Q.

A plastic circular disc of radius 10 cm is placed on a thin oil film of thickness 2 mm, spread over a flat horizontal surface. The torque (in 102Nm ) required to spin the disc about its central vertical axis with a constant angular velocity 8 rad/sec is (coefficient of viscosity of oil is 1.0 kg/ms ) (take  π = 3.14 )

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answer is 63.

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Detailed Solution

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τ=F.  r=0Rη(2πr  dr)(rwt).r

=2πηWR44t=2×3.14×1×8×1044×2×103=0.625Nm0.63Nm

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A plastic circular disc of radius 10 cm is placed on a thin oil film of thickness 2 mm, spread over a flat horizontal surface. The torque (in 10−2N−m ) required to spin the disc about its central vertical axis with a constant angular velocity 8 rad/sec is (coefficient of viscosity of oil is 1.0 kg/m−s ) (take  π = 3.14 )