Momentum Conservation

The laws of conservation of energy and momentum are among the most fundamental and useful laws of physics. They aid in the solution of many mechanics problems and come up frequently in many fields of science. What these laws say is that if there are no net forces on a system, then that system will have the same momentum, p = mv, at all times. In addition, if there are no external or internal forces acting in or on a system, then the energy of that system will remain constant. Newton's First Law is hidden in these conservation laws. Newton's First Law states that bodies at rest will remain at rest as long as no forces act upon them, and bodies in motion will remain in motion as long as no forces act upon them. As one can see, Newton's First Law is a statement about conservation of momentum and energy. Things stay the same, as long as they are left alone.