Question

A jet of water with area of cross-section 3cm${}^2$ strikes a wall at an angle $\theta ={60}^o$ to the normal and be rebounds elastically from the wall with the same speed. If the speed of water in the jet is 12 m/s, then the force acting on the wall is

• A. $4.31N$
• B. $4.32\times {10}^{-2}N$
• C. $4.32\times {10}^{-3}N$
• D. $43.2N$

Answer 1

The correct option is D $43.2N$

$A=3c{m}^2=3\times {10}^{-4}{m}^2$,
$\theta ={60}^o,u=12m/s$
F = Rate of change of momentum $=2mucos\theta =2\times A\times u\times \rho \times ucos\theta$
[$\therefore m=Volume\times density=Aux\rho$]
$=2\times 3\times {10}^{-4}\times 12\times 12\times {10}^3\times cos{60}^o$
$=2\times 3\times {10}^{-4}\times {10}^3\times 144\times \frac{1}{2}$
$=432\times {10}^{-1}N=43.2N$