💥 Conservation of Momentum

📖 The Physics

With no outside push, the total momentum of a system can't change — this is one of physics' deepest rules. In the linear case a still object bursts apart, yet the momenta of the two pieces stay equal and opposite so the total remains zero. That is recoil: a gun kicks back, a rocket pushes exhaust down to fly up.

The same idea applies to spin — this time it's angular momentumL = I·ω that stays constant.

In "recoil", the lighter piece flies off faster. In "skater", drawing the masses inward shrinks the moment of inertia, so the spin speeds up to keep L fixed — exactly how a figure skater accelerates a spin by pulling their arms in.