... One useful consequence of Newton's 3rd law is the conservation of momentum, as is shown by analyzing the recoil of a cannon. The example shows that while the gun and shell get equal amounts of momentum, by far most of the energy is given to the shell. Here mechanical energy is not conserved--some ends up as heat. ... We have discussed so far mass, velocity, acceleration, force and energy, and the way Newton's laws tie them together. ... momentum P = mass times velocity . ... Total mass . ...