The previous result might be surprising since we said at the outset
that we were going to decompose the total momentum into a sum
of linear plus angular momentum. Instead, we found that the total
momentum is simply that of the center of mass, which means any angular
momentum that might have been present just went away. (The center of
mass is just a point that cannot rotate in a measurable way.) Angular
momentum does not contribute to linear momentum, so it provides three
new ``degrees of freedom'' (three new energy storage dimensions, in 3D
space) that are ``missed'' when considering only linear momentum.
Whoops, No Angular Momentum!
To obtain the desired decomposition of momentum into linear plus angular momentum, we will choose a fixed reference point in space (usually the center of mass) and then, with respect to that reference point, decompose an arbitrary mass-particle travel direction into the sum of two mutually orthogonal vector components: one will be the vector component pointing radially with respect to the fixed point (for the ``linear momentum'' component), and the other will be the vector component pointing tangentially with respect to the fixed point (for the ``angular momentum''), as shown in Fig.B.3. When the reference point is the center of mass, the resultant radial force component gives us the force on the center of mass, which creates linear momentum, while the net tangential component (times distance from the center-of-mass) give us a resultant torque about the reference point, which creates angular momentum. As we saw above, because the tangential force component does not contribute to linear momentum, we can simply sum the external force vectors and get the same result as summing their radial components. These topics will be discussed further below, after some elementary preliminaries.
Circular Disk Rotating in Its Own Plane
Linear Momentum of the Center of Mass