1. An electric motor rotating a workshop grinding wheel at a rate of 1.46 10 2 r

Question

1. An electric motor rotating a workshop grinding wheel at a rate of 1.46 102 rev/min is switched off. Assume the wheel has a constant negative angular acceleration of magnitude 2.20 rad/s2.

(a) How long does it take for the grinding wheel to stop? ____ s

(b) Through how many revolutions has the wheel turned during the interval found in (a)? _____ revolutions

2. A sample of blood is placed in a centrifuge of radius 18.0 cm. The mass of a red blood cell is 3.0 1016 kg, and the magnitude of the net force acting on it as it settles out of the plasma is 4.0 1011 N. At how many revolutions per second should the centrifuge be operated? ______ rev/s

Explanation / Answer

a) a = - 2.20 rad/s2 , vi = 146 rev/min = 15.29 rad/sec

vf = vi + a t

t = 15.29 / 2.2

time for grinding wheel to stop = 6.95 s

b) d = 1/2 * (vf + vi) * t

d = 1/2 ( 0 + 15.29) * 6.95 s

d = 53.13 rad = 8.45 rev

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