**Angular momentum and Area**

It might seem surprising that a particle moving

in a straight lines has angular momentum.

**Conservation of angular momentum
L** **= ****r
X m v**

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 will move freely from left to right.

Different color shows the area it sweep out

with respect to some fix points.

Does 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).

Your suggestions are highly appreciated!

E-mail : please click *hwang@phy03.phy.ntnu.edu.tw*

Author¡GFu-Kwun Hwang, Dept. of physics, National Taiwan Normal University

Last modified :