Coefficient of Restitution (E)
Definition
The ratio of the relative velocity of separation to the relative velocity of approach in a collision; e = 0 for perfectly inelastic (maximum crush), e = 1 for perfectly elastic (no energy loss). Vehicle collisions typically give e between 0 and 0.3.
- Symbol
- e
- Range
- 0 (perfectly inelastic) to 1 (perfectly elastic)
- Typical vehicle collisions
- 0 to 0.3
- Formula
- Ratio of separation to approach relative velocity
Common questions
Why is e so low in typical vehicle-to-vehicle collisions?+
Most of the kinetic energy in a car crash goes into permanent crush deformation of the bodywork rather than being returned as rebound velocity, so vehicles behave close to perfectly inelastic, which is why measured e values cluster near the low end of the 0 to 1 range.
How is the coefficient of restitution used in momentum-based crash reconstruction?+
It appears alongside conservation of momentum equations to relate the vehicles' velocities before and after impact, letting a reconstructionist solve for unknown pre-impact speeds when post-impact positions, damage, and separation velocities are known or estimated from the scene evidence.
Related terms
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- IRTE
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- PC-Crash
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