Coefficient of restitution (1D) (e = (v₂f − v₁f)/(u₁ − u₂))

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e = (v₂f − v₁f)/(u₁ − u₂)

What each symbol means

SymbolWhat it stands for
eCoefficient of restitution (0 ≤ e ≤ 1 for passive collisions)
u_1, u_2Initial velocities along line of centres (m/s)
v_{1f}, v_{2f}Final velocities along same line (m/s)

When to use this

Central collision along a line; velocities along line of impact; sign convention must be consistent (same positive direction).