I\'m having trouble with a homework question.Any help would be much appreciated.

Question

I'm having trouble with a homework question.Any help would be much appreciated.

A stroboscope is a light that flashes on and off at a constantrate. It can be used to illuminate a rotating object, and if theflashing rate is adjusted properly, the object can be made toappear stationary. (a) What is the shortesttime between flashes of light that will make a three-bladedpropeller appear stationary when it is rotating with an angularspeed of 15.0 rev/s?


(b) What is the next shortest time?


Thanks!


(a) What is the shortesttime between flashes of light that will make a three-bladedpropeller appear stationary when it is rotating with an angularspeed of 15.0 rev/s?


(b) What is the next shortest time?


Thanks!

Explanation / Answer

a) A stroboscope is a source of flashing light that can besynchronized with any fast, repeating motion so that a rapidlymoving device seems to stand still, or to move slowly. Here the condition to appear three bladed propeller as stationaryis each blade must rotate through an angle of 120 degree. Then that three bladed propeller remains stationary. So given that = 120 degree   = 2 /3                = 15 rev/s = (15)(2) rad/s formula of the angular displacement:                    = / t t = /    = 2 / 3 * 15*2    = 0.022 s b) The next shortest time occurs when each blade rotates an angleof, 120 degree + 120 degree = 240 degree      ==> = 4 / 3                                       = (15)(2) rad/s Therefore t = / = 4 / 3 * 15*2                                 =0.044 s

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