A ball of mass 4kg falls to the floor it lands with a speed of 6m/s . It bounces off with the same speed. Show that is momentum has changed by 48kgm/s.

Momentum before the ball hits the floor = 4kg x 6m/s = 24kgm/s

Momentum after the ball bounces off the floor = 4kg x 6m/s = 24kgm/s
Change in momentum = 24kgm/s - 24kgm/s = 0kgm/s
Therefore, the momentum has not changed by 48kgm/s.

Oh yes it has.

Momentum is a vector. It has magnitude AND direction.
it was 24 kg m/s DOWN (negative)
then it had 24 kg m/s UP (positive)
change is 24 - (-24) = 48 kg m/s