Physics - Conservation of momentum
Conservation of momentum
This is an important and fundamental principle of physics - the total
momentum in the universe remains constant.
Also, in an isolated system,  momentum is conserved. This means that
the total momentum of the system before an interaction, such as a
collision, equals the total momentum during and after the interaction. An
isolated system is one in which the only forces affecting the collision are
those of the objects on each other. Often we can treat a real world
collision as an isolated system if we consider the motion immediately
before and after the collision. For instance, in a car collision, the cars
might stick together and then come to rest due to frictional forces. We
would only consider the motion of the cars just after the collision, before
the effects of the frictional forces were significant.
We can summarise the Law of Conservation of Momentum  with the
The total momentum is found by adding (using vector addition) the momenta
of all the objects. In problems involving the conservation of momentum it
is often necessary to define a positive direction.
If momentum  is conserved, then one object _loses_ momentum, the other
object must _gain_ momentum in order to keep the total momentum constant.
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