A boat of area `10 m^2` floating on the surface of a river is made to move horizontally with a constant speed 2m/s by applying a tangential force. If the river is 1m deep and the water in contact with the bed is stationary , find the tangential force needed `eta_(H_2 O) = 0.01 ` poise

Explanation

Velocity gradient `=(2m//s -0)/(1m-0) = 2s^(-1)` , from , Newton.s law of viscous force,
`F_v = eta_A (dV)/(dy) = (10^(-2) xx 10^(-1) SI) (10m^2) (2s^(-1)) = 0.02 N`
To keep the boat moving with constant velocity , net force on it must be made zero.
Therefore, we need to apply 0.02 N in the direction of motion.