Compound Units are units that require two different types of measurement: speed is a compound unit as it is defined with bothe distance and time. Acceleration, density and pressure are also compound units, as they require more than a single measurement to describe a quantity.
The table below shows some simple compound measurements. Compound measurements can be shown in different ways: a speed of 5 metres per second can be shown as 5ms-1 or 5m/s. In both instances, it means the same: a distance of 5 metres is covered in each second.
|Speed or Velocity||Distance cover in Time||kph m/s||5kph|
|Rates of Pay||Amount per Hour||£/hr||£7.80/hr|
|Prices||Price per unit||£/kg||38p/kg|
1. A train covers 128 miles in 2 hours 30 minutes. What is the average speed of the train?
Answer: 51.2 mph
2 hours 30 minutes is 2.5 hours.
The speed of the train is distance ÷ time = 128 ÷ 2.5 = 51.2 mph
2. Convert 45kph to ms-1.
45 kph-1 can be written as `frac(text(45 km))(text(1 hr))`.
Convert 45km to m, and hours to seconds:
`frac(45 times 1000)(1 times 60 times 60) = frac(45000)(3600) = 12.5ms^-1`