1. The problem statement, all variables and given/known data You drop a tennis ball on a hard floor, and it bounces upward. If the x-axis is directed upward, what is the sign of the acceleration while the ball is in contact with the floor? 2. Relevant equations a=v/t 3. The attempt at a solution I thought that the acceleration when the ball hits the floor would be 0 because the ball's speed is 0 at that point in time, and because there was no acceleration at that point then there would be no sign for it. The correct answer is that the acceleration is positive. As a side note, I believe the ball does not have a direction when it is in contact with the ground (please correct me if I'm wrong).