Calculate angular momentum and the speed change conservation implies when inertia changes.
With no external torque, angular momentum is conserved, so reducing inertia must raise angular speed proportionally. Note that rotational energy is not conserved — pulling mass inward requires work, which appears as extra energy. A skater pulling their arms in spins faster for exactly this reason, and the energy increase comes from the muscular work of pulling in.
Angular Momentum
L = I ω, conserved without external torque, so I₀ω₀ = I₁ω₁
L = I ω, conserved without external torque, so I₀ω₀ = I₁ω₁ With no external torque, angular momentum is conserved, so reducing inertia must raise angular speed proportionally. Note that rotational energy is not conserved — pulling mass inward requires work, which appears as extra energy.
A skater pulling their arms in spins faster for exactly this reason, and the energy increase comes from the muscular work of pulling in.
This calculator takes 3 inputs: Moment of inertia, Rotational speed, New moment of inertia. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.