Physics 125 Practice exam #3 Chapters 7,8,9 Name: Date:

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1 Physics 125 Practice exam #3 Chapters 7,8,9 Name: Date: 1. 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 provides the necessary force to stop the person. B) The bag reduces the impulse to the person. C) The bag increases the amount of time the force acts on the person and reduces the change in momentum. D) The bag decreases the amount of time during which the momentum is changing and reduces the average force on the person. E) The bag increases the amount of time during which the momentum is changing and reduces the average force on the person. 2. A kg tennis ball moving to the right with a speed of 15 m/s is struck by a tennis racket, causing it to move to the left with a speed of 15 m/s. If the ball remains in contact with the racquet for s, what is the magnitude of the average force exerted on the ball? A) zero newtons B) 98 N C) 160 N D) 240 N E) 320 N 3. In which one of the following situations is linear momentum not conserved? A) A bomb suspended by a string explodes into one hundred fragments. B) A bowling ball collides with a set of ten pins. C) A golf ball is struck by a club. D) An astronaut floating in space throws a hammer away and subsequently moves in the opposite direction. E) A tree limb is struck by lightning and falls to the ground. Page 1

2 4. A 50-kg toboggan is coasting on level snow. As it passes beneath a bridge, a 20-kg parcel is dropped straight down and lands in the toboggan. If KE1 is the original kinetic energy of the toboggan and KE2 is the kinetic energy after the parcel has been added, what is the ratio KE2/KE1. A) 0.4 B) 0.6 C) 0.7 D) 0.8 E) 1 5. A bullet of mass m is fired at speed v0 into a wooden block of mass M. The bullet instantaneously comes to rest in the block. The block with the embedded bullet slides along a horizontal surface with a coefficient of kinetic friction μ. Which one of the following expressions determines how far the block slides before it comes to rest (the magnitude of displacement s in the figure)? 2 mv s = 0 A) M μg 2 m v s 0 = m+ M μg B) 2 2 m v0 s = C) m+ M 2μ g 2 2 m v0 s = D) m+ M 2μ g E) 2 v 0 s = μg Page 2

6. A 0.10-kg cart traveling in the positive x direction at 10.0 m/s collides with a 0.30-kg cart at rest. The collision is elastic. What is the velocity of the 0.10-kg cart after the collision? A) +2.

3 6. A 0.10-kg cart traveling in the positive x direction at 10.0 m/s collides with a 0.30-kg cart at rest. The collision is elastic. What is the velocity of the 0.10-kg cart after the collision? A) +2.5 m/s B) 2.5 m/s C) +5 m/s D) 5 m/s E) +3.3 m/s 7. A 50.0-kg boy runs at a speed of 10.0 m/s and jumps onto a cart as shown in the figure. The cart is initially at rest. If the speed of the cart with the boy on it is 2.50 m/s, what is the mass of the cart? A) 150 kg B) 175 kg C) 210 kg D) 260 kg E) 300 kg 8. A 7.30-kg bowling ball strikes a 1.60-kg pin at rest head-on. Before the collision, the velocity of the ball is m/s. After the collision, the velocity of the ball is m/s. What is the velocity of the pin after the collision? A) +0.6 m/s B) +5.4 m/s C) +1.2 m/s D) +2.7 m/s E) +3.2 m/s 9. A wheel with a 0.10-m radius is rotating at 35 rev/s. It then slows uniformly to 15 rev/s over a 3.0-s interval. What is the angular acceleration of a point on the wheel? A) 2.0 rev/s 2 B) 0.67 rev/s 2 C) 6.7 rev/s 2 D) 42 rev/s 2 E) 17 rev/s 2 Page 3

4 10. A fan rotating with an initial angular velocity of 1000 rev/min is switched off. In 2 seconds, the angular velocity decreases to 200 rev/min. Assuming the angular acceleration is constant, how many revolutions does the blade undergo during this time? A) 10 B) 20 C) 100 D) 125 E) 1200 Use the following to answer questions 11-13: A grindstone of radius 4.0 m is initially spinning with an angular speed of 8.0 rad/s. The angular speed is then increased to 12 rad/s over the next 4.0 seconds. Assume that the angular acceleration is constant. 11. What is the average angular speed of the grindstone? A) 0.5 rad/s B) 2.0 rad/s C) 4.0 rad/s D) 8.0 rad/s E) 10 rad/s 12. What is the magnitude of the angular acceleration of the grindstone? A) 0.50 rad/s 2 B) 1.0 rad/s 2 C) 4.5 rad/s 2 D) 9.0 rad/s 2 E) 18 rad/s Through how many revolutions does the grindstone turn during the 4.0-second interval? A) 0.64 B) 3.8 C) 4.0 D) 6.4 E) 40 Page 4

5 14. A spinning disc rotating at 130 rev/min slows and stops 31 s later. How many revolutions did the disc make during this time? A) 34 B) 67 C) 8.4 D) 17 E) A string is tied to a doorknob 0.72 m from the hinge as illustrated in the figure. At the instant shown, the force applied to the string is 5.0 N. What is the magnitude of the torque on the door? A) 2.1 N m B) 3.0 N m C) 1.0 N m D) 0.78 N m E) 0.60 N m 16. A horizontal, 10-m plank weighs 100 N. It rests on two supports that are placed 1.0 m from each end as shown in the figure. How close to one end can an 800-N person stand without causing the plank to tip? A) 0 m B) 0.3 m C) 0.5 m D) 0.7 m E) 0.9 m Page 5

6 17. A meter stick is pivoted at the 0.50-m line. A 3.0-kg object is hung from the 0.15-m line. Where should a 5.0-kg object be hung to achieve equilibrium (the meter stick oriented horizontal and motionless)? A) 0.06-m line B) 0.24-m line C) 0.56-m line D) 0.71-m line E) A 5.0-kg object cannot be placed anywhere on the meter stick to result in equilibrium in this system. 18. A 45-N brick is suspended by a light string from a 2.0-kg pulley. The brick is released from rest and falls to the floor below as the pulley rotates through 5.0 rad. The pulley may be considered a solid disk of radius 1.5 m. What is the angular speed of the pulley? A) 17 rad/s B) 15 rad/s C) 9.4 rad/s D) 8.1 rad/s E) 7.3 rad/s 19. A solid cylinder of radius 0.35 m is released from rest from a height of 1.8 m and rolls down the incline as shown. What is the angular speed of the cylinder when it reaches the horizontal surface? A) 8.2 rad/s B) 14 rad/s C) 34 rad/s D) 67 rad/s E) This cannot be determined because the mass is unknown. Page 6

7 20. A solid sphere of radius R rotates about a diameter with an angular speed ω. The sphere then collapses under the action of internal forces to a final radius R/2. What is the final angular speed of the sphere? A) ω/4 B) ω/2 C) ω D) 2ω E) 4ω Page 7

8 Answer Key 1. E 2. B 3. E 4. C 5. C 6. D 7. A 8. D 9. C 10. B 11. E 12. B 13. E 14. A 15. B 16. C 17. D 18. A 19. B 20. E Page 8

PHY231 Section 2, Form A March 22, 2012. 1. Which one of the following statements concerning kinetic energy is true?

PHY231 Section 2, Form A March 22, 2012. 1. Which one of the following statements concerning kinetic energy is true? 1. Which one of the following statements concerning kinetic energy is true? A) Kinetic energy can be measured in watts. B) Kinetic energy is always equal to the potential energy. C) Kinetic energy is always