Force Due To Water Flow: When water from a horizontal pipe strikes a wall normally, a force is exerted on the wall. Suppose the water strikes the wall normally with velocity v and comes to rest on striking the wall, the change in velocity is then 0 – v = – v. from second law, the force equals the momentum change per second of water flow. If mass m of the water strikes the wall in time t then force F on the water is

From third law of motion, the reaction force exerted by the water on the wall is equal but opposite

 Point to ponder In thrill machine rides at amusement parks, there can be an without head rests, acceleration like this would not be safe; think why not?

Thus force can be calculated from the product of mass of sater striking normally per second and change in velocity. Suppose the water flow out from a pipe at 3 kgs-1 and its velocity changes from 5 ms-1 to zero on striking the balls, then,

Example 3.6: A hose pipe ejects water at a speed of  through a hole of area  , if the water strikes a wall normally, calculate the force on the wall, assuming the velocity of the water normal to the wall is zero after striking.

Solution:Mass per second striking the wall = volume x density