Axlet

Meet Axlet, an orange rod who finds it much harder to spin about his end than about his middle.

The moment of inertia says how hard it is to start or stop something rotating. It depends on how far the mass sits from the axis. Moving the axis away from the centre of mass always makes it bigger.

The parallel axis theorem says I = I_cm + Md², where d is the distance between the two axes. A uniform rod of mass 1.0 kg and length 1.0 m has I_cm = ML²/12 ≈ 0.083 kg·m². About one end, d = 0.50 m, so I = 0.083 + 0.25 ≈ 0.33 kg·m², four times as much.

This is why it is easier to twirl a baton by its middle than to swing a bat from its handle.

Engineers use the theorem to work out how parts of machines, doors and even planets resist turning about axes that do not pass through their centres.