# Dynamics Conservation of linear and angular momentum

1. Jan 9, 2014

### Dublin

Attached file shows where my problem occurs. I understand how to get the angular momentum just before the collision. But what confuses me is is within the square. Why mvg(r) is only on the left hand side when ive had similar equations with it on both sides. Why is Ig used and why isnt Ib used for rotational energy since isnt the ω thats used is about Ib.

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2. Jan 9, 2014

### tiny-tim

Hi Dublin!
You could use mvGr + IGωon the RHS also …

but it's much simpler to use IAω !
erm … no, ω is the same about any point

3. Jan 9, 2014

### Dublin

so why does mvr + Igw= Iaw? what is r suppose to represent? i thought it was just the distance of the mass center to the point of which the moment is being rotated about in this case Ia

Last edited: Jan 9, 2014
4. Jan 9, 2014

### Dublin

i think i figured it out thanks