Question

A steam of water flowing horizontally with a speed of $25\, m$ $s^{-1}$ gushes out of a tube of cross-sectional area $10^{-3}$ $m^{2},$ and hits at a vertical wall nearby. What is the force exerted on the wall by the impact of water?

• A. $125\, N$
• B. $625\, N$
• C. $-650 \, N$
• D. $-1125\, N$

Answer 1

Answer: (B)

Here, $u=25$ $m s^{-1},$ $v=0,$ $t=1 \,s,$ $A=10^{-3}$ $m^{2}$
Density of water, $\rho=1000\,kg$ $m^{-3}$
Mass of water gushed out per second, $m$
$=\frac{volume\,\times\,density}{time}=\frac{area\,\times\,distance\,\times\,density}{time}$
$=Area\,\times\,velocity \,\times\,density$
$=A\nu\rho=10^{-3}$ $\times25\times1000=25\, kg$
Force exerted on the wall by the impact of water,
$F=ma$ $=m$ $\left(\frac{v-u}{t}\right)$ $=25\times \left (\frac{-25}{1} \right)$ $=-625\, N$
$F'$ $=-F=625 \,N$