Video: Using Scales to Calculate Mass in Kilograms

Emma weighed a pumpkin and a watermelon. Figure out the mass of the pumpkin.

03:06

Video Transcript

Emma weighed a pumpkin and a watermelon. Figure out the mass of the pumpkin.

This is a really interesting question. We need to figure out or work out the mass of a pumpkin. And we’re given some pictures to help us. The only trouble is none of the pictures show that Emma has weighed a pumpkin on its own. She’s used this set of balancing scales to weigh a pumpkin and a watermelon together. And then, she’s just used a different set of scales to weigh the mass of a watermelon on its own. We need to use these two pieces of information to work out the mass of what the pumpkin weighs on its own.

First of all, let’s look at the pictures and work out what each set of scales is showing. Once we’ve done that, we can think about how we can work out the answer. Before we start, let’s make sure that we’re measuring in the same unit of measurement. Can you see the letters kg on both sets of scales? We know that kg is short for kilograms. And kilograms are a unit of measurement that we can use to measure the mass of heavier objects like pumpkins or watermelons.

Let’s look at our first set of scales. To begin with, we can see that they’re balanced. In other words, one end isn’t heavier than the other. They both weigh the same. It’s like being on a seesaw where someone on the other end is exactly the same mass as you. So we can use this set of scales to work out how heavy a pumpkin and a watermelon together are. Let’s count how many kilograms there are. We can see three weights on the bottom and two on the top. So that’s five kilogram weights altogether. The pumpkin and the watermelon weigh five kilograms together.

Now let’s read our second set of scales. These have got a dial on the front of them. And there’s a needle that moves. Can you see which number it’s pointing to? By putting the watermelon on these scales, it’s made the needle point to the number two. So we now know how heavy the watermelon is on its own. It’s two kilograms. Now, it’s time to use these two pieces of information to help work out the answer. Let’s use a part–whole model to help us.

We know that the total mass of a pumpkin and a watermelon is five kilograms. And we also know that the mass of a watermelon on its own is two kilograms. Can you see how we can work out the answer? We just need to think what goes with two to make five. We know that five take away two equals three. We didn’t need to weigh the pumpkin on its own. We used the information on both sets of scales to help us. The mass of a pumpkin on its own is three kilograms.

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