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The law states that in the absence of external force,the total momentum of the bodies remains the change.
Total momentum before collision = Total momentum after collision
Therefore total momentum is a constant.
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The Principle of the Conservation of Momentum states that: if objects collide, the total momentum before the collision is the same as the total momentum after the collision (provided that no external forces - for example, friction - act on the system).

That's amazingly useful because it means that you can tell what is going to happen after a collision before it has taken place.

Principle of Conservation of Energy: Of course, energy is also conserved in any collision, but it isn't always conserved in the form of kinetic energy, so be careful.

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in cae of absence of external force.