Video: Indirectly Comparing the Masses of Two Objects by Comparing Them Both to a Third Object

Which item is lightest?

01:59

Video Transcript

Which item is lightest, the bottle of milk or the apple?

In our question, we’re asked to find the item that’s lightest, in other words, the object that weighs the least. If we had a set of scales and we put the bottle of milk on one end and the apple on the other, we’d see straight away which item was lightest. But in this question, we’re not comparing these objects directly. Instead, we’re using a third object to help. Can you see what it is? It’s this pot of strawberry jam. By comparing both objects with the pot of jam, we should be able to work out which one’s the lightest.

Let’s look at the pictures. In the first picture, we can see that we’re comparing the bottle of milk with a pot of jam. The bottle of milk has pushed that end of the scales right down, hasn’t it? And this has lifted the pot of jam in the air. So, what can we say about the bottle of milk? The bottle of milk is heavier than the pot of jam. Now, let’s look at our second picture. This time, we’re comparing the apple with the pot of jam. And this time, it’s the end with the pot of jam on it that’s weighed down, and it’s the end with the apple on that’s moved upwards. So, what can we say about the apple? The apple is lighter than the pot of jam, isn’t it?

We’ve compared both our objects with a pot of jam. And we can use the information we’ve got to work out which item is lightest. We know that the bottle of milk is heavier than the pot of jam, but the apple is lighter than the pot of jam. So, we know that the apple must be lighter than the bottle of milk. The lightest item is the apple.

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