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A 250g , 25-cm-diameter plastic disk is spun on an axle through its center by an electric motor. What torque must the motor supply to take the disk from 0 to 1800 rpm in 4.1s ?
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Added Mon, 22 Jun '15
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You need to be able to calculate the moment of inertia. See here for Moment of Inertia (MOI )

http://en.wikipedia.org/wiki/List_of_moments_of_inertia

MOI (I) = (1/2) m r^2 =(1/2) * 0.25 kg *(0.25 m)^2 = 0.00781 kg-m^2
Angular Accelaration = Angular Speed/Time

The angular acceleration needed is

= 1800 rpm / 4.1 sec

= 439.024 rpm/s

= 439.024*2*?/60

Torque = MOI * Angular Accelaration
T = I * alpha = 0.00781 * 45.97 = 0.359 N-m

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Added Mon, 22 Jun '15
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