A flat plate moves normally towards a discharging jet of water at the rate of 3m/s. The jet discharges the water at the rate of $0.1{\mathrm{m}}^3/\mathrm{s}$ and at the speed of 18m/s. The force exerted on the plate due to the jet is

Relative velocity of the water jet with respect to the moving plate: $v=(18+3)=21 \mathrm{m}/\mathrm{s}$

mass of water hitting the plate per second: $m=\rho Av=21\rho A$

where, A is the cross-sectional area of the jet.

$A=\frac{volume flow rate}{speed of efflux}=\frac{0.1}{18} m^2$

Now, F = force normal to the plate = Rate of change of momentum along the direction of the water jet

$\Rightarrow F=m\times v=21\rho Av$

$=21\times 1000\times \frac{0.1}{18}\times 21=2450 N$