls along this direction after impact, and that the components of momenta perpendicular to this line of action cancel to zero after impact, the same value as before impact in this direction. You’ll see both the vector nature of momentum and its conservation more clearly when rotational skidding (“English”) is not imparted to the cue ball. When English is imparted by striking the cue ball off center, rotational momentum, which is also conserved, somewhat complicates analysis. But regardless of how the cue ball is struck, in the absence of external forces, both linear and rotational momentum are always conserved. Both pool and billiards offer a first-rate exhibition of momentum conservation in action.

Pour some dry sand into a tin can with a cover. Compare the temperature of the sand before and after you vigorously shake the can for a couple of minutes. Predict what occurs. What is your explanation?

Place a small rubber ball on top of a basketball or soccer ball, and then drop them together. If vertical alignment remains as they fall to the floor, you’ll see that the small ball bounces unusually high. Can you reconcile this with energy conservation?

Momentum = mv Show that the momentum for a 2-kg brick parachuting straight downward at a constant speed of 8 m/s is \(16 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}\). Text Transcription: 16 kg times m/s

Impulse = Ft Show that the impulse on a baseball that is hit with 100 N of force in a time of 0.5 s is 50 N.s.

Impulse = change in momentum; Ft = \(Delta\)mv Show that when a 10-kg cart undergoes a 2.0-m/s increase in speed, the impulse on the cart is \(20 \mathrm{~N} \cdot \mathrm{s}\). (The unit \(\mathrm{N} \cdot \mathrm{s}\) is equivalent to \(\mathrm{kg} \cdot \mathrm{m} / \mathrm{s}\).) Text Transcription: Delta 20 N times s N times s kg times m/s

Impulse = change in momentum; Ft = \(Delta\)mv Show that when an impulse produced by a 12-N force acts over 2.0 s on an ice puck initially at rest on an air table, the change in momentum is \(24 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}\). Text Transcription: Delta 24 kg times m/s

Work = \(\text { force } \times \text { distance }\); W = Fd Show that 2.4 J of work is done when a force of 2.0 N moves a book 1.2 m. (\(1 \mathrm{~N} \cdot \mathrm{m}=1 \mathrm{~J}\)) Text Transcription: force times distance 1 N times m = 1 J

Work = \(\text { force } \times \text { distance }\); W = Fd Calculate the work done when a 20-N force pushes a cart 3.5 m. Text Transcription: force times distance

Gravitational potential energy = \(\text { mass } \times \text { acceleration }\) due to \(\text { gravity } \times \text { height }\); PE = mgh Show that when a 3.0-kg book is lifted 2.0 m, its increase in gravitational potential energy is 60J. Text Transcription: mass times acceleration gravity times height

Gravitational potential energy = \(\text { mass } \times \text { acceleration }\) due to \(\text { gravity } \times \text { height }\); PE = mgh Show that the gravitational potential energy of a 1000-kg boulder raised 5 m above ground level is 50,000 J. (You can express g in units of N/kg, because m/s is equivalent to N/kg) Text Transcription: mass times acceleration gravity times height

Kinetic energy = \(1 / 2 \text { mass } \times \text { velocity }^{2} ; \mathrm{KE}=1 / 2 m v^{2}\) Show that the kinetic energy of a 1.0-kg book tossed across the room at a speed of 3.0 m/s is 4.5 J. (1 J is equivalent to \(1 \mathrm{~kg}(\mathrm{m} / \mathrm{s})^{2}\)) Text Transcription: 1/2 mass times velocity^2; KE = 1/2 mv^2 1 kg(m/s)^2

Kinetic energy = \(1 / 2 \text { mass } \times \text { velocity }^{2} ; \mathrm{KE}=1 / 2 m v^{2}\) Calculate the kinetic energy of a 84-kg scooter moving at 10 m/s. Text Transcription: 1/2 mass times velocity^2; KE = 1/2 mv^2

Work-energy theorem; \(W=\Delta \mathrm{KE}\) Show that 24J of work is done when a 3.0-kg block of ice is moved from rest to a speed of 4.0 m/s. Text Transcription: W=Delta KE

Work-energy theorem; \(W=\Delta \mathrm{KE}\) Show that a 2,500,000-J change in kinetic energy occurs for an airplane that is moved 500m in takeoff by a sustained force of 5000 N. Text Transcription: W=Delta KE

\(\text { Power }=\frac{\text { work }}{\text { time }}=\frac{W}{t}\) Show that 50 W of power is required to give a brick 100J of PE in a time of 2 s. Text Transcription: Power=work/time = w/t

