Internal and external torques.

[transparent]

How do I tell if a torque on a system is internal or external before conserving angular momentum? I know that if a force has its reactionary force in the system, then it is an internal force and we can conserve the linear momentum of the system. But I don't know how to recognize a reactionary torque. For example, if two rough rotating bodies (say cylinders) are brought into contact with each other, the torque experienced by them due to friction is different and hence the angular momentum cannot be conserved in this two-body system even though the forces are all internal.

 

[kuruman]

It's simple enough. Before you set out figuring out what's internal and what's external, you have to spell out to yourself what your system is. In this case, if your system is either one of the cylinders, then the other cylinder is external to it and neither system's angular momentum will be conserved. If your system is both cylinders together and this system is isolated, then the sum of the angular momenta of the cylinders will be conserved throughout. When, because of friction, the surfaces in contact come to rest relative to each other, the two-cylinder system will rotate about an axis passing through its center of mass. If the two-cylinder system is not isolated, say because the cylinders spin about shafts that are firmly attached to the Earth, then the angular momentum of the two-cylinder system is not conserved. However, the angular momentum of the two-cylinder system plus Earth will be conserved. Do you see how it woks?