Small imperfections in the spin accumulate and unbalance the spin.

Objects can spin around 3 axes. This effect *only* happens for objects in which the largest heaviest axis (of rotation) has lots more momentum than the middle weight axis, which is way more momentum than the lightest axis. Why? Because this distribution of weights allows tiny perturbations of kinetic energy (momentum, really) to *concentrate* in the middle axis, which accumulates for a while and then dissipates, in a cycle. If all the axes are about the same, there’s little “inbalance” that can leak, so the perturbations (that always exist) are not going to accumulate in one axis but will stay random among all the axes where you won’t notice it.

It always happens, in the same way. Most tennis players know about this, though they don’t know why.

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