A particle of mass m is moving along the line y=b, z=0 with constant speed u . State whether the angular momentum of particle about origin is increasing, decreasing or constant.

$\left|\mathrm{L}\right|=\mathrm{mvrsin\theta }={\mathrm{mvr}}_1=\mathrm{mvb}$

$\therefore$

$\left|\mathrm{L}\right|=$ constant as m, v and b all are constants.

Direction of $\overrightarrow{r}\times$ $\overrightarrow{v}$ also remains the same. Therefore, angular momentum of particle about origin remains constant with due course of time.