recoil
Recoil is the backward kick you feel when something is pushed forward. Fire a rifle and it thumps your shoulder; step off a small boat toward the dock and the boat scoots out from under you; blow up a balloon and let go, and it darts away as the air rushes out the back. Whenever you throw something one way, you get shoved the other way.
Recoil is conservation of momentum in action. Start with everything at rest, so the total momentum is zero. Send a mass forward with momentum +p, and the rest of the system must take momentum -p so the total stays zero: m_gun v_gun = -(m_bullet v_bullet). Because the gun is far heavier than the bullet, it recoils far more slowly than the bullet flies — same momentum, but shared out in inverse proportion to mass.
This one idea powers everything from a squid jetting water to a rocket climbing to orbit. It is also a vivid demonstration that you do not need to push against the ground or the air to move: expel some mass backward and you WILL go forward, even in empty space.
A 4.0 kg rifle fires a 0.010 kg bullet at 600 m/s. Momentum before is zero, so 4.0 times v_gun = -(0.010 times 600) = -6 kg m/s, giving v_gun = -1.5 m/s. The rifle recoils at 1.5 m/s — 400 times slower than the bullet, because it is 400 times heavier.
Equal and opposite momenta; the heavy gun barely moves while the light bullet races off.
Recoil does not mean the gun and bullet get equal speeds, or equal energies — they get equal and opposite MOMENTA. The light bullet, with the same momentum but far less mass, carries almost all of the kinetic energy (since kinetic energy = p^2 / 2m).