# Why do objects move back after collision?

When a ball collides with a wall, the ball moves in the opposite direction, why does this happen if Newton's third law operates. I mean the ball collided with the wall and imparted a force on it and simultaneously the wall imparted a force on the ball making the net force on the ball zero. So why is there still acceleration (motion) in the opposite direction?

The BALL exerted a force on the WALL, as a reaction the WALL exerted a force on the BALL. So the ball goes back. There is a non-zero net force on the ball and the wall individually

(Any force can be taken as action and the other as reaction though)

I mean the ball collided with the wall and imparted a force on it [the wall].

Here you are writing about the force on the wall.

and simultaneously the wall imparted a force on the ball.

Here you are writing about the force on the ball and this force is the Newton’s third law pair to the force on the wall.

making the net force on the ball zero.

This cannot be correct as you have only identified one force on the ball.

A point to remember is that Newton’s third law forces always act on different objects.

The force on object A due to object B is equal in magnitude and opposite in direction to the force on object B due to object A.

When the ball collided with the wall, it also collided with whatever the wall was attached to. In this case, perhaps you can see the collision with the wall as a collision with the earth.

You can see using this simulation that if something of low mass collides at high velocity with a stationary object of high mass, it is reflected in the opposite direction with roughly the same velocity (provided the collision is elastic): https://phet.colorado.edu/sims/collision-lab/collision-lab_en.html. In this case the mass of the ball is insignificant compared to that of the earth so its velocity following the collision can be approximated as you have mentioned. On the other hand, the earth and the wall moves in the opposite direction with a really small velocity.

If you assume that both momentum and kinetic energy due to movement remain constant in the collision, then the outcome of the collision must involve the ball moving in the other direction.

According to Newton's third law you are right that there exists an action reaction pair between the wall and the ball but Newton's third law also states that the members of the action -rxn pair act on two different objects so the force exerted by the wall on the ball is the only force along the axis of it's motion (obviously ignoring the friction) and hence causing an acceleration that is able to reverse its velocity. Also we must understand that when we see a force we must know it's applier(A) and the thing it is being applied on (B). And show only the forces being applied on it in the free body diagram and not the forces applied by it on other particles in the system.😊

I think that they should be regarded as a system (ball and wall). The net force within the system is zero. It did not say that the net force act on ball is zero. According to Newton's third law, there are action-reaction force. The ball is not equilibrium.
The magnitude of the force acts on ball and on wall are the same, but the directions are not the same, are opposite. And Based on second law, the net force is the product of mass of ball with acceleration of ball. Their mass are different, their acceleration is different and on the opposite directions.