A coin is standing upright on a table when a moment, M, is applied for 0.1 s. Th

Question

A coin is standing upright on a table when a moment, M, is applied for 0.1 s. The coin is 11.3 g and 30.6 mm in diameter with a thickness of 2.15 mm. Assume the coin’s axis of rotation remains perpendicular to the table

a. What is the magnitude of the moment, M, if the coin starts from rest and begins spinning at 2400 rad/s

b. If the coin slows to 1000 rad/s after 100 rotations, what was the average moment produced by friction and air resistance?

Explanation / Answer

mass moment of inertia of coin= 1/mr2

I= 1/2*13/1000*(6.8*10-3)2 =2.6125*10-7 kg-m2

Torque = moment= I   = 2.6125*10-7  *2400 =6.27*10-4   N-m = 0.627 N-mm

moment remainimg to spin 1000 rad/sec

= 2.6125*10-7  *1000 = 2.6125*10-4  N-m= 0.26125 N-mm

average moment prodused by air = 0.627-0.26125 =0.36572 N-mm

thanks

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