### Video Transcript

Comparing Masses Indirectly

In this video, we’re going to
compare how heavy two objects are. We’re going to do this by comparing
their masses to a third object. Let’s imagine you’re out shopping
for a new mobile phone, and there are two phones that you like the look of. There’s this black one here with a
big screen or this brightly colored red one with the buttons. Now, let’s imagine one of the main
things that’s gonna help you decide which one of these phones to buy is not the
price or what it can do, but how heavy it is. You want it to feel light in your
pocket. And so, you ask yourself, “Which
phone is the lightest?” Which one weighs less?

Now, of course, the quickest way to
answer this question will be to get a set of balancing scales like this and put one
phone on either end. And we’d be able to tell, just by
looking at how the scales move, which phone is the lightest. But let’s imagine that we can’t
compare the phones directly like this. Maybe they’re in different
shops. Whatever the reason, we’re going to
need to compare them, but not side by side. Is there a way we could work out
which phone is lightest without putting them on the same set of scales? Well, there is, and it involves
using a third object. Let’s use this mug.

Now, if we compare the mass of both
mobile phones one by one with the mug, it might help us see which one is
lightest. Let’s see if it helps. Here is a set of scales. And to begin with, we’re going to
compare the black mobile phone and the mug. Oh! Can you see what’s happened
here? The end with the mobile phone on it
has gone down. And this is meant that the end with
the mug on it has gone up. Now, we can look at these scales
and in a way we can read them. We can tell something about both
objects. We can see that the black phone is
heavier than the mug.

Now, let’s compare the red phone
with the mug. Something different happened this
time. This time, the end with the mug on
it has gone down, and this has made the end with the red mobile phone on it go up in
the air. Again, we can look at this set of
scales, and we can read it. What does the picture tell us? It tells us that the red phone is
lighter than the mug. We’ve compared both mobile phones
to a third object, and now we can see which phone is lightest. It’s the red one, isn’t it? The black phone is heavier than the
mug, but the red phone is lighter than the mug. And so, we can say that the red
phone is lighter than the black phone.

Another way we could compare the
masses of two objects without putting them directly on the same set of weighing
scales is by using units of measurement to help us. Let’s compare this blue mobile
phone with the black one. But instead of using a third object
to help, let’s use these plastic blocks. The blue phone weighs the same as
three plastic blocks. We know this because the scales are
balanced. But the black phone only weighs the
same as two plastic box. We know that two blocks are going
to be lighter than the three blocks, and so the black phone is lighter than the blue
phone. Again, we’ve managed to work out
which phone is lighter without having to compare them directly.

Let’s practice now what we’ve
learned, and we’re going to answer some questions where we need to compare the
masses of objects.

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.

Which item is heaviest? Is it the watermelon or the pot of
jam?

In this question, we need to spot
the heaviest item. We know that the word “heaviest”
means that it weighs the most. And If we had a set of balancing
scales like these and we put a watermelon on one end and that pot of jam on the
other, we’d be able to see straight away which item was heaviest. But in this question, we’re not
comparing the masses of these two objects directly like this, even though it’d be
easier. We’re comparing both objects with a
third object. Can you spot it? It’s a bottle of milk. First, we’re going to compare a pot
of jam with the bottle of milk. And then, we’re going to compare
the watermelon with the bottle of milk. And we’re going to use this to help
us work out which is heaviest out of the watermelon and the pot of jam.

So what can we see in the first
picture? Well, in this picture, we’re
comparing the pot of jam with the bottle of milk. And we can see that the end of the
scales that has the bottle of milk on it has gone down. This has meant that the end with
the pot of jam on it has gone up in the air. And this tells us something about
how heavy these items are, doesn’t it? What can we say about the pot of
jam? The pot of jam is lighter than the
bottle of milk.

On the second set of scales, we’re
comparing the watermelon with the bottle of milk. This time, it’s the end with the
watermelon on that’s gone down, and this has lifted the end with the bottle of milk
on it up in the air. What can we say about the
watermelon then? The watermelon is heavier than the
bottle of milk. And we can use what we’ve found to
compare our two objects. Because we know the pot of jam is
lighter than the bottle of milk but the watermelon is heavier than the bottle of
milk, we can tell which item is heaviest. It’s the watermelon. The watermelon must be
heaviest.

Order the animals on the scales, a
duck, a penguin, and a rabbit, from lightest to heaviest.

