It might seem surprising that a particle moving in a straight lines has angular momentum.

In the absence of a net external torque acting on a system, its angular momentum remains constant. Kepler's second law states:

During equal time interval, the radius vector from the sun to a planet sweeps out equal area.

Does a free particle moving in a straight lines sweep out equal area? (with respect to any origin.)

Press start to start the animation, and let the show begin

A black dot will move freely from left to right. Different color shows the area it sweeps out with respect to some fixed points. Do all the areas have the same size?

Click within each area and see what will happen. Certainly, from mathematic equation, we know

Area of a triangle = width * height /2

All the areas have the same height, and the same width (=Vx*dt).