3.2 Weighing things
It’s useful to have an idea of how much things weigh. It can help you to work out the weight of fruit or vegetables to buy in a market, for example, or whether your suitcase will be within the weight limit for a flight.
Try estimating the weight of something before you weigh it. It will help you to get used to measures of weight.
Hint: Remember to use appropriate units. Give the weight of small things in grams and of heavy things in kilograms.
Remember that:
- 1 g is approximately the weight of a paperclip.
- 1 kg is the weight of a bag of sugar.
- 1 kg = 1 000 g
Take a look at the example below before having a go at the activity.
Example: Weighing an apple
- Which metric unit would you use to weigh an apple?
- Estimate how much an apple weighs and then weigh one.
- How much would 20 of these apples weigh? Would you use the same units?
Method
- An apple is quite small, so it should be weighed in grams.
How much did you estimate that an apple weighs? A reasonable estimate would be 100 g.
When we weighed an apple, it was 130 g.
Twenty apples would weigh:
- 130 × 20 = 2 600 g
This answer could also be expressed in kilograms. To convert from grams to kilograms, you need to divide the figure in grams by 1 000 (1 kg = 1 000 g) . So the weight of the apples in kilograms is:
- 2 600 g ÷ 1 000 = 2.6 kg
We will look more at converting metric units of weight in the next section.
Activity 12: Weighing things
- How much do ten teabags weigh? Estimate and then weigh them.
- How heavy is a bottle of sauce? How much would a case of 10 bottles weigh?
Hint: The weight shown on the label is the weight of the sauce – it doesn’t include the weight of the bottle or jar that the sauce comes in. So for an accurate measurement, you need to weigh the bottle rather than read the label!
- How heavy is a book?
Discussion
Our suggestions are shown in the table below. Your estimates and measured weights might be different, but they should be roughly similar.
| Item | Estimated weight | Actual weight |
|---|---|---|
| Ten teabags | 25 g | 30 g |
| Bottle of sauce | 500 g | 450 g |
| Book | 900 g | 720 g |
A case of ten bottles of sauce would weigh:
- 450 × 10 = 4 500 g
As previously noted, 1 000 g = 1 kg, so 4 500 g = 4.5 kg, which is how you would more usually express this weight.
If your book weighed more than ours, you might have given its weight in kilograms. If you chose a small book, it may have weighed a lot less.