Conservation of momentum
From Conservapedia
Conservation of momentum is a principle in physics that momentum, like energy, is a conserved quantity in an closed system. This due to symmetry in space, that the laws of physics are identical at all points in space.
It can also be explained by considering that in a closed system, there can be no net force on the system. From Newton's second law 
, it is clear that if the force is zero, the momentum must be constant. Hence momentum is a conserved quantity.
