EngineeringFormulaMomentum (p)
Linear momentum is the product of an object's mass and velocity - a measure of how hard it is to stop.
p = mv. It's conserved in a closed system (the basis of collision analysis), and it's related to force by Newton's second law in its original form: force is the rate of change of momentum. Base SI unit is kg·m/s, equivalent to the newton-second (N·s).
Conservation of momentum is what makes momentum useful for analyzing collisions: the total momentum of a closed system is the same immediately before and after impact, even though kinetic energy is typically lost to heat and deformation. That's why momentum, not energy, is the standard starting point for reconstructing a collision.
Typical Momentum by Context
| Context | Mass | Velocity | Momentum |
|---|---|---|---|
| Walking adult | 75 kg | 1.4 m/s | 105 kg·m/s |
| 9mm handgun bullet | 8 g | 360 m/s | 2.9 kg·m/s |
| Car at highway speed | 1,500 kg | 30 m/s | 45,000 kg·m/s |
Used In These Calculators
Related Terms
Frequently Asked Questions
Is momentum the same as kinetic energy?
No, and they don't even scale the same way with velocity: momentum is p = mv (linear in velocity), kinetic energy is KE = ½mv² (quadratic in velocity). Doubling an object's speed doubles its momentum but quadruples its kinetic energy - which is part of why higher-speed collisions are disproportionately more damaging.
Why is momentum conserved but kinetic energy usually isn't, in a real collision?
Momentum conservation follows directly from Newton's third law (equal and opposite forces between colliding objects) and holds for any collision. Kinetic energy is only conserved in an idealized "elastic" collision; in most real collisions some kinetic energy converts irreversibly into heat, sound, and permanent deformation, so total kinetic energy after impact is typically less than before.
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