Conservation of Momentum
Definition
In a collision, the vector sum of all momenta (mass × velocity) before impact equals the vector sum after, provided no significant external forces act during the brief impact interval. This is the central equation in multi-vehicle reconstruction.
- Field
- Accident reconstruction physics
- Quantity conserved
- Vector sum of mass times velocity
- Assumption
- External forces negligible during impact
- Complements
- Crush-energy analysis, skid-to-stop calculations
Common questions
Why is momentum used instead of energy for reconstructing a crash?+
Momentum is conserved exactly in any collision, elastic or not, as long as external forces during the impact are small. Kinetic energy is not conserved in most vehicle crashes because deformation absorbs it, so energy methods need separate crush-stiffness data while momentum methods work from pre- and post-impact speeds and directions alone.
What breaks the conservation of momentum assumption in a real crash?+
Braking or steering forces that act over a longer interval than the impact itself, a vehicle striking a fixed object that absorbs momentum into the ground, or a multi-stage collision where a third vehicle enters before the first exchange finishes. Reconstructionists check the impact duration against these factors before applying the equation.
Related terms
- Coefficient of Friction (Mu)
- The dimensionless ratio of frictional force to normal force between tire and road. It determines how quickly a skidding vehicle decelerates. Typical...
- Critical Speed Yaw Mark
- Curved scuff marks deposited when a vehicle slides sideways. The radius of curvature and friction coefficient yield the vehicle's minimum speed at...
- Delta-V
- The change in vehicle velocity during the crash event, recorded by the EDR as a time-history. It is the integral of the...
- Perception-Reaction Time (PRT)
- The elapsed time from when a driver can first perceive a hazard to when the brakes begin to apply. Olson and Farber...
- Skid Mark
- A dark rubber deposit left when locked wheels slide along the road surface. Length combined with the friction coefficient allows calculation of...