Systems of Particles and Rotational Motion — Physics STD 11 Science — Question

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