Question

If there is no external force acting on a nonrigid body, which of the following quantities must remain constant?
(a) angular momentum
(b) linear momentum
(c) kinetic energy
(d) moment of inertia.

Answer 1

(a) angular momentum
(b) linear momentum

$$\begin{gathered} {\overrightarrow{F}}_{ext}=0 \\ \Rightarrow {\overrightarrow{\tau }}_{ext}=0 \end{gathered}$$
That is, the change in linear momentum and angular momentum is zero. This is because:
$$\begin{gathered} \frac{\mathit{d}\overrightarrow{P}}{\mathit{d}t}\mathit{=}{\overrightarrow{F}}_{\mathrm{ext}} \\ \mathrm{And}\frac{\mathit{d}\overrightarrow{L}}{\mathit{d}t}={\overrightarrow{\tau }}_{\mathrm{ext}} \end{gathered}$$