If a ball hits a wall at a constant velocity, it will slow down and eventually come to rest. Since the ball was at constant velocity, it was not accelerating, and so there was no force. Also, the force of the wall on the ball equals the force of the wall on the ball, presumably equals zero. Where did the force to decelerate the ball to 0 velocity come from?
[Physics] Where does the force to stop a constant velocity object come from
collisionforcesfree-body-diagramkinematicsnewtonian-mechanics
Best Answer
It came from the wall.
If a ball hits a wall at constant velocity, then it's not going to "slow down and eventually come to rest". It's going to bounce back, and in fact, if the collision is elastic, it won't even necessarily slow down. The force on the ball from the wall serves to change the direction of the velocity (and possibly decrease its magnitude if the collision was in-elastic). This force is mostly elecro-magnetic in nature, occurring at the microscopic level between the molecules that make up the ball and the molecules that make up the wall.
Julian is right. Technically speaking, the velocity of the ball does not go from $v$ to $-v$ instantaneously, but does so over a very small amount of time. This means that sometime during impact, the instantaneous velocity of the ball will be $0$. But this does not imply that the instantaneous force on it is $0$: $F=ma$, not $F=mv$. Also note that the net change, from $v$ to $-v$, is a change in velocity of magnitude $2v$. In particular, the ball is not at constant velocity during the impact time.