Question

a bowling machine is throwing tennis balls of mass m and velocity u each towards a wall . the balls hits the wall at 90 degree and rebounds with same velocity. the machine throws n balls per second. the force on the wall is

Answer 1

n balls per second are thrown on the wall at right angles to it.
mass = m
initial velocity = - u  before hitting the wall, towards the wall
final velocity =  u after hitting the wall and rebounding, away from the wall
Change of momentum of the ball during collision = m u - (-m u)  = 2 m u
 
  As n balls are hitting the wall in 1 sec,  the total change of momentum of all the balls together in one second  =  2 n m u

       Force = change of momentum per second.

 Force exerted by the wall on all the balls = 2 n m u

  Hence the force experienced by the wall = 2 n m u, as action and reaction forces are equal and opposite in direction.