# Collision of two objects

Suppose there are two objects, denoted by A and B. When they collide with each other with the condition that A is stationary and B is in motion, B exerts an action force on A due to which A exerts a reaction force on B. Thus the net force exerted on A is FactionAB and the net force exerted on B is FreactionBA. Same is the case when A is in motion and B is at rest. But when A and B collide when they are in motion, the net force exerted on A is FactionAB + FreactionAB and the net force exerted on B is FactionBA + FreactionBA.
Question: Is my reasoning correct for the case when A and B collide when they are in motion? Is the net force exerted on A and B when they collide when they are in motion greater in magnitude than the net force exerted on A and B when they collide when only one of them is in motion?

PeroK
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The force depends how fast they are moving relative to each other.

In fact, all three scenarios are essentially the same: the same collision in different frames of reference. If you wanted to analyse a collision where both objects are moving it's a good idea to change your reference frame to one in which only one object is moving.

Finally, for this reason, there is only really one pair of action -reaction forces in each case.

• donaldparida
@PeroK, Is it possible to explain that there is one pair of action-reaction forces without changing the frame of reference?

A.T.
Question: Is my reasoning correct for the case when A and B collide when they are in motion?
No, you got confused by the physically meaningless "action / reaction" terminology.

Is it possible to explain that there is one pair of action-reaction forces ...
There is one pair of equal but opposite forces, because there is one interaction. Forget the "action / reaction" labels. The case where both move shows why they are meaningless.

Chandra Prayaga