And if we look at the bottom of the
question, we can see that we have three possible answers to choose from. In this question, we’re shown some
animals on some sets of weighing scales. There’s a duck, which appears
twice. Then, we have a penguin and a
rabbit. And although we don’t weigh all
three objects at the same time, we’ve got enough information here for us to be able
to put these animals in order from lightest to heaviest.

Let’s draw a line to help us. Because we’ve been asked to go from
lightest to heaviest, let’s put the word lightest on the left and heaviest on the
right. Now, the idea behind this line is
it is going to work a little bit like a number line. But instead of putting numbers in
order, we’re going to be putting these animals in order. Let’s look at our first set of
weighing scales. What can we see? We can see the end with the penguin
on it has gone down, and this has pushed the end with a duck on it upwards. Although we’ve only compared two
animals so far, we can tell which one’s heavier, can’t we? The penguin belongs somewhere at
this end of our comparing line because it’s heavier than the duck. And we’ll put the duck at this end
because it’s lighter than the penguin.

Now, let’s look at our second set
of scales. On these scales, we’re comparing
the rabbit and the duck. This time, it’s the end with the
duck on that’s moved downwards, and the end with the rabbit on is lifted up. We can say then that the duck is
heavier than the rabbit. And if we look at our line and we
know that the duck is heavier than the rabbit, we’re going to have to draw the
rabbit to the left of the duck towards the lighter end of our line, aren’t we? The rabbit is lighter than the
duck, and the duck is lighter than the penguin.

Now that we’ve put our animals in
order from lightest to heaviest, can you spot the correct order in the answers? It’s this one. We found that the penguin was
heavier than the duck, but that the duck was heavier than the rabbit. And so, if we order the animals
from lightest to heaviest, the correct order is the rabbit, then the duck, then the
penguin.

Here are three objects. The feather is lightest, and the
car is heaviest. Which scales represent the correct
masses?

In this question, we need to
compare the masses of three objects. There’s a ball, a feather, and this
toy car here. Now, we don’t need to weigh these
objects to be able to put them in order of mass. We’re given some clues to help
us. Firstly, we’re told that the
feather is lightest. We could draw a line to help us
here. At this end, we can put the objects
that are lightest and at the other end the objects that are heaviest. So, our line goes from lightest to
heaviest. Now, we’re told that the feather is
lightest, so let’s sketch our feather in the correct place. It belongs here, doesn’t it?

The next piece of information we’re
given is that the car is heaviest. Let’s sketch the car on our line
then. It belongs at this end. Now because we know the feather is
lightest and the car is heaviest, we know where to draw the ball too. We know that the ball weighs
somewhere in between. It must be heavier than the feather
but also lighter than the car. Now, we know what order to put our
objects in according to their mass. We can look at the sets of scales
that we’re shown. We’re asked which scales represent
the correct masses. Sketching that line was useful,
wasn’t it? So, let’s do the same as we compare
these scales.

On the very first set of scales, we
can see that the end that has the toy car on it has gone down and the end with the
ball on it has lifted up. This tells us that the toy car is
heavier than the ball. So wherever we draw the ball on our
line, we need to draw the car to the right of it. It’s heavier than the ball. The second set of scales compares
the ball and the feather. And we can see that the end that
has the ball on it has gone down and this has lifted the end with the feather up in
the air. We can say that the feather is
lighter than the ball. This means that we need to draw the
feather to the left of the ball, don’t we? We’ve used our first picture to put
the objects in order.

Now, let’s look at our second
picture. On the first set of scales, we’re
comparing the ball with the feather. And the end with the feather on has
gone down, and this has lifted the end with the ball on up in the air. We can see in this picture the
feather is heavier than the ball. So wherever we draw the ball on our
line, we need to draw the feather to the right of it. In this picture, it’s heavier. The second set of scales compares
the ball with the car. This time, it’s the end with the
ball on that’s gone down, and the end with the car has been lifted up. We can say then that the ball is
heavier than the car. Or If we say it in a different way,
the car is lighter than the ball. So, we need to draw the car now to
the left of the ball.

We were told to begin with that the
feather was lightest and the car was heaviest. And we used these two facts to put
the objects in order from lightest to heaviest: feather, ball, car. By looking at all the sets of
scales, we can see that only one picture represents the correct masses. Because the car is heavier than the
ball and the feather is lighter than the ball, if we put the objects on these scales
in order, it matches the facts we already know. The feather is lightest, and the
car is heaviest.

What have we learned in this
video? We’ve learned how to compare the
masses of two objects by using scales to compare them to a third object.