Animation shows a particle (m) moving at v along a straight line. It's angular momentum vector about a fixed point O is shown to be related to the area swept by the radius vector of the particle relative to that fixed point.
![](Eqn01.svg)
The shaded portions are areas swept by the radius vector in equal intervals of time t. Since the velocity is uniform, vt is a constant. The bases of these shaded triangles are equal to Vt each and heights are h (=r sinθ), the perpendicular distance of the line of motion from O. The constancy of angular momentum of the particle about o implies the constancy of areal velocity of radius vector of the paticle about O.
![](02.svg)