A small ball of mass m and density ρ is dropped in a viscous liquid having a density ρ0. After some time, the ball falls with a constant velocity. Calculate viscous force acting on the ball.
Solution: Volume of the ball, v=m/ρ; a mass of liquid displaced by the ball. m′=(ρm)ρ0 ρ = density of ball When the ball falls with a constant velocity ⇒ viscous force = effective weight of the ball or F=mg=m’g=(m–m’)g =(m−ρmρ0)g=mg(1−ρρ0)