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CW10 - takes 1 6 h to stop aFter the motor power is turned...

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boland (kbb544) – CW10 – mackie – (10611) 1 This print-out should have 3 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A car traveling at 36.4 m/s undergoes a con- stant deceleration of 2.93 m/s 2 when the brakes are applied. How many revolutions does each tire make before the car comes to a stop? Assume that the car does not skid and that each tire has a radius of 0.319 m. Correct answer: 112 . 807 rev. Explanation: Let : v t = 36 . 4 m / s , a t = - 2 . 93 m / s 2 , and r = 0 . 319 m . From linear-angular relationships, v t = ω r , and a t = α r and from kinematics, ω 2 f = ω 2 i + 2 α Δ θ = 0 since ω f = 0 rad / s. Thus Δ θ = - ω 2 i 2 α = - parenleftBig v t r parenrightBig 2 2 parenleftBig a t r parenrightBig · r 2 r 2 = - v 2 t 2 r a t = - (36 . 4 m / s) 2 2(0 . 319 m)( - 2 . 93 m / s 2 ) · 1 rev 2 π rad = 112 . 807 rev . 002 (part 1 of 2) 10.0 points A flywheel with a very low friction bearing
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Unformatted text preview: takes 1 . 6 h to stop aFter the motor power is turned o³. The ²ywheel was originally rotating at 56 rpm. What was the initial rotation rate in radi-ans per second? Correct answer: 5 . 86431 rad / s. Explanation: Let : f = 56 rpm . ω = (56 rpm) 2 π rev 1 min 60 sec = 5 . 86431 rad / s . 003 (part 2 of 2) 10.0 points How many revolutions does the ²ywheel make beFore it stops? Correct answer: 2688 rev. Explanation: Let : t = 1 . 6 h . The angular acceleration oF the ²ywheel is α =-ω t , so the angle is θ = ω t + 1 2 α t 2 = ω t + 1 2 ±-ω t ² t 2 = 1 2 ω t = 1 2 (5 . 86431 rad / s) (1 . 6 h) 3600 s 1 h 1 rev 2 π = 2688 rev ....
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