3.1 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.
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?
- 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
Remember the metric conversion diagram? To convert from grams to kilograms, you need to divide the figure in grams by 1,000. So the weight of the apples in kilograms is:
- 2,600 g ÷ 1,000 = 2.6 kg
Now try the following activity. Remember to check your answers once you have completed the questions.
Activity 7: 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?
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.