This law has passed the test of experiment so often that physicists have deep faith in it. In every case, further experimentation has shown that the theory of energy conservation is valid. This is how the neutrino was first detected, as missing energy. This says that in the absence of external forces the total momentum of a system is conserved. Newton's second law tells us that forces change an object's momentum, hence we need the no external forces qualifier.
Pool balls are a commonly used example.
If one ball strikes another ball, the momentum of each ball is changed because each felt the force of the collision. If we drop one of the pool balls, its momentum increases since its velocity increases. The force of gravity was external to our two pool ball system. If we included the earth in our system then total momentum would have once again been conserved as the pool balls' momentum change would have been countered by the earth's momentum change. Here is an important point.