A coin placed on a rotating turntable just slips if it is placed at a distance of 4 cm from the centre.

Monjur Alahi

A coin placed on a rotating turntable just slips if it is placed at a distance of 4 cm from the centre. If the angular velocity of the turntable is doubled, it will just slip at a distance of
(a) 1 cm (b) 2 cm (c) 4 cm (d) 8 cm.


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Salim Ahmed Kawsar

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The right answer is (a) 1 cm

Explanation:

The friction force to keep it rotating =mω²r. At 4 cm away from the center when it just slips, the static frictional force is at its limiting value. When the angular speed of the turntable is doubled the magnitude of the required centripetal force increases four times. So to keep it to limiting value of friction force, 'r' should be made one fourth. It will be clear as below m(2ω)².r/4 =mω²r. 

That is why it should be placed at 1 cm.

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Salim Ahmed Kawsar

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Correct option is (A) 1 cm

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