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Unformatted text preview: Version 018 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 A circular disk with moment of inertia 1 2 mR 2 , mass m and radius R is mounted at its center, about which it can rotate freely. A light cord wrapped around it supports a weight mg . R I m T g Find the total kinetic energy of the system when the weight is moving at a speed v . 1. K = 1 3 mv 2 2. K = 5 2 mv 2 3. K = 3 4 mv 2 correct 4. K = 1 2 mv 2 5. K = 5 4 mv 2 6. K = 3 2 mv 2 7. K = mv 2 8. K = 4 5 mv 2 9. K = 2 3 mv 2 Explanation: v = r , so the total kinetic energy is K tot = K m + K rot = 1 2 mv 2 + 1 2 I 2 = 1 2 mv 2 + 1 2 parenleftbigg 1 2 mR 2 parenrightbigg parenleftBig v R parenrightBig 2 = 3 4 mv 2 . 002 10.0 points A childs top is spinning as shown on a table- top. Which vector qualitatively indicates the di- rection of the tops angular velocity vector ? 1. horizontally, CW from above 2. horizontally, CCW from above 3. downward correct 4. upward Explanation: Since vector = vectorr vectorv r 2 , the direction of the tops angular velocity is downward. 003 10.0 points Suppose a physics instructor seated him- self at rest on a low-friction piano stool and held a spinning bike wheel with its angular ve- locity vector initially horizontal . Suppose the instructor then turned the bike wheel so that Version 018 Quiz 3 Coker (56625) 2 its angular velocity vector pointed vertically downward . When this turn was completed, the instructor would be spinning rapidly on the stool. What would be the final, total angular mo- mentum of the system of instructor plus bike wheel, projected along the vertical axis, the only axis along which the system can spin? 1. Zero, as it was initially. correct 2. The same, nonzero value it had initially. 3. Twice its initial value. 4. Equal but opposite to what the wheel had initially. Explanation: The system originally had zero angular mo- mentum about a vertical axis, so no matter what happens within the system of person, bike wheel and stool, as long as no external torque is involved, the angular momentum along this axis, say z , is always zero. When the bike wheels angular momentum vector is twisted downward by an internal torque, the rest of the system develops an angular momentum vector of equal magnitude in the opposite direction to maintain a total angular momentum about the vertical axis of zero....
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