Speed

Speed is calculated using the equation:

\text{speed}=\frac{\text{distance}}{\text{time}}

Compound measure speed, distance and timeAn equation triangle is a tool that can be used to help rearrange equations

This is an example of a compound measure because distance and time can’t be measured in the same unit. Speed can be measured in metres per second, kilometres per hour and several other forms too.

curriculum-key-fact
Different measures have different sizes. 10 km/h is not the same as 10 m/s.
Question

Calculate the average speed of a mongoose if it travels 4 km in 0.25 hours.

\text{speed}=\frac{\text{distance}}{\text{time}}

\text{speed}=\frac{\text{4~km}}{\text{0.25~hours}}

\text{speed = 16 km/h}

Question

Calculate the average speed of a cheetah if it travels 300 m in 10 s.

\text{speed}=\frac{\text{distance}}{\text{time}}

\text{speed}=\frac{\text{300~m}}{\text{10~s}}

\text{speed = 30 m/s}

The equation for speed can also be rearranged into either of the following forms:

\text{distance}=\text{speed}\times{\text{time}}

or

\text{time}=\frac{\text{distance}}{\text{speed}}

This allows you to calculate either distance or time.

curriculum-key-fact
Distance and time are not compound measures as they are both measured in a single unit.
Question

A car travels from Cardiff to London with an average speed of 50 mph. If the journey takes three hours, estimate the distance from Cardiff to London.

\text{distance}=\text{speed}\times{\text{time}}

\text{distance}=\text{50~mph}\times{\text{3~hours}}

\text{distance}=\text{150~miles}