# 2.1 Mass: SI units

Unlike all the other SI base units, the base unit for mass has a prefix. You may have noticed that in Week 1, when you looked at the summary of the seven SI base units.

The base unit for mass in the SI is the kilogram (abbreviated to ‘kg’), not the gram as you may have logically thought. However, the gram can be seen as the base unit from the point of view of adding prefixes to produce the other units.

Adding the prefixes gives the milligram (mg), as well as the kilogram (kg). Note that you could have a centigram as well, but this is not a unit that is commonly used.

Use your knowledge from the previous section on length in this next short activity.

## Activity _unit3.2.1 Activity 1 SI units of mass

- a.What would you need to do to convert from kilograms (kg) to grams (g)?

### Answer

Kilo means a thousand, so kilogram is the same as saying 1000 grams.

Hence to convert from kilograms to grams you should multiply by 1000.

- b.What would you need to do to convert from grams (g) to milligrams (mg)?

### Answer

Milli means a thousandth, so milligram is the same as saying a thousandth of a gram. This means that 1 gram = 1000 milligrams.

Hence, to convert from grams to milligrams you should multiply by 1000.

- c.What would you need to do to convert from grams (g) to kilograms (kg)?

### Answer

From part (a) to convert from kilograms to grams you needed to multiply by 1000, so the opposite must be true to convert from grams to kilograms.

Hence, to convert from grams to kilograms you should divide by 1000.

The findings from this activity can be summarised as:

- kilograms (kg) to grams (g) – multiply by 1000
- grams (g) to kilograms (kg) – divide by 1000
- grams (g) to milligrams (mg) – multiply by 1000
- milligrams (mg) to grams (g) – divide by 1000.

Use this information to help you in the next activity, as well as the steps from Week 1 to convert between different units. These are summarised again here for you:

- Find out how the two units are related to each other – the conversion factor.
- Ask yourself if the final answer should be bigger or smaller than the original value.
- Divide or multiply by the conversion factor to get your answer.

You can click on ‘reveal comment’ if you would like some additional guidance.

## Activity _unit3.2.2 Activity 2 Everyday mass

- a.During a shopping trip you buy 1.5 kg of apples, 450 g of bananas and 0.75 kg of pears. How much fruit in grams have you bought altogether?

### Discussion

Remember that 1 kg = 1000 g.

### Answer

a.As there is a mixture of different units, start by converting all the units into the same one. The final answer should be in grams, so it makes sense to convert all the masses into grams to start with.

So,

- b.One batch of mortar mix required for building a brick wall requires 250 g of cement and 750 g of sand. If you need 5 batches to finish the wall, how much sand and cement will you need and what is the total mass in kilograms?

### Answer

b.

This time you need to divide by 1000 to convert from g to kg

So,

- c.The daily recommended intake of vitamin C in the UK for adults is 0.04 g. Convert this to milligrams, the more usual way to quote this value.

### Answer

c.1 milligram (mg) is a thousandth of a gram.

This tells us that:

So, the

As with length, imperial units for mass are not quite as straightforward as the SI units. So, you may need to take your time over this next section.