Inelastic Collision Calculator
Work out inelastic collision instantly with clear inputs, formula shown and shareable results.
About Inelastic Collision Calculator
In a perfectly inelastic collision the bodies move off together, so momentum conservation alone fixes the answer: v = (m1u1 + m2u2)/(m1+m2). Kinetic energy is not conserved - it goes into deformation, heat and sound. A 1200 kg car at 15 m/s hitting a 900 kg car at -5 m/s leaves the pair at 6.43 m/s, having destroyed about 51% of the kinetic energy.
Formulas
Combined velocity
v = (m1 u1 + m2 u2) / (m1 + m2)
Energy lost
loss = KE before - KE after
Worked example
With the default values below, Inelastic Collision Calculator returns Velocity After They Stick Together of 6.4286. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Mass of the first bodyminimum 0 kg
- 1,200 kg
- Velocity of the first body
- 15 m/s
- Mass of the second bodyminimum 0 kg
- 900 kg
- Velocity of the second body
- -5 m/s
Results
- Velocity After They Stick Together
- 6.4286 m/s
- Kinetic Energy Lost
- 102,857.1 J
- Share Of Energy Lost
- 70.33%
- Total Momentum (Conserved)
- 13,500.0 kg m/s
Step by step
- 1Values used
- 2Combined velocity
- 3Energy lost
- 4Velocity After They Stick Together= 6.4286 m/s
- 5Kinetic Energy Lost= 102,857.1 J
- 6Share Of Energy Lost= 70.33
- 7Total Momentum (Conserved)= 13,500.0 kg m/s
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why is momentum conserved but energy not?
Momentum conservation follows from Newton's third law and holds in every collision. Energy can hide as heat and deformation, which momentum cannot.
Is a real car crash perfectly inelastic?
Close to it when the vehicles lock together. Crumple zones are designed to maximise energy absorption for exactly this reason.