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Unformatted text preview: homework 07 – JACOBS, AMBER – Due: Mar 18 2008, 1:00 am 1 Question 1, chap 10, sect 10. part 1 of 1 10 points Given: 1 hp=746 W An automobile engine develops a torque of 500 N m and is rotating at a speed of 2000 rev / min. What horsepower does the engine generate? Correct answer: 140 . 375 hp (tolerance ± 1 %). Explanation: ω = (2000 rev / min) parenleftbigg 2 π rad 1 rev parenrightbiggparenleftbigg 1 min 60 sec parenrightbigg = 209 . 44 rad / s . We have P = τ ω = (500 N m) (209 . 44 rad / s) = (104720 W) parenleftbigg 1 hp 746 W parenrightbigg = 140 . 375 hp . Question 2, chap 11, sect 11. part 1 of 2 10 points A child of mass 51 . 6 kg sits on the edge of a merrygoround with radius 2 m and moment of inertia 150 . 672 kg m 2 . The merrygoround rotates with an angular velocity of 1 . 5 rad / s. What radial force does the child have to exert to stay on the merrygoround? Correct answer: 232 . 2 N (tolerance ± 1 %). Explanation: Basic Concepts: Conservation of Angu lar Momentum. Centripetal Force: F = m v 2 r = mω 2 r The force the child needs to exert in the radial direction to hold on is simply the centripetal force: F = m v 2 r = mω 2 r Question 3, chap 11, sect 11. part 2 of 2 10 points The child then walks towards the center of the merrygoround and stops at a distance . 88 m from the center. Now what is the angular velocity of the merrygoround? Correct answer: 2 . 80966 rad / s (tolerance ± 1 %). Explanation: When the child moves inward, the moment of inertia of the system i MGR + i child (the merry goround plus the child) changes. Therefore, to conserve angular momentum, the angular velocity of the system must change. Specifi cally: L init = L final ( i child + i MGR ) ω = ( i child, 2 + i MGR ) ω 2 The moment of inertia of the child is mr 2 . Therefore ω 2 = mr 2 + i MGR mr 2 2 + i MGR ω . Question 4, chap 11, sect 11. part 1 of 1 10 points If the polar ice caps of the Earth were to melt, the oceans would be deeper by about 30m. What effect would this have on the Earth’s rotation? 1. It depends on mass. 2. Faster. 3. No change. 4. Slower. correct Explanation: In accord with the conservation of angular momentum, if mass moves farther from the axis of rotation, rotational speed decreases. So the Earth would slow in its daily rotation. Question 5, chap 11, sect 11. part 1 of 1 10 points homework 07 – JACOBS, AMBER – Due: Mar 18 2008, 1:00 am 2 A figure skater on ice spins on one foot. She pulls in her arms and her angular speed increases. Physically, this happens because ... ? Choose the reason from the list below; if two or more reasons seem plausible, choose the most important reason. 1. Her angular speed increases because she is undergoing uniformly accelerated angular motion....
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This note was uploaded on 04/28/2008 for the course PHY 317k taught by Professor Kopp during the Spring '07 term at University of Texas.
 Spring '07
 KOPP
 Physics, Power, Work

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