This section explains how to calculate the force involved in changing the momentum of an object.
Calculating force
Force can be calculated using this equation.
\(\text{force}~=~\frac{\text{change in momentum}}{\text{time}}\)
where
force is measured in newtons (N)
change in momentum is measured in kilogram metres per second (kg m/s)
time is measured in seconds (s)
Question
What force is needed to get a 25 kg stationary bicycle moving at 12 m/s in 5 s?
Momentum at start = 0 × 12 = 0 kg m/s
Momentum at end = 25 × 12 = 300 kg m/s
Change in momentum = 300 – 0 = 300 kg m/s
Force = change in momentum ÷ time = 300 ÷ 5 = 60 N
To change the momentum of an object you can apply a small force over a long time, or a larger force over a shorter time. Changing the direction of an oil tanker at sea is difficult because a large change in momentum is needed, but the force from the propeller is only relatively small, so it takes a long time.