# Calculating a 2D collision between two perfectly circular disks [duplicate]

Assume I have two disks, $p_1$ and $p_2$, of radius $r$, with their own velocities (preferably in $(x,y)$ form, but $(m, \theta)$ works too) and masses (unit-less, but same unit) collide in two dimensions, how can I compute their resulting velocities?

I was looking around on the internet and it seems like every calculation assumes that one of them is at rest, but both of mine will be moving.

Wikipedia has this bit, but it assumes that I know how to calculate the angle of deflection of the system, $\theta$, but that's even more confusing.

I'm pretty lost. What do I do?

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## marked as duplicate by tpg2114, akhmeteli, centralcharge, Chris White, Qmechanic♦Nov 10 '13 at 16:07

You can perform a Galilei transformation, such that one of the particles is at rest. Then, the problem just reduces to one that you knew already. If you don't want this, you could use conservation of moment and energy. – Bernhard Feb 26 '13 at 17:46
Do the particles have the same radii? Also, since you said you've found calculations with one body at rest: why don't you try doing your calculation by transforming to the rest frame of one particle, using the sources you've found to calculate the result, then transform back to your original frame? – jeffdk Feb 26 '13 at 17:48
Possible duplicate: physics.stackexchange.com/q/53877/2451 – Qmechanic Feb 26 '13 at 17:52
@Bernhard I am not sure what a Galilei transformation is. – tekknolagi Feb 26 '13 at 21:32
@jeffdk they do not have the same radii. – tekknolagi Feb 26 '13 at 21:33