Shear stress:

$\overline{){\mathbf{\tau}}{\mathbf{=}}{\mathbf{\mu}}\frac{\mathbf{d}\mathbf{u}}{\mathbf{d}\mathbf{y}}}$

where μ is the viscosity of air, du is the relative velocity, and dy is the thickness of the air layer.

Retarding force:

$\overline{){\mathbf{F}}{\mathbf{=}}{\mathbf{\tau}}{\mathbf{A}}}$

A cart is moving along and very close to a level air track at a speed of 0.48 m/s. The surface area of the ear's underside is 2.1 x 10^{-2} m^{2} and the thickness of the air layer between the cart and the track is 6.2 x 10^{-5 }m. The air temperature is 20°C

Find the magnitude of the retarding force, in newtons, acting on the cart due to the viscosity of the air layer between the cart and the air track. Take the viscosity of air at the given temperature to be 1.81 x 10^{-5} Pa-s

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