Physics 2010: Exam 3 (Final) Sample Questions - Key True/False Questions: Boxed are true 1. An object moving to the right may have a net force acting on it either to the right or left. 2. A newton is a unit of mass. 3. For a given object, the force of gravity and the normal force are not an action-reaction pair. 4. Angular momentum is conserved as long as no work is done inside the system. 5. Increasing the amount of thermal energy created in a process will always make that process less efficient. 6. If two objects have the same momentum, they might also have the same potential energy. 7. No work is done on an object that is in uniform circular motion. 8. If three objects collide, they each have the same change in momentum during the collision. 9. Entropy is the energy associated with temperature. 10. Even without changing the operating temperatures, better engineering could eventually bring the energy efficiency of the internal combustion engine up to nearly 100%. Multiple Choice Questions: 11. Two carts (A and B), having spring bumpers, collide as shown. Cart A has a mass of 2 kg and is initially moving to the right. Cart B has a mass of 3 kg and is initially stationary. When the separation between the carts is a minimum which of the following is true? A) Cart B is still at rest. B) Cart A has come to rest. C) Both carts have the same momentum. D) Both carts have the same kinetic energy. E) The kinetic energy of the system is at a minimum. 12. A phonograph record is dropped onto a freely spinning turntable. Then: A) neither angular momentum nor mechanical energy is conserved because of the frictional forces between record and turntable B) the frictional force between record and turntable increases the total angular momentum C) the frictional force between record and turntable decreases the total angular momentum D) the total angular momentum remains constant E) the sum of the angular momentum and rotational kinetic energy remains constant 13. A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height? A) The heavy object, because it has greater kinetic energy. B) The lightweight object, because it weighs less. C) They both slide to the same height. D) This cannot be determined from the information given Short Answer Question: 14. Please read this question carefully so that you don’t miss any of the parts. A basketball player is jumping vertically upward in order to land a shot. Her legs are flexed and pushing on the floor so that her body is accelerated upward. a) Draw free-body diagrams of the player’s body and Earth. Show the relative magnitudes of the various forces and indicate which object exerts each force and on what. Identify the action-reaction pairs. b) Repeat this exercise for the situation immediately after the player’s body loses contact with the floor. c) Finally, repeat this again for the situation at the top of her jump. a) For her diagram: One arrow up (F on her by ground), one down (F on her by Earth). The up arrow is larger. For the Earth’s diagram: One arrow down (F on ground by her), one arrow up (F on Earth by her). The down arrow is larger. The two larger arrows are paired, as are the two smaller arrows. b) Start with what we had for a) but remove the forces between her and the ground. c) Exactly the same as b) Quantitative Questions: 15. A playground “ride” is a 3.0 meter long, 80.0 kg, level plank that can rotate around a vertical axis through its center (as shown from a top view). When the plank is at rest a 20.0 kg child runs at 5.0 m/s along a line perpendicular to the plank and jumps onto the very end. a) Immediately afterwards, what is the angular speed of the plank? axis b) Immediately afterwards, what is the speed of the child? a) Use conservation of angular momentum. Result is 1.4 rad/s. b) 2.1 m/s child 16. A rigged puck is sliding across some ice at 5.00 m/s when it explodes into two pieces. One piece moves off with a speed of 15.0 m/s in a direction perpendicular to the initial direction of movement as shown. The other piece has three times the mass of the smaller piece. a) What is final velocity of the larger piece? b) If the original puck had a mass of 4.00 kg, what is the minimum amount of energy released by the explosion? 15 m/s 5 m/s a) Use conservation of momentum along x & y axes. Combine components to get final velocity of 8.33 m/s at 36.9 degrees below the original direction. b) Compare initial and final kinetic energies. The difference is 167J. 17. In a factory, 50.0 kg crates are released on a frictionless ramp, 3.0 m above the bottom of the ramp (as shown). The crates slide down the frictionless ramp and emerge onto the level section. The problem is that the crates end up going too fast at the bottom of the ramp. In order to slow them down a rough section of track is inserted. If we want the crate to end up moving at 5.0 m/s after the rough section, what length of rough track should be inserted? Assume the coefficient of kinetic friction between the crate and the rough track is 0.40. Use conservation of energy with friction accounted for as an external work ( L and you should get 4.3 m. ). Solve for 18. Two blocks are connected to a rope that passes over a pulley shaped like a disk, as shown. The pulley has a mass of 3.40 kg, a radius of 11.0 cm and is on a frictionless axle. Block A has a mass of 9.00 kg and block B has a mass of 20.0 kg. The system is released from rest when block A is 1.40 m above the floor. a) How fast is Block A moving when it has risen 1.00 meters? (Hint: There are at least two different concepts you can use here.) B b) When Block A has risen 1.00 m the string is cut! How fast will block A be moving when it hits the floor? A a) Use conservation of (mechanical) energy with A gaining K and Ug, B gaining K and losing Ug, and the pulley gaining Krot. vf = 2.28 m/s. b) Use conservation of mechanical energy again, with just block A. It starts with K and Ug, and ends with just K. v at floor = 7.23 m/s. 19. A “drop leaf” table has hinged sections (called leaves) on each side that can swing down to make the table smaller. Assume that the leaf of the table shown is 0.34 meters wide (from the edge to the hinges) and has a mass of 8.3 kg. a) You pull up the leaf so that it is horizontal and you place a 12 kg flower pot so that its center is 10.0 cm from the outer edge of the leaf. What is the minimum force you have to exert on the edge of the leaf to keep it horizontal? b) You then slide the flower pot back onto the main part of the table and let the leaf drop. At the moment you release it, what is its angular acceleration? c) What angular velocity will the leaf have when it has swung down 90.0° and is about to smack into the table legs? [Hints: Be sure to measure all positions using the center of mass of the leaf and note that since torque changes as the leaf falls, you can’t use torque concepts to answer this part.] a) Static equilibrium problem. Sum of the torques equals zero (using hinge as axis). Min. force = 120 N b) Newton’s 2nd law for rotations (∑ ). There is only one torque remaining (from gravity). Look up moment of inertia. c) Use conservation of energy! Start with finish with . Solve for