Momentum Calculator
Calculate momentum (mass × velocity) and the average force needed to bring the object to rest in a given time.
How hard something is to stop — and the force that stopping takes.
How the momentum calculator works
Momentum is mass times velocity, in kilogram-meters per second. Newton's second law in its original form says force equals the rate of change of momentum, so stopping an object in time t needs an average force of p ÷ t — the impulse–momentum theorem. That is why crumple zones and airbags work: they stretch the stopping time.
Enter the speed in km/h (or m/s, mph in Detailed mode) and, optionally, a stopping time to see the force involved.
Formula: p = m v · F = Δp ÷ Δt
Worked examples
| Inputs | Momentum | Note |
|---|---|---|
| 1,500 kg car at 60 km/h | 25,000 kg·m/s | 25,000 kg·m/s |
| 1,500 kg car at 60 km/h stopped in 0.1 s | 25,000 kg·m/s | a crash: 250 kN, 17 g |
| 0.145 kg baseball at 145 km/h | 5.8 kg·m/s | 5.8 kg·m/s |
FAQFrequently asked questions
What is the difference between momentum and kinetic energy?
Momentum (mv) is a vector that is always conserved in collisions; kinetic energy (½mv²) is a scalar that is only conserved in perfectly elastic ones. A heavy slow object and a light fast one can share the same momentum with very different energies.
Why do airbags reduce injury?
They lengthen the stopping time. The momentum change is fixed, so a longer time means a smaller average force on the body.
What is impulse?
Force × time — equal to the change in momentum. A 25,000 kg·m/s momentum needs 25,000 N·s of impulse to remove, however it is delivered.
Does momentum have a direction?
Yes. In collisions the vector sum of momenta before equals the sum after, which is how crash investigators reconstruct speeds.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.