Question

A flat plate 30 cm x 50 cm slides on oil (μ = 0.8 N-s/m2) over a large   plane surface. What is the force required to drag the plate at 2m/s if separating oil film is 0.4 mm thick ?​

Answer 1

Answer:

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Answer 2

Velocity gradient dzdV=22.5×10−20.5=40

As force on the plate due to viscosity is from upper as well as lower portion of the oil, and both are equal, using the expression for viscous force,

Fviscous=ηAdzdV

Since there are two surfaces, we get

Fviscous=η×2AdzdV

∴1N=∴η×2×0.5×40

∴η=2.5×10−2kgsecm−2