The velocity of liquid (v) in steady flow at a location through cylindrical pipe is given by `v=v_(0)(1-(r^(2))/(R^(2)))`, where r is the radial distance of that location from the axis of the pipe and R is the inner radius of pipe. If R = 10 cm. volume rate of flow through the pipe is `pi//2 xx 10^(-2) m^3s^(-1)` and the coefficient of viscosity of the liquid is 0.75 N s`m^(-2)`, find the magnitude of the viscous force per unit area, in N`m^(-2)` at r = 4 cm.