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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.
Solution
(a) angular momentum
(b) linear momentum
\[\overrightarrow{F}_{ext} = 0\]
\[ \Rightarrow \overrightarrow{\tau}_{ext} = 0\]
That is, the change in linear momentum and angular momentum is zero. This is because:-
\[\frac{d \overrightarrow{P}}{d t} = \overrightarrow{F}_{ext} \]
\[\text{And }\frac{d \overrightarrow{L}}{d t} =\overrightarrow{\tau}_{ext}\]
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