HW 11 - johnson(mjj622 HW 11 Coker(56625 This print-out...

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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 : ω 0 = 0 rev / s , ω 1 = 2 rev / s , Δ t 1 = 3 . 2 s , ω 2 = 0 rev / s , and Δ t 2 = 6 . 4 s . θ = ω av Δ t = ω i + ω f 2 Δ t = ω 1 2 Δ t in each case since either the initial or final speed is zero. While speeding up, θ 1 = 2 rev / s 2 (3 . 2 s) = 3 . 2 rev and while decelerating, θ 2 = 2 rev / s 2 (6 . 4 s) = 6 . 4 rev , so θ = θ 1 + θ 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 ω = (0 . 1 m)(10 1 / s) = 1 m / s and its acceleration is a r = R ω 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 vectorα . Which sketch qualitatively shows the direc- tion of the acceleration vector vectora of the bug? 1. vectora 2. vectora
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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 . 004 10.0 points A disk is spinning about an axis through its center of mass, and at a given moment has an angular velocity vector vectorω as shown.
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