= Smooth equal-disc oblique collision
Let one equal-mass disc arrive with <velocity> $\mathbf v$ while the other is stationary, and let $\mathbf n$ be the unit line of centres at contact. A smooth central elastic <impulse> exchanges their normal <velocity> components and leaves tangential components unchanged: $\mathbf v_A'=\mathbf v-(\mathbf v\cdot\mathbf n)\mathbf n$ and $\mathbf v_B'=(\mathbf v\cdot\mathbf n)\mathbf n$. The final translational <velocities> are orthogonal when both are nonzero, and no rotation about a disc centre is generated by the normal <impulse>. This result concerns nonrelativistic smooth discs; roughness introduces tangential contact dynamics.
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