I am from HK. Hope you guys can understand my poor English! ^^ actually, this is not for my homework but my preparation for the public exam. 1. The problem statement, all variables and given/known data The situation is that a ball bouncing up and down on the ground in vertical direction. The question is to choose which graphs best describes the variation of its acceleration a. It is a MC-question. Althought I know the ans., I want to have a full explanation but I am not sure if I am correct. 2. Relevant equations No 3. The attempt at a solution We all know that there is a sudden change in acceleration to the opposite sign at the moment that the ball hits ground. I want to explain it. Please comment on my explanation. u↓ ↑v Ｏ ↓+ve －－－－－－－ ground where u ≥ -v ps. the ball with mass m. Let's consider momentum. impact force = (mv - mu)/t ≥ [m(-u) - mu]/t (since v ≥ -u) = -2mu/t which is negative net Force = ma impact force - mg = ma ma ≤ -2mu/t - mg a ≤ -2u/t - g hence, when the ball hits the ground, it experiences acceleration in the sign opposite to gravitational field. Am I correct? Can I explain in other way(s)? or simplier way(s)? I want explanation as much as possible.