Why Does a Raw Egg Start Spinning Again After You Stop It?

Place an intact raw egg on a level surface, spin it, briefly stop the shell with a finger and release it immediately. The egg will often begin turning slowly again, while a hard-boiled egg usually stays still. The difference lies not in the shell but in whether the contents remain liquid or have set into something closer to a single solid body.

When a raw egg begins to spin, the shell moves first and gradually drags the white and yolk through viscous friction. A brief touch stops the shell, but the liquid inside retains some rotational motion. After release, viscous interaction between the moving liquid and the shell transfers some angular momentum back to the shell, so the egg starts turning again. In a hard-boiled egg, the contents and shell largely rotate and stop together; there is no continuing internal flow to restart the shell.

This also explains why judging only how steadily an egg spins at first is not the most reliable test. A raw egg’s behaviour depends on its speed, internal flow and friction with the surface. The stop-and-release test is more direct because it exposes the difference between a stationary shell and moving contents.

There is nothing mysterious about the restart. The experiment shows that an object which appears to have one state of motion can contain parts moving differently. An external force acts first at the boundary, while friction gradually changes motion within the whole object.

https://arxiv.org/abs/2202.10243