Force and momentum

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?

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.

Force can also be calculated using this equation.

\(\text{force} = {\text{mass}}\times{\text{acceleration}}\)

In the question above, the acceleration of the bicycle is (12 – 0) ÷ 5 = 2.4 m/s²

Force = 25 × 2.4 = 60 N (the same answer as before).

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Further motion concepts – WJECForce and momentum