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Test 3 Review Chapters 7-9 7.1.5. Consider the following objects: (1) an electron (m = 9.1 × 1031 kg, v = 5.0 × 107 m/s) (2) the Hubble Space Telescope (m = 1.1 × 104 kg, v = 7.6 × 103 m/s) (3) a snail (m = 0.02 kg, v = 0.0003 m/s) (4) the largest super oil tanker (m = 1.5 × 108 kg, v = 2.0 m/s) (5) a falling rain drop (m = 0.0002 kg, v = 9.5 m/s) Which one of these objects requires the greatest change in momentum to stop moving? a) 1 b) 2 c) 3 d) 4 e) 5 7.1.3. During the filming of a movie, a stunt person jumps from the roof of a tall building, but no injury occurs because the person lands on a large, air-filled bag. Which one of the following best describes why no injury occurs? a) The bag increases the amount of time the force acts on the person and reduces the change in momentum. b) The bag increases the amount of time during which the momentum is changing and reduces the average force on the person. c) The bag decreases the amount of time during which the momentum is changing and reduces the average force on the person. d) The bag provides the necessary force to stop the person. e) The bag reduces the impulse to the person. 7.1.2. A football of mass m, initially at rest, is kicked so that leaves the foot at a speed v. If t represents the duration of the collision between the ball and the foot, which one of the following expressions determines the magnitude of the average force exerted on the ball? a) mvt b) (1/2)mv2 c) (1/2)mv2/t d) mv/t e) (1/2)mvt2 7.1.1. An astronaut drops a golf ball that is initially at rest from a cliff on the surface of the moon. The ball falls freely under the influence of gravity. Which one of the following statements is true concerning the ball as it falls? Neglect any frictional effects. a) The ball will gain an equal amount of kinetic energy during each second. b) The ball will gain an equal amount of momentum during each second. c) The ball will gain an equal amount of momentum during each meter through which it falls. d) The ball will gain twice the amount of speed for each meter through which it falls. e) The amount of momentum the ball gains will be directly proportional to the amount of potential energy that it loses. 7.1.6. A 28-kg beginning roller skater is standing in front of a wall. By pushing against the wall, she is propelled backward with a velocity of 1.2 m/s. While pushing, her hands are in contact with the wall for 0.80 s. Ignoring frictional effects, find the magnitude and direction of the impulse acting on the skater. a) 34 Ns b) +34 Ns c) 42 Ns d) +42 Ns e) 53 Ns 7.1.8. A small asteroid collides with a planet. Which one of the following statements concerning what happens during the collision is correct? a) The asteroid exerts a smaller force on the planet than the planet exerts on the asteroid. b) The planet exerts a force on the asteroid, but the asteroid does not exert a force on the planet. c) The asteroid exerts the same amount of force on the planet as the planet exerts on the asteroid. d) The asteroid exerts a force on the planet, but the planet does not exert a force on the asteroid. e) The planet exerts a smaller force on the asteroid than the asteroid exerts on the planet. 7.1.9. Three events are observed at a baseball game: I. A baseball is thrown by a pitcher. It starts from rest and is traveling at +38 m/s as it flies toward the catcher. II. A baseball is traveling at +38 m/s when it enters the catcher’s glove and stops. III. A baseball is traveling at +38 m/s when it hits a wall and bounces away from the wall at 38 m/s. The change in the momentum of the baseball has the largest magnitude in which case(s)? a) I only b) II only c) III only d) I and II only e) II and III only 7.1.10. A stone of mass m is dropped and falls freely under the influence of gravity. Ignoring any effects of air resistance, which one of the following expressions gives the momentum of the stone as a function of time t? The local acceleration due to gravity is g. a) b) gt c) d) mgt e) 7.2.1. A 9-kg object is at rest. Suddenly, it explodes and breaks into two pieces. The mass of one piece is 6 kg and the other is a 3-kg piece. Which one of the following statements concerning these two pieces is correct? a) The speed of the 6-kg piece will be one eighth that of the 3-kg piece. b) The speed of the 3-kg piece will be one fourth that of the 6-kg piece. c) The speed of the 6-kg piece will be one forth that of the 3-kg piece. d) The speed of the 3-kg piece will be one half that of the 6-kg piece. e) The speed of the 6-kg piece will be one half that of the 3-kg piece. 