This applies to any particle that moves in any path in relation to a fixed point, not just rotating systems. P is a particle in this closed system. It position is at a distance r from the reference point of O. O can be any reference point and the distance to the particle from that point will be r . Remember that this just isn t only for rotating objects. Every object has a chance to share in this equation.
Angular momentum is defined as .
Choosing a good reference point, such as point O , will become important when dealing with Equilibrium type of equations as seen from Chapter 13. Just like Newton s Second law when dealing with forces, remember , Angular momentum has its own version of this law, which is . This is for a single particle. However for a system of particles the equation is pretty much the same thing where . For a system of particles, the total angular momentum is equal to the total of all the angular momentum of each particle in the system.