\(\text { Power }=\frac{\text { work }}{\text { time }}=\frac{W}{t}\) Show that nearly 786 W of power is expended when a 500-N barbell is lifted 2.2 m above the floor in 1.4 s. Text Transcription: Power=work/time = w/t

\(\text { Efficiency }=\frac{\text { useful energy output }}{\text { total energy input }}\) The parameter is incorrect.Image size is not supported. Each image dimension must be between 40 and 2600 pixels. Text Transcription: Efficiency=useful energy output/total energy input

In Chapter 1 we learned that acceleration \(a=\frac{\Delta v}{\Delta t}\), and in Chapter 2 we learned that the cause of acceleration involves net force, where \(a=\frac{F}{m}\). Equate these two a = equations for acceleration and show that, for constant mass, \(F \Delta t=\Delta(m v)\). Text Transcription: a=Delta v/Delta t a=F/m F Delta t=Delta mv

A 10-kg bag of groceries is tossed onto a table at 3 m/s and slides to a stop in 2 s. Begin with the equation you derived in Problem 50, and show that the force of friction is 15 N.

An ostrich egg of mass m is tossed at a speed v into a sagging bed sheet and is brought to rest in a time t. (a) Show that the force acting on the egg when it hits the sheet is \(\frac{m v}{t}\). (b) Show that if the mass of the egg is 1 kg, its initial speed is 2 m/s, and the time to stop is 1 s, then the average force on the egg is 2 N. Text Transcription: mv/t

A 6-kg ball rolling at 3 m/s bumps into a pillow and stops in 0.5 s. (a) Show that the force exerted by the pillow is 36 N. (b) How much force does the ball exert on the pillow?

At a baseball game, a ball of mass m = 5 0.15 kg moving at a speed v = 5 30 m/s is caught by a fan. (a) Show that the impulse supplied to bring the ball to rest is \(4.5 \mathrm{~N} \cdot \mathrm{s}\). (b) Show that if the ball is stopped in 0.02 s, then the average force of the ball on the catcher’s hand is 225 N. Text Transcription: 4.5 N times s

Jeannie Beanie (mass 40 kg), standing on slippery ice, catches her leaping dog, Daisy (mass 20 kg), who leapt into her arms after running across an adjacent field at 6 m/s. Use the conservation of momentum to show that the speed of Jeannie and her dog after the catch is 2 m/s.

A railroad diesel engine weighs four times as much as a freight car. The diesel engine coasts at 5 km/h into a freight car that is initially at rest. Use the conservation of momentum to show that after they couple together, the engine + car coast at 4 km/h.

A 5-kg fish swimming at 1 m/s swallows an absentminded 1-kg fish swimming toward it at a velocity that brings both fish to a halt. Show that the speed of the smaller fish before lunch was 5 m/s.

Little Hudson (next page) holds the 10-kg barbell 0.3 m above the floor. (a) What is the PE of the barbell relative to the floor? (b) How much additional work will Hudson do when he lifts the barbell to a height 1.0 m above the floor? (c) If Hudson then drops the barbell, what will be its KE of impact with the floor?

If you push a crate horizontally with a force of 100 N across a 10-m factory floor, and the friction force between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate?

A simple lever is used to lift a heavy load. When a 60-N force pushes one end of the lever downward 1.2 m, the load rises 0.2 m. Show that the weight of the load is 360 N.

The following questions refer to Problem 60: (a) How much work is done by the 60-N force? (b) What is the gain in PE of the load? (c) How does the work done compare with the increased PE of the load?

In raising a 6000-N piano with a pulley system, the movers note that for every 2 m of rope pulled down, the piano rises 0.2 m. Show that, ideally, the force required to lift the piano is 600 N.

The girl steadily pulls her end of the rope upward a distance of 0.4 m with a constant force of 50 N. By how much does the PE of the block increase? Show that the mass of the block is 10 kg.

How many watts of power do you expend when you exert a force of 6 N that moves a book 2 m in a time interval of 3 s?

Show that 480 W of power is expended by a weightlifter who lifts a 60-kg barbell a vertical distance of 1.2 m in a time interval of 1.5 s.

