If there is no external force acting on a non-rigid body which of the following quantities must remain constant?
Let L be the Angular momentum.
Then
External torque
When external force is zero, , that means
This condition becomes true when, L remains constant because differentiation of a constant becomes zero.
Hence, angular momentum, L remains constant if external torque is zero
Similarly,
Linear momentum p-
External force,
Hence
P remains constant if external force is zero
Hence, in the absence of external forces, angular momentum and linear momentum of system is conserved.
A change in Kinetic Energy can be seen as in case of inelastic collision.
Similarly, moment of inertia may also change in this case.