ω′
Selected axis

Rotational Physics Lab

The idea

A rigid body really rotates about one default axis (through the centre of mass, or the door hinges) with angular speed ω.

You can also observe parallel axes a distance d away. By the parallel-axis theorem, inertia about that axis is I′ = I₀ + md². For the same angular momentum, the related speed is ω′ = I₀ω / (I₀ + md²).

So farther axes mean larger I′ and a smaller ω′ compared with ω.

How to use

  1. Pick an object — door, sphere, or dice.
  2. Click an axis (or use − / +) to select it. Selected axis is solid; others are dotted.
  3. View → Ground shows real spin about the default axis. View → Axis shows spin about the selected axis.
  4. Read ω′ on the right: how it relates to ω and distance d.
  5. Drag to orbit, scroll to zoom. Use fullscreen for a larger view.
AXIS 0
Ground — real motion about the default axis only.
Axis — show spin about the selected axis (for comparison).