How to use it
- Choose what to find: momentum, mass, velocity, or impulse and force.
- Enter the known values and pick the mass and velocity units.
- For impulse, enter the velocity before and after (with a minus for the opposite direction) and how long the impact lasted.
- Read the result in the unit you picked, plus the kinetic energy.
How it works
- Momentum: p = m × v, in kg·m/s.
- Mass and velocity: m = p ÷ v and v = p ÷ m.
- Impulse: J = m × (v₂ − v₁), in N·s.
- Average force: F = J ÷ Δt, in newtons.
Inputs in grams, pounds, km/h or mph are converted to kilograms and metres per second first (1 lb = 0.45359237 kg, 1 mph = 0.44704 m/s).
Worked example
A 1500 kg car at 72 km/h: 72 km/h = 20 m/s, so p = 1500 × 20 = 30,000 kg·m/s and the kinetic energy is ½ × 1500 × 20² = 300,000 J.
A 0.5 kg ball hits a wall at 10 m/s and bounces back at 5 m/s in 0.02 s: the velocity changes by −5 − 10 = −15 m/s, the impulse is 0.5 × −15 = −7.5 N·s and the average force is −7.5 ÷ 0.02 = −375 N. The minus sign shows that the wall pushes the ball back.
Common mistakes
- Forgetting the direction. A bounce reverses the velocity, so the change is bigger than either speed.
- Mixing units. 72 km/h is 20 m/s, not 72 m/s; pick the right velocity unit.
- Confusing momentum with energy. Doubling the speed doubles the momentum but quadruples the kinetic energy.