If a ball collides with a ball of same mass the first ball stops and the second ball gets the velocity of first ball.The first ball stops due to the reaction force acting on it. But when a ball collides with a wall why doesn't it stop due to the reaction force?
The blue arrow shows the force on the object.
For this scenario to happen it is important that the collision is elastic (all energy is conserved). I used a force that is proportional to penetration depth. This way the balls feel a force that is the same during the deceleration and recoil. In inelastic collisions it is possible for two balls to stick together after the collision. Imagine two pieces of clay colliding in mid air. In inelastic collisions the force is smaller (or zero) during the recoil than during the deceleration.
Because the reaction force is larger.
The force needed to make an object reach a certain speed is the same as the force needed to slow it down from that speed to zero over the same time. This is Newton's 2nd law.
So, when two equal and movable objects collide, the (action) force that is required to make one object speed up to a certain speed is exactly the same as the (reaction) force required to slow the impacting object down to precisely zero.
When two non-equal objects collide, this is not necessarily so, and thus you will not see any of the objects stopping. Instead you might see that when
- a lighter object hits a heavier object, the action/reaction forces between them are larger than they would be between equal objects. And thus, the reaction force that stops the lighter object is larger than what is needed to precisely slow it down to zero. Thus, the lighter object slows down to zero and then start moving backwards, as if it bounces off.
- When an object hits a wall, you can consider this a collision between a light object and a very heavy object, and so, this bouncing-back effect is much stronger (the reaction force is much bigger). It might actually be so strong that the impacting light object bounces backwards with exactly the speed it came with, just directed oppositely.
Try considering momentum conservation. The wall won’t be moving before or after the collision with the ball, and presuming the collision is elastic (no energy lost to heat, sound etc.) then the sum of the momenta will be conserved before and after the collision and you should find that the ball must have the same momentum before and after the collision. So it must be in moving too.