The SI units are standard units that all scientists use around the world. It allows scientists to be able communicate their results without any miscommunications.

The standard units are:

Units, figure 1

All other units are made up from these standard units.

Order of Magnitude

We can put prefixes in front of a unit in order to make the number easier to write.

For example;

If we had a value of 1000 m we can write is as 1 km. The ‘k’ is a multiple of 1000.

Here is a table of all the ones we need to know:

Units, figure 1

Unit Conversions

You need to able to convert between different units with ease. It is easy to be caught out or lose marks if you can’t do this correctly.

In all the equations we will come across, the number that we use must be in the same unit as the SI unit. This means that if we have a distance of 5km, we must convert this to be 5000m.

Time is a tricky one as it does not follow the table above. This is because we have multiples of 60 and 24, rather than factors of a 1000.

If we need to convert 10mins into seconds; we do

10 x 60 = 600s

This is because there are 60s in one minute, but we want to know 10mins! In which case it must be 60 x 10.

If we have to convert 3 hours into seconds then:

3 x 60 x 60 = 10,800s

There are 60mins in 1 hour, so in 3 hours there are 3 x 60 mins. For seconds we then multiply this by another 60 as there are 60s in one minute.

Standard Form

Standard Form is really useful for writing out really large and small numbers.

It looks like this 4 x 105 or this 4 x 10-5

All you do is move the decimal point the number of times as the number is multiplied by 10.

  • If the number is positive, move the decimal point to the right (bigger number)
  • If the number is negative, move the decimal to the left (smaller number)

Examples:Units, figure 1