Momentum Conservation
Definition
In the absence of external horizontal impulses, the total linear momentum of a two-vehicle collision system (sum of mass times velocity for each vehicle) is the same before and after the collision.
- Applies to
- Two-vehicle collision system
- Condition
- No external horizontal impulses
- Quantity conserved
- Total linear momentum (mass times velocity)
- Used in
- Pre- and post-impact speed calculation
Common questions
Why do reconstructionists need momentum conservation in addition to skid-mark analysis?+
Skid marks only estimate post-impact speed from friction and distance, while momentum conservation links the pre-impact speeds of both vehicles to the impact geometry, giving a second, independent method to cross-check the reconstructed speeds.
What real-world factor can make strict momentum conservation inaccurate at a scene?+
External horizontal impulses such as a vehicle striking a fixed object, rolling, or being deflected by road camber during the collision violate the closed-system assumption, so reconstructionists must confirm the collision was a clean two-vehicle exchange before applying it.
Related terms
- Coefficient of Kinetic Friction (Mu)
- The dimensionless ratio of the sliding friction force to the normal force between a tyre and the road; typically 0.55-0.70 for dry...
- Coefficient of Restitution (E)
- The ratio of the relative velocity of separation to the relative velocity of approach in a collision; e = 0 for perfectly...
- Daubert Standard
- The US federal evidentiary standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requiring that expert testimony be based on scientifically valid methods with...
- Energy-Equivalent Speed (EES / Delta-V)
- The SAE J2080 metric quantifying the kinetic energy absorbed by vehicle crush deformation, expressed as the speed change equivalent; used in the...
- Event Data Recorder (EDR)
- An onboard module that records pre-crash vehicle kinematics (speed, throttle, brake, steering) in the seconds before airbag deployment; governed by SAE J1698...
- Head-Throw Distance (HTD)
- The horizontal distance from the vehicle's front bumper contact zone to the final rest position of the pedestrian's head; used in the...
- IRTE
- Institute of Road Traffic Engineers (UK); publishes the Forensic Collision Investigation manual that governs police and expert-witness reconstruction methodology in England and...
- PC-Crash
- Accident reconstruction simulation software (Steffan and Moser, Austria) that models vehicle dynamics, pedestrian multibody kinematics, and collision geometry; validated against crash-test data...
- SAE J1739
- Society of Automotive Engineers standard for traffic-accident reconstruction covering momentum, skid, and crush-energy methods; the primary scientific-community reference for admissibility arguments in...
- Skid-Mark Formula
- v = sqrt(2 mu g d), where v is the pre-skid speed (m/s), mu is the coefficient of kinetic friction between tyre...
- Yaw Mark
- A curved, striated tyre impression produced when a vehicle slides at an angle to its direction of travel during a steered or...