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