7.2.3. Consider the recoil of a rifle when it is fired. Recoil is an example of which of the following concepts? a) Newton’s Third Law of Motion b) Impulse c) Conservation of Momentum d) all of the above e) none of the above 7.3.6. Two identical cars were involved in a collision at an icy intersection. Car A was stopped at a traffic light. Car B was moving at a speed v when it suffered a perfectly inelastic collision with the back end of car A. With what speed did the two cars slide into the intersection after the collision? a) 2v b) v c) v/2 d) v/4 e) v/8 7.5.1. Consider the Earth and Mars in their orbit around the Sun. Where is the center of mass located for this three body system? a) It is closer to the Earth, than it is to either the Sun or Mars. b) It is at the center of a triangle that has the Sun at one apex, the Earth at another apex, and Mars at the third apex. c) It is half of the distance between the Sun and Mars. d) It is closer to the Sun, than it is to either the Earth or Mars. e) It is closer to Mars, than it is to either the Earth or the Sun. 8.1.1. Over the course of a day (twenty-four hours), what is the angular displacement of the second hand of a wrist watch in radians? a) 1440 rad b) 2880 rad c) 4520 rad d) 9050 rad e) 543 000 rad 8.2.2. Complete the following statement: For a wheel that turns with constant angular speed, a) each point on its rim moves with constant acceleration. b) the wheel turns through “equal angles in equal times.” c) each point on the rim moves at a constant velocity. d) the angular displacement of a point on the rim is constant. e) all points on the wheel are moving at a constant velocity. 8.4.3. A long, thin rod of length 4L rotates counterclockwise with constant angular acceleration around an axis that is perpendicular to the rod and passes through a pivot point that is a length L from one end as shown. What is the ratio of the tangential acceleration at a point on the end closest to the pivot point to that at a point on the end farthest from the pivot point? a) 4 b) 3 c) 1/2 d) 1/3 e) 1/4 8.5.1. An airplane starts from rest at the end of a runway and begins accelerating. The tires of the plane are rotating with an angular velocity that is uniformly increasing with time. On one of the tires, Point A is located on the part of the tire in contact with the runway surface and point B is located halfway between Point A and the axis of rotation. Which one of the following statements is true concerning this situation? a) Both points have the same tangential acceleration. b) Both points have the same centripetal acceleration. c) Both points have the same instantaneous angular velocity. d) The angular velocity at point A is greater than that of point B. e) Each second, point A turns through a greater angle than point B. 8.6.3. Which one of the following statements concerning a wheel undergoing rolling motion is true? a) The angular acceleration of the wheel must be zero m/s2. b) The tangential velocity is the same for all points on the wheel. c) The linear velocity for all points on the rim of the wheel is non- zero. d) The tangential velocity is the same for all points on the rim of the wheel. e) There is no slipping at the point where the wheel touches the surface on which it is rolling. 8.6.5. A bicycle wheel of radius 0.70 m is turning at an angular speed of 6.3 rad/s as it rolls on a horizontal surface without slipping. What is the linear speed of the wheel? a) 1.4 m/s b) 28 m/s c) 0.11 m/s d) 4.4 m/s e) 9.1 m/s 8.7.1. A packaged roll of paper towels falls from a shelf in a grocery store and rolls due south without slipping. What is the direction of the paper towels’ angular velocity? a) north b) east c) south d) west e) down 8.7.3. A top is spinning counterclockwise and moving toward the right with a linear velocity as shown in the drawing. If the angular speed is decreasing as time passes, what is the direction of the angular velocity of the top? a) upward b) downward c) left d) right 9.1.1. You are using a wrench in an attempt to loosen a nut by applying a force as shown. But this fails to loosen the nut. Which one of the following choices is most likely to loosen this tough nut? a) Tie a rope of length 2L to the wrench at the same location and apply the same force as shown. b) Place a pipe of length 2L over the handle of the wrench and apply the same force to the opposite end (farthest from the nut). c) Double the force. d) Doubling the length or doubling the force will have the same result, but doubling the length is easier. e) Continue applying the same force as in the drawing and eventually the nut will loosen. 