# Some Concept Questions

Hi

I was wondering if anyone could explain why the momentum of the center of mass of a system is different than the sum of the independent momentums of every object.

I also don't understand the concepts of the first and second moment of area. I would appreciate it if someone explained it to me.

Thanks for any help that you can provide

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Doc Al
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I was wondering if anyone could explain why the momentum of the center of mass of a system is different than the sum of the independent momentums of every object.
Why do you think it's different?

Why do you think it's different?
Because on a question for my homework, which asked for linear momentum of the system, I got two different answers when i did the momentum of each separately than doing the momentum of the center of mass.

Doc Al
Mentor
Because on a question for my homework, which asked for linear momentum of the system, I got two different answers when i did the momentum of each separately than doing the momentum of the center of mass.
I'd have to see the details, but I suspect you made an error.

I'd have to see the details, but I suspect you made an error.
If they are indeed the same then I probably did. I'll try to work out what I did wrong. Thanks.

If they are indeed the same then I probably did. I'll try to work out what I did wrong. Thanks.
I think I figured out what I did but I don't really understand why. I did the change in position of the center of mass, divided it by the time interval, and multiplied the result by the total mass of the system and I got a very off answer. Can anyone explain why I can't do it this way? Thanks

Doc Al
Mentor
I think I figured out what I did but I don't really understand why. I did the change in position of the center of mass, divided it by the time interval, and multiplied the result by the total mass of the system and I got a very off answer. Can anyone explain why I can't do it this way? Thanks
You can do it that way. In order to find out where you went wrong, please describe the actual problem.