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.
