Momentum is one way to quantify – that means "put a numerical value to" – the motion of an object. The two pieces you need to know about an object to calculate its momentum are mass and velocity.

Scientists usually use the first letter of a quantity as the variable to represent it in equations, but since momentum starts with an m, they had a little trouble. Lower case m is already used for mass and as the prefix milli-, upper case M is used in chemistry for molarity and as the prefix Mega-, and even the Greek letter μ has already been used as the coefficient of friction and as the prefix micro-. The variable for momentum is lower case p.

The equation for momentum is:

In a triangle equation, you cover the variable that you want to solve, and the other two line up like they are in the correct equation. This equation is sometimes called a 'triangle equation.' Put your mouse on the triangle for more information.

From the equation, you can see that the units of momentum are (kg) * (m/s), usually just written as kg m/s.

Example:

A 1250 kg car is driving at 7 m/s. What is the car's momentum?
Given: m = 1250 kg v = 7 m/s
Equation: p = mv
Solution: p = (1250) (7) = 8750 kg m/s

Momentum is a vector quantity. That means that it has magnitude and direction. Be careful that motion in the negative direction (usually left, down, south or west) is given a negative momentum.