Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | Two forces and are acting on the box shown in the drawing, causing the box to move across the floor. The two force vectors are drawn to scale. Which force does more work? Justify your answer.

Question 1

| 2 | A box is being moved with a velocity by a force (parallel to ) along a level horizontal floor. The normal force is , the kinetic frictional force is , and the weight of the box is . Decide which forces do positive, zero, or negative work. Provide a reason for each of your answers.

| 3 | A force does positive work on a particle that has a displacement pointing in the direction. This same force does negative work on a particle that has a displacement pointing in the direction. In what quadrant does the force lie? Account for your answer.

| 4 | ssm A sailboat is moving at a constant velocity. (a) Is work being done by a net external force acting on the boat? Explain. (b) Recognizing that the wind propels the boat forward and the water resists the boat’s motion, what does your answer in part (a) imply about the work done by the wind’s force compared to the work done by the water’s resistive force?

| 5 | A ball has a speed of 15 m/s. Only one external force acts on the ball. After this force acts, the speed of the ball is 7 m/s. Has the force done positive or negative work? Explain.

| 6 | A slow-moving car may have more kinetic energy than a fast-moving motorcycle. How is this possible?

| 7 | A net external force acts on a particle. This net force is not zero. Is this sufficient information to conclude that (a) the velocity of the particle changes, (b) the kinetic energy of the particle changes, and (c) the speed of the particle changes? Give your reasoning in each case.

| 8 | The speed of a particle doubles and then doubles again because a net external force acts on it. Does the net force do more work during the first or the second doubling? Justify your answer.

| 9 | ssm A shopping bag is hanging straight down from your hand as you walk across a horizontal floor at a constant velocity. (a) Does the force that your hand exerts on the bag’s handle do any work? Explain. (b) Does this force do any work while you are riding up an escalator at a constant velocity? Give a reason for your answer.

| 10 | In a simulation on earth, an astronaut in his space suit climbs up a vertical ladder. On the moon, the same astronaut makes the same climb. In which case does the gravitational potential energy of the astronaut change by a greater amount? Account for your answer.

| 11 | A net external nonconservative force does positive work on a particle, and both its kinetic and potential energies change. What, if anything, can you conclude about (a) the change in the particle’s total mechanical energy and (b) the individual changes in the kinetic and potential energies? Justify your answers.

| 12 | Suppose the total mechanical energy of an object is conserved. (a) If the kinetic energy decreases, what must be true about the gravitational potential energy? (b) If the potential energy decreases, what must be true about the kinetic energy? (c) If the kinetic energy does not change, what must be true about the potential energy?

| 13 | ssm In Example 10 the Steel Dragon starts with a speed of 3.0 m/s at the top of the drop and attains a speed of 42.9 m/s when it reaches the bottom. If the roller coaster were to then start up an identical hill, would its speed be 3.0 m/s at the top of this hill? Assume that friction is negligible. Explain your answer in terms of energy concepts.

| 14 | A person is riding on a Ferris wheel. When the wheel makes one complete turn, is the net work done by the gravitational force positive, negative, or zero? Justify your answer.

| 15 | Consider the following two situations in which the retarding effects of friction and air resistance are negligible. Car A approaches a hill. The driver turns off the engine at the bottom of the hill, and the car coasts up the hill. Car B, its engine running, is driven up the hill at a constant speed. Which situation is an example of the principle of conservation of mechanical energy? Provide a reason for your answer.

| 16 | A trapeze artist, starting from rest, swings downward on the bar, lets go at the bottom of the swing, and falls freely to the net. An assistant, standing on the same platform as the trapeze artist, jumps from rest straight downward. Friction and air resistance are negligible. (a) On which person, if either, does gravity do the greatest amount of work? Explain. (b) Who, if either, strikes the net with a greater speed? Why?

| 17 | The drawing shows an empty fuel tank about to be released by three different jet planes. At the moment of release, each plane has the same speed and each tank is at the same height above the ground. However, the directions of travel are different. In the absence of air resistance, do the tanks have different speeds when they hit the ground? If so, which tank has the largest speed and which has the smallest speed? Explain.

| 18 | Is it correct to conclude that one engine is doing twice the work of another just because it is generating twice the power? Explain, neglecting friction and taking into account the time of operation of the engines.

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