Rigid Body Net Torques Are From External Forces

Torques from internal forces besides magnetism in rigid bodies must cancel due to Newton's third law. Consider internal forces F and -F between two particles at positions r₁ and r_2:

r₁ x F + r₂ x (-F) = (r₁ - r₂) x F = 0

Therefore, rigid body net torques are from external forces.