Physics · SYSTEMS OF PARTICLES AND ROTATIONAL MOTION

For a rigid body rotating about a fixed \( z \)-axis, why do internal forces between particles do zero net work during rotation?

1. Because the distance between any pair of particles remains completely invariant in a perfectly rigid body
2. Because the angular velocity is identical for all particles
3. Because internal forces act only perpendicular to the axis of rotation
4. Because the net torque due to internal forces about any axis is non-zero