When an average force F is exerted over a certain distance on a shopping cart of mass m, its kinetic energy increases by \(1 / 2 m v^{2}\). (a) distance over which the force acts is Use the work-energy theorem to show that the \(\frac{m v^{2}}{2 F}\). (b) If twice the force is exerted over twice the distance, how does the resulting increase in kinetic energy compare with the original increase in kinetic energy? Text Transcription: 1/2mv^2 mv^2/2F

Emily holds a banana of mass m over the edge of a bridge of height h. She drops the banana, and it falls to the river below. Use the conservation of energy to show that the speed of the banana just before hitting the water is \(v=\sqrt{(} 2 g h)\). Text Transcription: v=sqrt 2gh

Starting from rest, Megan zooms down a frictionless slide from an initial height of 4.0 m. Show that her speed at the bottom of the slide is \(\sqrt{80} \mathrm{~m} / \mathrm{s}\), or 8.9 m/s Text Transcription: sqrt 80 m/s

The balls have different masses and speeds. Rank the following from greatest to least: (a) Momentum. (b) The impulses needed to stop the balls.

Jogging Jake runs along a train flatcar that moves at the velocities shown. In each case, Jake’s velocity is given relative to the car. Call direction to the right positive. Rank the following from greatest to least: (a) The magnitude of Jake’s momentum relative to the flatcar. (b) Jake’s momentum relative to an observer at rest on the ground.

Starting from rest, Marshall pushes crates across the floor of his classroom for 3 seconds with a net force as shown. For each crate, rank the following from greatest to least: (a) Impulse delivered. (b) Change in momentum. (c) Final speed. (d) Momentum at the end of 3 seconds.

A ball is released from rest at the left of the metal track shown. Assume it has only enough friction to roll rather than to slide, but not enough to lessen its speed. Rank, from greatest to least, the following quantities at each point: (a) Momentum. (b) KE. (c) PE.

The roller coaster ride starts from rest at point A. Rank, from greatest to least, the following quantities at each point: (a) Speed. (b) KE. (c) PE.

Rank the scale readings from greatest to least. (Ignore friction.)

Momentum and Impulse A lunar vehicle is tested on Earth at a speed of 12 km/h. When it travels at that same speed on the Moon, is its momentum more, less, or the same?

Momentum and Impulse In terms of impulse and momentum, why do airbags in cars reduce the chances of injury in accidents?

Momentum and Impulse Why are today’s autos designed to crumple upon impact?

Impulse Changes Momentum If you throw a raw egg against a wall, you’ll break it; but if you throw it with the same speed into a sagging sheet, the egg won’t break. Explain, using concepts from this chapter.

Impulse Changes Momentum In terms of impulse and momentum, when a boxer is being hit, why is it important that he or she move away from the punch? Why is it disadvantageous to move into an oncoming punch?

Impulse Changes Momentum To throw a ball, do you exert an impulse on it? Do you exert an impulse to catch it if it’s traveling at the same speed? About how much impulse do you exert, in comparison, if you catch it and immediately throw it back again? (Imagine yourself on a skateboard.)

Conservation of Momentum Bronco dives from a hovering helicopter and finds his momentum increasing. Does this violate the conservation of momentum? Explain.

Conservation of Momentum A pair of skaters who are initially at rest push against each other so that they move in opposite directions. What is the total momentum of the two skaters as they move apart? Is there a different answer if their masses are not the same?

Conservation of Momentum When you are traveling in your car at highway speed, the momentum of a bug is suddenly changed as it splatters onto your windshield. Compared with the change in momentum of the bug, by how much does the momentum of your car change?

Conservation of Momentum You are at the front of a floating canoe near a dock. You leap, expecting to land on the dock easily. Instead you land in the water. Explain in terms of momentum conservation.

Conservation of Momentum If you throw a ball horizontally while standing on a skateboard, you roll backward with a momentum that matches that of the ball. Will you roll backward if you hold on to the ball while going through the motions of throwing it? Explain in terms of momentum conservation.

Conservation of Momentum A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must get to shore. How can this be accomplished? Explain in terms of momentum conservation.

Conservation of Momentum The examples of Exercises 84, 85, and 86 can be explained in terms of momentum conservation. Now explain them in terms of Newton’s third law.

Conservation of Momentum In the previous chapter, rocket propulsion was explained in terms of Newton’s third law. That is, the force that propels a rocket is from the exhaust gases pushing against the rocket, the reaction to the force that the rocket exerts on the exhaust gases. Now explain rocket propulsion in terms of momentum conservation.

Conservation of Momentum When vertically falling sand lands in a horizontally moving cart, the cart slows. Ignore any friction between the cart and the tracks. Give two reasons for the slowing of the cart, one in terms of a horizontal force acting on the cart, and one in terms of momentum conservation.

Conservation of Momentum In a movie, the hero jumps straight down from a bridge onto a small boat that continues to move with no change in velocity. What physics is being violated here?

