The moment of external force on the rotating shaft is zero, so the moment of momentum of the system is conserved. Without considering the wheel mass, (Va_wr)mr=(Vb+wr)mr, (VA and Vb are relative to the speed of the rope, and the absolute speed is used above).

The moment of a particle to a fixed point (or axis) is always equal to zero in the absence of force or force, and the moment of momentum of a particle to this point (or axis) remains unchanged. The particle system is affected.

The sum of moments of external force to a fixed point (or axis) is always equal to zero, and the moment of momentum of a particle system to that point (or axis) remains unchanged. For example, the attraction of the sun to the planet always points to the center of the sun, so if the attraction of other stars is ignored, the moment of momentum of the planet to the center of the sun is conserved.

Extended data:

Conservation condition of momentum moment: also known as conservation of angular momentum, the resultant moment is zero, and the resultant force is not necessarily zero. There are two routes to describe the motion of an object. Newton found that momentum change is equal to the product of resultant force and time. The change of kinetic energy developed by Leibniz is the product of external force and displacement.

The two development routes have been debated for many years, and finally we know that both routes can describe the motion state of objects. However, it was later found that momentum could not describe the state of rotating objects. Both the stationary disk and the rotating disk have zero momentum, but the stationary object and the rotating object are indistinguishable. Therefore, the state of an object is described by angular momentum, resulting in the law of conservation of angular momentum.