Figure shows a column of water of length
15 m and cross - section area
10−2 m2 at rest. Let us calculate the mass of water flowing out per second and hitting the wall. When the stream of water has a speed of
15 m/s, the particle of water at section B would have travelled a distance of
15 m in one second and arrived at the section A. The water contained in this column would have gone out in one second and hit the wall.
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/800361/original_21_S.png)
Thus, the volume of water coming out of A in one second = volume of water contained in a cylinder of length
15 m and area of cross - section
0.01 m2
=
Length×Area of cross - section
=15×10−2=0.15 m3
Mass of water gushing out per second
=
Volume of water×Density of water
=0.15×1000=150 kg
[density of water
=1000 kg/m3]
|Change in momentum|
=2250−0=2250 N
Since change in momentum per second is the force, hence
force exerted on the wall will be
2250 N.