Question

A body of mass m collides against a wall with a velocity v and retraces its path with the same speed. The change in momentum is

Answer 1

when a body collides with wall,
initial momentum = mu
final momentum = -mu ( - indicates change in  direction)
total momentum = 2mu
therefore, change in momentum = mu

Answer 2

initial momentum=mu
final momentum=-mu(- sign indicates change in direction)
net change= initial momentum - final momentum
=mu - (- mu )
= 2 mu