HW 11 - johnson (mjj622) HW 11 Coker (56625) 1 This...

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Unformatted text preview: johnson (mjj622) HW 11 Coker (56625) 1 This print-out should have 15 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. This assignment covers the fairly short Ch. 12, dealing with rigid body rotation, rota- tional inertia and rotational kinetic energy. 001 10.0 points The tub of a washer goes into its spin-dry cycle, starting from rest and reaching an an- gular speed of 2 rev / s in 3 . 2 s . At this point the person doing the laundry opens the lid, and a safety switch turns off the washer. The tub slows to rest in 6 . 4 s . Through how many revolutions does the tub turn? Assume constant angular accelera- tion while it is starting and stopping. Correct answer: 9 . 6 rev. Explanation: Let : = 0 rev / s , 1 = 2 rev / s , t 1 = 3 . 2 s , 2 = 0 rev / s , and t 2 = 6 . 4 s . = av t = i + f 2 t = 1 2 t in each case since either the initial or final speed is zero. While speeding up, 1 = 2 rev / s 2 (3 . 2 s) = 3 . 2 rev and while decelerating, 2 = 2 rev / s 2 (6 . 4 s) = 6 . 4 rev , so = 1 + 2 = 3 . 2 rev + 6 . 4 rev = 9 . 6 rev . 002 (part 1 of 2) 10.0 points A disk with a radius of 0.1 m is spinning about its center with a constant angular speed of 10 rad/sec. What are the speed and magnitude of the acceleration of a bug clinging to the rim of the disk? 1. 0.1 m/s and 1 m/s 2 2. 10 m/s and 10 m/s 2 3. 1 m/s and 10 m/s 2 correct 4. 1 m/s and 0 m/s 2 (Disk spins at constant speed.) Explanation: The speed of the bug is v = R = (0 . 1 m)(10 1 / s) = 1 m / s and its acceleration is a r = R 2 = (0 . 1 m)(10 1 / s) 2 = 10 m / s 2 . 003 (part 2 of 2) 10.0 points Let the turntable spin faster and faster, with constant angular acceleration vector . Which sketch qualitatively shows the direc- tion of the acceleration vector vectora of the bug? 1. vectora 2. vectora johnson (mjj622) HW 11 Coker (56625) 2 3. vectora correct 4. vectora Explanation: The bug has an acceleration vectora r directed toward the center of the circle, and an accel- eration vectora t in the same direction as its velocity (counterclockwise); the net acceleration is the vector sum of vectora r and vectora t ....
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This note was uploaded on 02/10/2011 for the course PHY 303K taught by Professor Turner during the Fall '08 term at University of Texas.

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HW 11 - johnson (mjj622) HW 11 Coker (56625) 1 This...

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