Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | Two identical automobiles have the same speed, one traveling east and one traveling west. Do these cars have the same momentum? Explain.

| 2 | ssm In Times Square in New York City, people celebrate on New Year’s Eve. Some just stand around, but many move about randomly. Consider a system consisting of all of these people. Approximately, what is the total linear momentum of this system at any given instant? Justify your answer.

| 3 | Two objects have the same momentum. Do the velocities of these objects necessarily have (a) the same directions and (b) the same magnitudes? Give your reasoning in each case.

| 4 | (a) Can a single object have kinetic energy but no momentum? (b) Can a system of two or more objects have a total kinetic energy that is not zero but a total momentum that is zero? Account for your answers.

| 5 | An airplane is flying horizontally with a constant momentum during a time interval . (a) With the aid of Equation 7.4, decide whether a net impulse is acting on the plane during this time interval. (b) In the horizontal direction, both the thrust generated by the engines and air resistance act on the plane. What does the answer in part (a) imply about the impulse of the thrust compared to the impulse of the resistive force?

| 6 | You have a choice. You may get hit head-on either by an adult moving slowly on a bicycle or by a child that is moving twice as fast on a bicycle. The mass of the child and bicycle is one-half that of the adult and bicycle. Considering only the issues of mass and velocity, which collision do you prefer? Or doesn’t it matter? Account for your answer.

| 7 | An object slides along the surface of the earth and slows down because of kinetic friction. If the object alone is considered as the system, the kinetic frictional force must be identified as an external force that, according to Equation 7.4, decreases the momentum of the system. (a) If both the object and the earth are considered to be the system, is the force of kinetic friction still an external force? (b) Can the frictional force change the total linear momentum of the two-body system? Give your reasoning for both answers.

| 8 | The energy released by the exploding gunpowder in a cannon propels the cannonball forward. Simultaneously the cannon recoils. Which has the greater kinetic energy, the launched cannonball or the recoiling cannon? Explain, assuming that momentum conservation applies.

| 9 | The drawing shows a garden sprinkler that whirls around a vertical axis. From each of the three arms of the sprinkler, water exits through a tapered nozzle. Because of this nozzle, the water leaves each nozzle with a speed that is greater than the speed inside the arm. (a) Apply the impulse–momentum theorem to deduce the direction of the force applied to the water. (b) Then, with the aid of Newton’s third law, explain how the water causes the whirling motion.

| 10 | Concept Simulation 7.1 at reviews the concepts that are pertinent in this question. In movies, Superman hovers in midair, grabs a villain by the neck, and throws him forward. Superman, however, remains stationary. Using the conservation of linear momentum, explain what is wrong with this scene.

| 11 | A satellite explodes in outer space, far from any other body, sending thousands of pieces in all directions. How does the linear momentum of the satellite before the explosion compare with the total linear momentum of all the pieces after the explosion? Account for your answer.

| 12 | You are a passenger on a jetliner that is .ying at a constant velocity. You get up from your seat and walk toward the front of the plane. Because of this action, your forward momentum increases. What, if anything, happens to the forward momentum of the plane? Give your reasoning.

| 13 | ssm An ice boat is coasting on a frozen lake. Friction between the ice and the boat is negligible, and so is air resistance. Nothing is propelling the boat. From a bridge someone jumps straight down into the boat, which continues to coast straight ahead. (a) Does the total horizontal momentum of the boat plus the jumper change? (b) Does the speed of the boat itself increase, decrease, or remain the same? Explain your answers.

| 14 | On a distant asteroid, a large catapult is used to throw chunks of stone into space. Could such a device be used as a propulsion system to move the asteroid closer to the earth? Explain.

| 15 | A collision occurs between three moving billiard balls such that no net external force acts on the three-ball system. Is the momentum of each ball conserved during the collision? If so, explain why. If not, what quantity is conserved?

| 16 | In an elastic collision, is the kinetic energy of each object the same before and after the collision? Explain.

| 17 | Concept Simulation 7.2 at illustrates the concepts that are involved in this question. Review Multiple-Concept Example 7. Now, suppose both objects have the same mass, . Describe what happens to the velocities of both objects as a result of the collision, using the algebraic expressions in Equations 7.8a and 7.8b to justify your answers.

| 18 | Where would you expect the center of mass of a doughnut to be located? Why?

| 19 | Would you expect the center of mass of a baseball bat to be located halfway between the ends of the bat, nearer the lighter end or nearer the heavier end? Provide a reason for your answer.

| 20 | ssm A sunbather is lying on a floating raft that is stationary. She then gets up and walks to one end of the raft. Consider the sun-bather and raft as an isolated system. (a) What is the velocity of the center of mass of this system while she is walking? Why? (b) Does the raft itself move while she is walking? If so, what is the direction of the raft’s velocity relative to that of the sunbather? Provide a reason for your answer.

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