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Unformatted text preview: Version 054 Quiz 3 Coker (56625) 1 This print-out should have 23 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. This examination covers momentum, cen- ter of mass, impulse, collisions, rigid body rotations, rotational KE, torque, rotational 2nd Law and conservation of energy, rolling, angular momentum conservation, and static equilibrium 001 10.0 points Two rigid rods of length L and mass M are attached as shown. pivot What is the rotational inertia of this sys- tem of two rods about an axis through the end of a rod, as indicated in the sketch? The rota- tional inertia of a rod about an axis through one end is I = 1 3 M L 2 , while the rotational inertia about an axis through the center of mass is I CM = 1 12 M L 2 . 1. 1 3 + 1 12 M L 2 . 2. 17 12 M L 2 . correct 3. 9 12 M L 2 . 4. 13 12 M L 2 . Explanation: Using the parallel axis theorem, I = 1 3 M L 2 + 1 12 M L 2 + M L 2 = 4 + 1 + 12 12 M L 2 = 17 12 M L 2 . 002 10.0 points A spool (similar to a yo-yo) is pulled in three ways as shown. There is sufficient friction for rotation. a b c In what direction will each spool move (in the order spool a, spool b, spool c)? 1. right; left; right 2. right; left; left 3. left; right; right 4. right; right left 5. None of these 6. right; right; right correct Explanation: In (a) there is a clockwise torque about the point of contact with the table, so the spool rolls to the right. In (b) the line of action extends through the point of table contact, yielding no lever arm and therefore no torque; with a force component to the right, the spool slides to the right without rolling. In (c) the torque produces clockwise rota- tion so the spool rolls to the right. 003 10.0 points A uniform, rigid rod of mass M and length L is pivoted frictionlessly at its upper end. If the rod is dropped from an initially horizontal position, it swings freely through a vertical position, as shown. Version 054 Quiz 3 Coker (56625) 2 A B C D In which of the four positions illustrated is the net torque about the pivot on the rod greatest, and in which of the four positions is it the least? 1. Greatest at A ; least (zero) at D correct 2. Not enough information is provided. 3. Greatest at D ; least (zero) at A 4. The same in all four positions, since the force M g acts with lever arm L 2 in all four positions. Explanation: The only forces acting on the rod are the force of the pivot keeping the rod in place, and the gravitational force. The pivot force has a zero lever arm and as such exerts no net torque. At A, the lever arm is L 2 and is perpendicular to the gravitational force at A , yielding the maximum torque M g L 2 . As the rod falls, the lever arm shortens until at position D it is 0 since the line of force passes through the pivot point....
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