Conservation of Momentum Freddy Frog drops vertically from a tree onto a horizontally moving skateboard. The skateboard slows. Give two reasons for the slowing, one in terms of a horizontal friction force between Freddy’s feet and the skateboard, and one in terms of momentum conservation.

Energy and Work If your friend pushes a stroller four times as far as you do while exerting only half the force, which one of you does more work? How much more?

Energy and Work Which requires more work: stretching a strong spring a certain distance or stretching a weak spring the same distance? Defend your answer.

Energy and Work Two people of the same weight climb a flight of stairs. The first person climbs the stairs in 30 s, and the second person climbs them in 40 s. Which person does more work? Which uses more power?

Energy and Work Why do you run out of breath when running up the stairs but not when walking up the stairs?

Work–Energy Theorem A friend says that when twice as much work is done on a wagon, it will gain twice as much kinetic energy. Another friend say it will gain four times as much kinetic energy. Which friend do you agree with?

Work–Energy Theorem Compared with a pickup truck moving at a certain speed and braking to a stop, how much work must the brakes do to stop the truck when it is moving at twice that speed?

Work–Energy Theorem When a cannon with a long barrel is fired, the force of expanding gases acts on the cannonball for a long distance. What effect does this have on the velocity of the emerging cannonball? (Do you see why long-range cannons have such long barrels?)

Conservation of Energy At what point in its motion is the KE of a pendulum bob at a maximum? At what point is its PE at a maximum? When its KE is at half its maximum value, how much PE does it possess?

Conservation of Energy A physics instructor demonstrates energy conservation by releasing a heavy pendulum bob, as shown, allowing it to swing to and fro. What would happen if, in his exuberance, he gave the bob a slight shove as it left his nose? Explain.

Conservation of Energy On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?

Conservation of Energy Consider the identical balls released from rest on tracks A and B, as shown. When they reach the right ends of the tracks, which will have the greater speed? Why is this question easier to answer than the similar one (Discussion Question 107) in Chapter 1?

Conservation of Energy If a golf ball and a Ping-Pong ball both move with the same kinetic energy, can you say which has the greater speed? Explain in terms of the definition of KE. Similarly, in a gaseous mixture of heavy molecules and light molecules with the same average KE, can you say which have the greater speed?

Conservation of Energy In the absence of air resistance, a snowball thrown vertically upward with a certain initial KE returns to its original level with the same KE. When air resistance is a factor affecting the snowball, does it return to its original level with the same, less, or more KE? Does your answer contradict the law of energy conservation?

Conservation of Energy You’re on a rooftop and you throw one snowball downward to the ground below and another upward. The second snowball, after rising, falls and also strikes the ground below. If air resistance can be neglected, and if downward and upward initial speeds are the same, how do the speeds of the snowballs upon striking the ground compare? (Use energy conservation to arrive at your answer.)

Conservation of Energy When a driver applies the brakes to keep a car going downhill at constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy go? Where does most of it appear in a hybrid vehicle?

Conservation of Energy When the mass of a moving object is doubled, with no change in speed, by what factor is its momentum changed? By what factor is its kinetic energy changed?

Conservation of Energy When the velocity of an object is doubled, by what factor is its momentum changed? By what factor is its kinetic energy changed?

Conservation of Energy Which, if either, has greater momentum: a 1-kg ball moving at 2 m/s or a 2-kg ball moving at 1 m/s? Which has greater kinetic energy?

Conservation of Energy If an object’s kinetic energy is zero, what is its momentum?

Conservation of Energy If your momentum is zero, is your kinetic energy necessarily zero also?

Conservation of Energy Two lumps of clay with equal and opposite momenta have a head-on collision and come to rest. Is momentum conserved? Is kinetic energy conserved? Why are your answers the same (or different)?

Conservation of Energy Consider Charlotte’s swinging-balls apparatus. If two balls are lifted and released, momentum is conserved because at impact, two balls pop out the other side with the same speed as the released balls. But momentum would also be conserved if one ball popped out at twice the speed. Explain why this never happens.

Machines Discuss the physics that explains how the girl in Figure 3.29 can jack up a car while applying so little force.

Machines Why bother using a machine if it cannot multiply work input to achieve greater work output?

Machines In the pulley system shown, Block A has a mass of 10 kg and is suspended precariously at rest. Assume that the pulleys and string are massless and there is no friction. No friction means that the tension in one part of the supporting string is the same as in any other part. Discuss why the mass of Block B is 20 kg.

