Egg Science

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Dr. Keith Moffatt, a mathematical physicist at the University of Cambridge in England, provides this scientific explanation of why a hard-boiled egg will flip from a horizontal position to spin like a top:

1. If you place a hard-boiled egg at rest on a table, its center of mass, O, will be vertically above the point of contact with the table, P.

2. When you spin the egg about a vertical axis, its axis of symmetry gets inclined to the horizontal, and the point of contact P is displaced horizontally relative to O.

3. This leads to slipping and a horizontal frictional force acting on the egg at P, which tends to reduce the rate of precession of its axis of symmetry about the vertical. At the same time there is a build-up of spin about the axis of symmetry, a consequence of gyroscopic forces.

4. The frictional force exerts no leverage about the line OP through which it acts; from this (and here a bit of maths is unavoidable) it follows that the height of O above the table is inversely proportional to the rate of precession.

5. It follows further that as the rate of precession decreases, the center of mass O must rise. This continues until O is at maximum height, at which stage the axis of symmetry is vertical, and the egg spins about this axis in a stable condition.

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