Rotational inertia

Science

Definition

Rotational inertia (moment of inertia) is the resistance of an object to changes in its rotational motion. It depends on both the mass of the object and how that mass is distributed relative to the axis of rotation — mass farther from the axis means greater rotational inertia.

Examples

  • A figure skater spinning faster by pulling arms in close to the body (reducing rotational inertia)
  • A solid disk rolling down a ramp faster than a hollow ring of the same mass
  • A tightrope walker using a long pole to increase rotational inertia for better balance
Key Fact

I = Σmr² (point masses); for a solid disk I = ½MR², for a hollow ring I = MR²

Related Terms

Torque Angular momentum Angular velocity Centripetal force

Study This Concept

Practice rotational inertia with free review games in these units:

AP Physics 1 Unit 5: Rotational Motion