# Angular momentum

I have a system that consists of a cycle wheel on which a vertical thin rod is welded. to this rod another thin rod is welded perpendicular. It is something like a cross fitted on the wheel and as the wheel rotates, whole thing rotates. now on the top rod if two eqaul masses are slided one from each side and fixed. Can i take the angular momentum of the whole system to be angular momentum of the wheel + the angular momentum of the two mass system?

Andrew Mason
Homework Helper
rgshankar76 said:
I have a system that consists of a cycle wheel on which a vertical thin rod is welded. to this rod another thin rod is welded perpendicular. It is something like a cross fitted on the wheel and as the wheel rotates, whole thing rotates. now on the top rod if two eqaul masses are slided one from each side and fixed. Can i take the angular momentum of the whole system to be angular momentum of the wheel + the angular momentum of the two mass system?
Yes. Angular momentum is a vector quantity. The angular momentum vector of the whole is equal to the sum of the angular momenta vectors of its parts.

AM

moment of inertia

is it also applicable in the case of moment of inertia? means, MI of wheel + MI of masses = MI of whole system?

Doc Al
Mentor
Yes. The MOI of a system = Sum of the MOIs of its parts. (You seem to be ignoring the MOI of the cross piece. If it's light enough, that may be OK.)

Yes. The MOI of a system = Sum of the MOIs of its parts. (You seem to be ignoring the MOI of the cross piece. If it's light enough, that may be OK.)
Be carefull! This is true, IF the centroids of the composite shape are along the same axis! In general, you can add them and subtract them, but you have to include the parallel axis theorem as well!

Doc Al
Mentor
I'll rephrase my statement: The MOI of a system about an axis = Sum of the MOIs of its parts about that same axis. 