Measurement and Motion
Measuring length
Say why standard units are needed, convert between kilometre, metre, centimetre and millimetre, and read a scale the right way.
Five children measure the same table with their handspans and get five different answers — 13, 14, and three answers in between. Nobody has made a mistake. The trouble is the unit: a handspan is a different size on each of them. Units taken from the body are convenient and ancient, but they cannot settle an argument.
The standard units
Countries settled this by agreeing on one shared set, the International System of Units, or SI. The SI unit of length is the metre, symbol m. Around it sit the kilometre for long distances, and the centimetre and millimetre for short ones: 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm.
Have a play
Slide along this 15-centimetre scale. Between each pair of big marks there is 1 cm, and each of those is cut into 10 parts of 1 mm.
Tap anywhere on the line.
Worked example
A pencil is measured against a scale whose end has snapped off, so there is no zero. One end of the pencil sits at 2.0 cm and the other at 11.5 cm. How long is it?
Notice what the broken end costs you: the reading at the pencil's start is not 0, so the reading at its far end is not the length.
Reading 11.5 straight off would count the 2 cm of scale sitting before the pencil even begins.
Try it together
Now convert a length down through the units, one step at a time.
A rope is 2 km long. Work down to millimetres, answering with a bare number each time.
1.First to metres. How many metres is 2 km?
Have a go
Have a go on your own. A scale marking reads in inches as well. How many centimetres is 1 inch? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.