Engineering
Why doesn't a spinning top fall?
A spinning top stays upright, but the same top falls over at once when it is not spinning. Why?
- Hint 1What does gravity do to a tilted top: a force, or a turning effect?
- Hint 2A torque changes angular momentum. Which way does the spinning top's angular momentum point?
- Gravity acting on a tilted top gives a torque about the point of contact.
- A torque changes angular momentum in the direction of the torque.
- A still top has no angular momentum, so the torque simply starts it toppling.
- A spinning top has large angular momentum along its axis. The torque is sideways to that, so it swings the axis round in a circle (precession) instead of tipping it.
- Friction slowly removes the spin; as it drops, the precession speeds up and the top wobbles and falls.
Where it lands: Its angular momentum turns gravity's torque into precession instead of a fall.
The trap: Saying spinning 'balances' the forces: the torque is still there; it changes the direction of the axis instead.
What a tutor might ask next: Why is it easier to balance on a moving bicycle than a still one, and is it the same reason?
Now do one out loud. In the interview you think aloud with a tutor. Try an Engineering problem with hints, follow-ups and Mia's feedback on how you think.