Question

Does the angular momentum of planet revolving around the sun remain constant or not? Prove

Answer 1

There is no external torque about the sun since the force of the sun and position vector are always at an angle 180 ∘

180∘

since τ ¯ =r ¯ ×F ¯

τ¯=r¯×F¯

, so angular momentum is conserved or remains constant.

But since the path in not circular but elliptical, the position vector is not perpendicular to direction of motion hence some work is done, which changes the momentum indirectly by changing magnitude of velocity directly.