Short version: The principle of Conversation of Momentum demands that the Momentum (mass x velocity) of the projectile (here: several tons times several tens of mph) is equal to, but opposite, the Momentum of the contraption that does the projection (assuming all start at rest. This is why guns experience recoil. In guns, of course, the gun itself is much heavier than the projectile, and thus the projectile ends up being much faster than the gun. Also, the gun is usually fixed to something even heavier ... Read even more
