# How to Find the Resulting Angles in a Lab-Frame Collision between Particles of Equal Mass

Say you have two particles of equal mass colliding in a lab frame (where one particle starts at rest). You want to show that the two particles end up traveling at right angles with respect to each other in the lab frame. Using quantum physics, you can.

Note that if *m*_{1} = *m*_{2}, then

Note also that

You know that

tells you that the following is true:

So substituting

into the preceding equation gives you

Therefore,

the angles of the particles in the lab frame after the collision, add up to

are at right angles with respect to each other. Cool.

In this case, you can use the relations you’ve already derived to get these relations in the special case where *m*_{1} = *m*_{2}: