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August 4, 2008 PHY 2053 Discussion Practice Exam 2 (Chapter 6-8) 1. A crane drops a 0.30 kg steel ball onto a steel plate. The ball’s speeds just before impact and after are 4.5 m/s and 4.2 m/s respectively. If the ball is in contact with the plate for 0.030 s, what is the magnitude of the average force that the ball exerts on the plate during impact? (Impulse-Momentum Theorem) 2. a. 87 N b. 133 N c. 3.0 N d. 3.5 N A machine gun is attached to a railroad flatcar that rolls with negligible friction. If the railroad car has a mass of 6.25×104 kg, how many bullets of mass 25 g would have to be fired at 250 m/s off the back to give the railroad car a forward velocity of 0.5 m/s? (Conservation of Momentum) b. 2 000 3. c. 3 000 d. 5 000 In a two-body collision, if the kinetic energy of the system is conserved, then which of the following best describes the momentum after the collision? (Collisions) conserved 4. a. 400 c. may also be conserved a. must be less b. must also be d. is doubled in value A 20-kg object sitting at rest is struck elastically in a head-on collision with a 10-kg object initially moving at +3.0 m/s. Find the final velocity of the 20-kg object after the collision. (Elastic Collisions) a. -1.0 m/s 5. b. -2.0 m/s c. +1.5 m/s d. +2.0 m/s A 2 500-kg truck moving at 10.0 m/s strikes a car waiting at a traffic light, hooking bumpers. The two continues to move together at 7.00 m/s. What is the mass of the struck car? (Perfectly Inelastic Collisions) 6. a. 1 730 kg b. 1 550 kg c. 1 200 kg d. 1 070 kg A Ferris wheel, rotating initially at an angular speed of 0.500 rad/s, accelerates over a 7.000s interval at a rate of 0.040 0 rad/s². What angular displacement does the Ferris wheel undergo in this 7-s interval? (Kinematics for Rotational Motions) a. 4.48 rad b. 2.50 rad c. 3.00 rad d. 0.500 rad 7. Calculate the linear speed due to the Earth’s rotation for a person at the equator of the Earth. The radius of the Earth is 6.40×106 m. (Relations bet. Linear and Angular Quantities) 233 m/s 8. c. 465 m/s a. 74.0 m/s b. d. 73.0 m/s A roller coaster, loaded with passengers, has a mass of 2 000 kg; the radius of curvature of the track at the bottom point of the dip is 24 m. If the vehicle has a speed of 18 m/s at this point, what force is exerted on the vehicle by the track? (Centripetal Acceleration/Force-rotational motion in a vertical a. 2.3×104 N plane) 9. b. 4.7×104 N c. 3.0×104 N d. 1.0×104 N A 1 500-kg car rounds an unbanked curve with a radius of 52 m at a speed of 12 m/s. What minimum coefficient of friction must exist between the road and tires to prevent the car from slipping? (Centripetal Acceleration/Force-rotational motion in a horizontal plane) 0.28 a. 0.18 b. 0.30 c. d. 0.37 10. If a planet has 3 times the radius of the Earth, but has the same density as the Earth, what is the gravitational acceleration at the surface of the planet? (Newtonian Gravitation) 88.2 m/s² c. 265 m/s² a. 29.4 m/s² b. d. 3.27 m/s²s 11. An 800-N billboard worker stands on a 4.0-m scaffold supported by vertical ropes at each end. If the scaffold weighs 500 N and the worker stands 1.0 m from one end, what is the tension in the rope nearest the worker? (Rigid bodies in Equilibrium-1D) a. 450 N b. 500 N c. 800 N d. 850 N 12. A 100-N uniform ladder, 8.0 m long, rests against a smooth vertical wall. The coefficient of static friction between ladder and floor is 0.40. What minimum angle can the ladder make with the floor before it slips? (Rigid Bodies in Equilibrium-2D) a. 22˚ b. 51˚ c. 18˚ d. 42˚ 13. A solid cylinder (I = MR²/2) has a string wrapped around it many times. When I release the cylinder, holding on to the string, the cylinder falls and spins as the string unwinds. What is the downward acceleration of the cylinder as it falls? (Rigid Bodies in Motion) a. 0 b. 4.9 m/s² c. 6.5 m/s² d. 9.8 m/s² 14. A cylinder (I = MR²/2) is rolling along the ground at 7.0 m/s. It comes to a hill and starts going up. Assuming no losses to friction, how high does it get before it stops? (Rotational Kinetic Energy) 1.2 m b. 3.7 m c. 4.2 m a. d. 5.9 m 15. A turntable has a moment of inertia of 3.00×10-2 kg m² and spins freely on a frictionless bearing at 25.0 rev/min. A 0.300-kg ball of putty is dropped vertically onto the turntable and sticks at a point 0.100 m from the center. What is the new rate of rotation of the system? (Angular Momentum) 40.8 rev/min Answers: 1-a 2-d b. 22.7 rev/min 3-b 4-d 5-d c. 33.3 rev/min 6-a 7-c 8-b d. 27.2 rev/min 9-c 10-a 11-d 12-b 13-c 14-b 15-b a.