Which law explains why more power is required to accelerate to a target RPM than to maintain it?

• A. Newton's third law
• B. Bernoulli's principle
• C. Thermodynamics
• D. Inertia

Answer: (D)

Rotational inertia resists changes in how fast the rotor spins. To spin up, you must apply torque to create angular acceleration, and the relationship is τ = I α. The power you provide is P = τ ω, so as you speed up (ω increases) you’re having to push against inertia with increasing power. In addition, you’re adding rotational kinetic energy, ΔK = 1/2 I ω^2, which must be supplied during the acceleration period.

Once you reach the target RPM, you mainly need to overcome ongoing losses like drag and friction, so the required power drops to the amount needed to keep those losses balanced. That’s why more power is needed to accelerate to a target RPM than to maintain it.