Question

A ball of mass ’m’ moves perpendicularly to a wall with a speed v, strikes it and rebounds with the same speed in the opposite direction. What is the direction and magnitude of the average force acting on the ball due to the wall? (Ans: 2mv/t away from the wall.)

Answer 1

a ball of mass m moves perpendicularly to wall with a speed v and sritkes it .so, momentum of ball before striking, $P_i=mv$

after striking the ball in wall it rebounds with same speed in the opposite direction.so, momentum of ball after striking the wall $P_f=m(-v)$

according to Newton's 2nd law,
rate of change of momentum is known as force.
$so,\frac{P_f-P_i}{t}=F\\\\\frac{-mv-mv}{t}=F\implies F =- \frac{2mv}{t}\: N$
here negative sign shows that force acts away from the wall [ because we assume intially, velocity of approaching the wall is positive then, velocity of separating the wall]

Answer 2

Answer:

total force = mv/t + mv/t= 2mv/t