Sun March 13, 2011 By: Rohan Mehra

According to law of conservation of momentum,total momentum before collision=total momentum after collision.Suppose a ball of mass m moving with velocity v strikes the wall and rebounds back with the same velocity.Then according to law of conservation of momentum: mv+0=-mv+0.But is not correct.Why?

Expert Reply
Sat March 12, 2011
Dear student,
Since the forces between the two objects are equal in magnitude and opposite in direction, and since the times for which these forces act are equal in magnitude, it follows that the impulses experienced by the two objects are also equal in magnitude and opposite in direction.

since each object experiences equal and opposite impulses, it follows logically that they must also experience equal and opposite momentum changes. As an equation, this can be stated as

The above equation is one statement of the law of momentum conservation. In a collision, the momentum change of object 1 is equal to and opposite of the momentum change of object 2. That is, the momentum lost by object 1 is equal to the momentum gained by object 2. In most collisions between two objects, one object slows down and loses momentum while the other object speeds up and gains momentum. If object 1 loses 75 units of momentum, then object 2 gains 75 units of momentum. Yet, the total momentum of the two objects (object 1 plus object 2) is the same before the collision as it is after the collision. The total momentum of the system (the collection of two objects) is conserved.
Hope this helps.
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