Mechanics · Quick Reference

Momentum Cheat Sheet

Mechanics · Lesson 12/19

In one line: momentum captures how much 'motion' a moving object carries — a heavy truck and a light bicycle at the same speed have very different momentum, and very different effects in a collision.

Key Ideas

1Momentum Definition. p = m*v, the product of an object's mass and velocity; like velocity, momentum is a vector quantity.

2Units of Momentum. Measured in kg*m/s (there's no separate named unit).

3Conservation of Momentum. In a closed system with no external forces, total momentum before a collision or interaction equals total momentum after.

4Elastic vs. Inelastic Collisions. In an elastic collision, both momentum AND kinetic energy are conserved. In an inelastic collision, momentum is conserved but some kinetic energy converts to other forms (like heat or sound).

5Perfectly Inelastic Collisions. A special case where the colliding objects stick together and move with a common final velocity after impact.

Worked Examples

Find the momentum of a 5 kg object moving at 8 m/s.

40 kg*m/s

A 2 kg ball moving at 6 m/s collides with a stationary 4 kg ball, and they stick together. Find their common velocity after collision.

2 m/s

Is a collision where two cars crash and crumple together (sticking) elastic or inelastic?

Perfectly inelastic

Formulas

p = m*v

Conservation: m1*v1 + m2*v2 = m1*v1' + m2*v2'

Practice Yourself

Find momentum of a 3 kg object at 10 m/s.

30 kg*m/s

Find momentum of a 10 kg object at 2 m/s.

20 kg*m/s

A 1 kg ball at 5 m/s hits a stationary 1 kg ball and they stick. Find final velocity.

2.5 m/s (using conservation: 5=2v)