There is a 1mm thick layer of glycerine between a flat plate of area 100 cm² and a big fixed plate. If the coefficient of viscosity of glycerine is 1.0 kg m⁻¹ s⁻¹ then how much force is required to move the plate with a velocity of 7 cm s⁻¹?
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Question

There is a 1mm thick layer of glycerine between a flat plate of area 100 cm² and a big fixed plate. If the coefficient of viscosity of glycerine is 1.0 kg m⁻¹ s⁻¹ then how much force is required to move the plate with a velocity of 7 cm s⁻¹?

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G1 Physics · Accepted Answer

Step 1: Use viscous-force relations: shear F = ηA(dv/dy), Stokes force 6πηrv, or Reynolds number Re = ρvD/η as appropriate. Step 2: At terminal speed set net weight/buoyancy equal to viscous drag and use the given geometry. Applying these relations to the given data gives B: 0.7 N. Final answer: B.