# HW 009 - Version PREVIEW – hk 9 – Opyrchal –(11105 1...

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Unformatted text preview: Version PREVIEW – hk 9 – Opyrchal – (11105) 1 This print-out should have 14 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Spin Dry Cycle 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 5 . 3 rev / s in 7 . 4 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 7 . 3 s . Through how many revolutions does the tub turn? Assume constant angular accelera- tion while it is starting and stopping. Correct answer: 38 . 955 rev. Explanation: Let : ω = 0 rev / s , ω 1 = 5 . 3 rev / s , Δ t 1 = 7 . 4 s , ω 2 = 0 rev / s , and Δ t 2 = 7 . 3 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 = 5 . 3 rev / s 2 (7 . 4 s) = 19 . 61 rev and while decelerating, θ 2 = 5 . 3 rev / s 2 (7 . 3 s) = 19 . 345 rev , so θ = θ 1 + θ 2 = 19 . 61 rev + 19 . 345 rev = 38 . 955 rev . Grinding Wheel Rotation 002 (part 1 of 2) 10.0 points A grinding wheel, initially at rest, is ro- tated with constant angular acceleration of 7 . 04 rad / s 2 for 4 . 97 s. The wheel is then brought to rest with uniform deceleration in 14 . 9 rev. Find the angular deceleration required to bring the wheel to rest. Correct answer:- 6 . 53825 rad / s 2 . Explanation: Let : ω = 0 rad / s , α 1 = 7 . 04 rad / s 2 , t 1 = 4 . 97 s , and θ 2 = 14 . 9 rev . From kinematics ω f = ω + α t. First find the speed attained before the wheel begins to slow down. Since ω = 0 rad / s for the acceleration, the final speed ω 1 = α 1 t 1 for the acceleration is also the initial speed for the deceleration....
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HW 009 - Version PREVIEW – hk 9 – Opyrchal –(11105 1...

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