Two worked examples
Working out momentum is a simple multiplication. The one place to be careful is the units.
Example 1
The problem
A vehicle of mass 2000 kg travels at a velocity of 20 m s−1. What is its momentum?
Solution
momentum = m v
= 2000 kg × 20 m s−1
= 40 000 kg m s−1
Example 2
The problem
A bullet of mass 10 g fired from a gun travels at a velocity of 400 m s−1. What is its momentum?
Solution
Here the value of the mass must be converted into kilograms before it is substituted into the equation.
momentum = m v
= (10 ÷ 1000) kg × 400 m s−1
= 0.01 kg × 400 m s−1
= 4 kg m s−1
Comparing the two
The vehicle's momentum is 10 000 times the bullet's — even though the bullet's velocity is 20 times the vehicle's. That is how large the difference in mass is.
So why is the bullet dangerous? Because it is stopped over a very small area, in a very short time.
Check your units
The mass must always be in kg. If it is given in grams, divide by 1000.