Law of conservation of momentum

This law states that when two or more body collides the total momentum before collision is equal to the total momentum after collision, provided there is no net external force on the system.

Consider a lorry of mass Mkg and a car of mass mkg moving in the same direction with velocities of Vm/s and vm/s collide together.

 The total momentum before collision is MV + mv  and the total momentum after collision is MU+mu  . where u and U are the velocities after collision.

Then the law of conservation of momentum states that ;

MV + mv  = MU+mu 

If this two bodies move together after collision with common velocity u then we have

MV + mv  = (M+m)u

If they move in opposite direction and collide together then we have

MV -mv  = MU+mu

 If they move together after collision we have

MV - mv  = (M+m)u

The velocities must be measure with correct positive or negative signs

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