Manipal Entrance Test 2013 Physics Paper

Force experienced by the surface is equal to change in momentum in one second.
From Newton's second law of motion, the rate of change of momentum of a body is equal to the external force applied on the body and the change in momentum always takes place in the direction of the force.
Momentum of ball before impact = mu
Momentum after impact = - mu
Change in momentum = mu - (- mu) = 2mu
Change in momentum due to n balls in one second is = 2 mnu.
Change of momentum = force experienced per second = 2 mnu.