# Please help me, I'm really in need.

1. Jan 4, 2010

### TaintedLove

Okay, so what I'm confused with is.
A 2D collision took place...
It was an inelastic collision in the sense that energy was lost, but the objects did not stick together.
So, I have two initial velocities and two final velocities

When calculating the speed at the centre of mass, I know I have to multiply the velocities by the mass and divide it by the total mass... but do I only do that with the initial velocities? or both? Also, if I do that with both, do I only add the masses twice (in the divisor) or four times if i'm adding all the velocities?

Also, after that step, to find the momentum, I know it's your mass x velocity... so the velocity that I'll plug in here will the velocity at the centre of mass... but for the actual masses itself, do I add the two initial masses?

Let's say, the two masses are 20 and 50 kg.
so do I multiply the velocity at the centre of gravity by 70?

Please help! Thanks.

2. Jan 5, 2010

### vela

Staff Emeritus
You only use one set of velocities at a time. If you use the velocities from before the collision, you will get the velocity of the center of mass before the collision. Likewise, if you use the final velocities, you get the velocity of the center of mass after the collisions.

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