9.1.2. A 1.5-kg ball is tied to the end of a string. The ball is then swung at a constant angular velocity of 4 rad/s in a horizontal circle of radius 2.0 m. What is the torque on the stone? a) 18 Nm b) 29 Nm c) 36 Nm d) 59 Nm e) zero Nm 9.1.6. An object with a triangular cross-section is free to rotate about the axis represented by the black dot shown. Four forces with identical magnitudes are exerted on the object. Which one of the forces, if any, exerts the largest torque on the object? a) 1 b) 2 c) 3 d) 4 e) The same torque is exerted by each force. 9.2.3. Consider the three situations shown in the figure. Three forces act on the triangular object in different ways. Two of the forces have magnitude F and one of the forces has a magnitude 2F. In which case(s), if any, will the object be in equilibrium? In each case, the forces may act at the center of gravity or at the center of a corner. a) A only b) B only c) C only d) A and C e) A and B 9.2.7. Which one of the following pictures best represents the forces that prevent the ladder from slipping while someone is standing on it? 9.4.1. Two solid disks, which are free to rotate independently about the same axis that passes through their centers and perpendicular to their faces, are initially at rest. The two disks have the same mass, but one of has a radius R and the other has a radius 2R. A force of magnitude F is applied to the edge of the larger radius disk and it begins rotating. What force must be applied to the edge of the smaller disk so that the angular acceleration is the same as that for the larger disk? Express your answer in terms of the force F applied to the larger disk. a) 0.25F b) 0.50F c) F d) 1.5F e) 2F 9.6.8. Two ice skaters are holding hands and spinning around their combined center of mass, represented by the small black dot in Frame 1, with an angular momentum L. When the skaters are at the position shown in Frame 2, they release hands and move in opposite directions as shown in Frame 3. What is the angular momentum of the skaters in Frame 3? a) zero kg m2/s b) a value that is greater than zero kg m2/s, but less than L c) a value less than L and decreasing as they move further apart d) a value that is greater than L e) L 9.6.4. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. As Joe holds the dumbbells out as shown, the professor temporarily applies a sufficient torque that causes him to rotate slowly. Then, Joe brings the dumbbells close to his body and he rotates faster. Why does his speed increase? a) By bringing the dumbbells inward, Joe exerts a torque on the stool. b) By bringing the dumbbells inward, Joe decreases the moment of inertia. c) By bringing the dumbbells inward, Joe increases the angular momentum. d) By bringing the dumbbells inward, Joe increases the moment of inertia. e) By bringing the dumbbells inward, Joe decreases the angular momentum. 9.6.5. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. Joe holds the dumbbells out as shown as the stool rotates. Then, Joe drops both dumbbells. How does the rotational speed of stool change, if at all? a) The rotational speed increases. b) The rotational speed decreases, but Joe continues to rotate. c) The rotational speed remains the same. d) The rotational speed quickly decreases to zero rad/s. 9.6.6. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. Joe holds the dumbbells out as shown as the stool rotates. Then, Joe drops both dumbbells. Then, the angular momentum of Joe and the stool changes, but the angular velocity does not change. Which of the following choice offers the best explanation? a) The force exerted by the dumbbells acts in opposite direction to the torque. b) Angular momentum is conserved, when no external forces are acting. c) Even though the angular momentum decreases, the moment of inertia also decreases. d) The decrease in the angular momentum is balanced by an increase in the moment of inertia. e) The angular velocity must increase when the dumbbells are dropped. 9.5.1. Four objects start from rest and roll without slipping down a ramp. The objects are a solid sphere, a hollow cylinder, a solid cylinder, and a hollow sphere. Each of the objects has the same radius and the same mass, but they are made from different materials. Which object will have the greatest speed at the bottom of the ramp? a) Since they are all starting from rest, all of the objects will have the same speed at the bottom as a result of the conservation of mechanical energy. b) solid sphere c) hollow cylinder d) solid cylinder e) hollow sphere 9.4.4. A long board is free to rotate about the pivot shown in each of the four configurations shown. Weights are hung from the board as indicated. In which of the configurations, if any, is the net torque about the pivot axis the largest? a) 1 b) 2 c) 3 d) 4 e) The net torque is the same for all four situations.

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