Some Concept Questions

**Red_CCF** · Nov14-09, 04:05 PM

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

**Doc Al** · Nov14-09, 04:13 PM

Originally Posted by Red_CCF

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?

**Red_CCF** · Nov14-09, 04:14 PM

Originally Posted by Doc Al

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** · Nov14-09, 04:25 PM

Originally Posted by Red_CCF

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.

**Red_CCF** · Nov14-09, 04:27 PM

Originally Posted by Doc Al

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.

**Red_CCF** · Nov14-09, 04:50 PM

Originally Posted by Red_CCF

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** · Nov15-09, 08:08 AM

Originally Posted by Red_CCF

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.