Efficiency If an automobile had a 100% efficient engine, transferring all of the fuel’s energy to work, would the engine be warm to your touch? Would its exhaust heat the surrounding air? Would it make any noise? Would it vibrate? Would any of its fuel go unused? Discuss.

Efficiency The energy we need to live comes from chemically stored potential energy in food, which is transformed into other energy forms during the metabolism process. What happens to a person whose combined work and heat output is less than the energy consumed? What happens when the person’s combined work and heat output is greater than the energy consumed? Can an undernourished person perform extra work without extra food? Discuss and defend your answers.

Efficiency To combat wasteful habits, we often speak of “conserving energy” by turning off lights, heating or cooling systems, and hot water when these are not being used. In this chapter, we also speak of “energy conservation.” Distinguish between these two uses of the term conservation.

Sources of Energy What is the argument that dry-rock geothermal power is a form of nuclear power?

Sources of Energy What is the argument that the energy provided by a hydrogen fuel cell is actually solar energy?

Sources of Energy What is the fuel that powers a fuel cell in forklifts, busses, and trains?

Sources of Energy Imagine that you’re in a completely dark room with no windows except a \(1-\mathrm{ft}^{2}\) round hole in the roof. When the Sun is high in the sky, about 100 W of solar power enters the hole. On the floor where the light hits, you place a beach ball covered with aluminum foil, with the shiny side out. Discuss the illumination in your room compared with that of a 100-W incandescent light bulb. Text Transcription: 1-ft^2

Railroad cars are loosely coupled so that there is a noticeable delay between the time the first car is moved and the time the last car is moved from rest by the locomotive. Discuss the advisability of this loose coupling and slack between cars from the point of view of impulse and momentum.

Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and gains momentum. Your friend wants to discuss this. What do you say?

An ice sailcraft is stalled on a frozen lake on a windless day. The skipper sets up a fan as shown on the next page. If all the wind bounces backward from the sail, discuss whether or not the craft will be set in motion. If so, in what direction?

Will your answer to Question 126 be different if the air is brought to a halt by the sail without bouncing? Discuss.

Discuss the advisability of simply removing the sail in the situation discussed in Questions 126 and 127.

Suppose that three astronauts outside a spaceship decide to play catch. All three equally strong astronauts have the same mass. The first astronaut throws the second astronaut toward the third one and the game begins. Describe the motion of the astronauts as the game proceeds. How long will the game last?

Discuss whether something can have energy without having momentum. Discuss whether something can have momentum without having energy

Arrange the following four energy-transforming devices or processes in the correct order for converting solar energy to the kinetic energy of vehicles: (1) fuel cell, (2) electrolysis, (3) electric motor, (4) photovoltaic cell.

What are the roles of water, energy, hydrogen, and oxygen in (a) electrolysis, (b) operation of a fuel cell?

Does a car burn more fuel when its lights are turned on? Does the overall consumption of fuel depend on whether the engine is running while the lights are on? Discuss and defend your answers.

This may seem like an easy question for a physics buff to answer: With what force does a 10-N rock strike the ground if it is dropped from a rest position 10 m high? In fact, the question cannot be answered unless you have more information. What information is needed to answer this question, and why?

Your friend says that one way to improve air quality in a city is to synchronize traffic lights so that motorists can travel long distances at constant speed. Discuss the physics that supports this claim.

Does the electric power produced by wind-powered generators affect the speed of the wind? That is, would locations behind the wind blades be windier if the generators weren’t there?

A red ball of mass m and a blue ball of mass 2m have the same kinetic energy. Explain which of the two has the larger momentum, letting equations guide your discussion.

No work is done by gravity on a bowling ball resting or moving on a bowling alley, because the force of gravity on the ball acts perpendicular to the surface. But on an incline, the force of gravity has a vector component parallel to the alley, as sketch b shows. Discuss two ways in which this component (a) accounts for acceleration of the ball, and (b) accounts for work done on the ball to change its kinetic energy.

Consider a bob attached by a string-a simple pendulum-that swings to and fro. (a) Why does the tension force in the string not do work on the pendulum? (b) Explain, however, why the force due to gravity on the pendulum at nearly every point does do work on the pendulum. (c) Where is the single position of the pendulum where “no work by gravity” occurs?

Consider a satellite in a circular orbit above Earth’s surface. In Chapter 4 we will learn that the force of gravity changes the direction of motion of a satellite (and keeps it in a circle) but does NOT change its speed. Work done on the satellite by the gravitational force is zero. What is your explanation?