Question

In which of the following case(s), the angular momentum is conserved ?

• A. The planet Neptune moves in elliptical orbit around the sun with sun at one focus
• B. A solid sphere rolling on an inclined plane
• C. An electron revolving around the nucleus in an elliptical orbit
• D. An $\alpha$ particle approaching a heavy nucleus from sufficient distance.

Answer 1

Answer: (A), (C), (D)

The correct options are
A The planet Neptune moves in elliptical orbit around the sun with sun at one focus
C An electron revolving around the nucleus in an elliptical orbit
D An $\alpha$ particle approaching a heavy nucleus from sufficient distance.

Angular momentum is conserved if net external torque is zero
(A) torque about sun is zero , $\because$ line of focus passer though it and no external force is present.
(B) Torque of friction is present
(C) Force on the electron is internal , $\therefore$ torque is zero
(D) No external force is present which may produce changes in angular momentum.