= Recoil angle in a two-body elastic collision
{title2=$\theta_2=(\pi-\theta^*)/2$}
For a nonrelativistic <elastic collision> with the target initially at rest, let $\theta^*$ be the outgoing projectile angle in the <centre-of-mass frame>. The laboratory target <velocity> is proportional to $(1-\cos\theta^*,-\sin\theta^*)$. Its unsigned recoil angle is therefore $(\pi-\theta^*)/2$, independent of the mass ratio. At zero deflection the target is stationary and its direction is undefined; the right-angle value is a limit.
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