Video Transcript
Comparing Masses in Grams or
Kilograms
In this video, we’re going to learn
how to use scales to measure the mass of objects in grams or kilograms. And once we’ve done that, we’re
then going to compare the masses of two objects.
Let’s imagine you’re in a shop and
you see two chocolate bars. When you look more closely, you can
see that one of the chocolate bars has got a label by it. It says that it’s the lightest,
fluffiest chocolate ever. And your mouth starts to water, and
you start to think that sounds delicious. But I wonder whether the label that
they’ve put next to that bar is true. When I ate one of those pink bars
the other day, that was light. How could we compare the mass of
these two objects?
If we have a set of weighing scales
like this, then we could just put one bar of chocolate on its each end and see which
one is lightest by the way that the scales move. But in this video, we’re going to
do something slightly different. We’re going to compare the masses
of these objects by weighing each one separately and then comparing the masses. So to begin with, we need to make a
choice. Are we going to measure the mass of
these chocolate bars in grams or kilograms? One kilogram weighs about the same
as a bag of rice. And so we would normally choose
grams to measure lighter objects like these chocolate bars.
To give you an idea, one gram
weighs about the same as a paper clip. But if we were going to weigh these
chocolate bars in paper clips, it might take a while. Instead, let’s use one of
these. It’s a plastic weight that weighs
10 grams. We’ll start by putting our first
chocolate bar on the weighing scales. Can you see how this end has gone
down? This is because it’s got the
chocolate bar on it. And the other end has nothing. We need to start putting 10-gram
weights on the other end until the scales balance.
Let’s count in 10s as we put each
plastic weight on. 10 grams — there’s been no change
in our weighing scales. So we know the green chocolate bar
must weigh more than 10 grams. Let’s add another plastic
weight. 20 grams — still no change — 30
grams, 40 grams. Look what’s happened to our
weighing scales! They’re now balanced. This means that the mass of the
chocolate bar is exactly the same as the mass of the plastic weights on the other
end. It’s a little bit like a seesaw
when the two of you on either end are exactly the same mass.
So now, we can tell how heavy the
green chocolate bar is. We’ve weighed it on a set of scales
and found that it weighs 40 grams. In order to compare these two
masses, we now need to do exactly the same thing with the pink chocolate bar. Let’s start putting our 10-gram
weights on. 10, 20, 30 grams. And our scales are balanced. The pink chocolate bar weighs 30
grams.
And now that we’ve weighed both of
these chocolate bars, we can compare the masses. We know that if we were to mark
these two numbers on a number line, the number 40 would be further to the right of
the number 30.
And we know this: as we read a
number line from left to right, the numbers get larger, don’t they? 40 is larger than 30. And so we can say 40 grams is
heavier than 30 grams. Or in other words, 30 grams is
lighter than 40 grams. And we need to march into the shop
and talk to the man behind the tale because that sign that he’s got up is not
correct.
Did you notice, by the way, how
easy it was to compare these masses? We just looked to the numbers,
didn’t we? But the reason we can do this is
because we’ve measured both masses in the same unit of measurement. Because we weighed them both using
grams, we can see straightaway 40 grams is heavier than 30 grams.
And it’s just as easy to compare
mass if we’ve used kilograms too. These two bowling balls are quite
heavy, wouldn’t make sense to measure their mass in grams. So let’s compare them using
kilograms. And we’re going to use a different
sort of weighing scales here. We can see that these weighing
scales have got a dial on them, which is labeled kg for kilograms. And there’s also a red needle which
hopefully is going to move around the dial to point to the number of kilograms that
each bowling ball weighs.
Let’s weigh the orange bowling ball
first. The needle is pointing to the
number four. The orange bowling ball weighs four
kilograms. Now, we can do the same with the
blue bowling ball. This time, the needles move further
around the scale. It’s pointing to the number
six. The blue bowling ball weighs six
kilograms. Now, again, because we’ve weighed
both of our objects in the same unit of measurement — in other words, we’ve used
kilograms both times — it’s really quick to compare these masses. We don’t need to draw a number line
to know that four is less than six. And so we can say straightaway that
four kilograms is lighter than six kilograms.
Let’s put into practice what we’ve
learned now. We’re going to answer some
questions where we have to compare the mass of two objects in grams and
kilograms.
Fill in the blank: The mass of the
box of books is what kilograms heavier than the mass of the box of toys. And then we’re given two sentences
to help us. The mass of the box of books is 31
kilograms. The mass of the box of toys is 25
kilograms.
In this question, we’re shown two
boxes. And we don’t need to weigh them to
find out how heavy they are because we’re told that the mass of the box of books is
31 kilograms and the mass of the box of toys is 25 kilograms. Now, we know that the number 31 is
a larger number than 25. And because both masses are
measured in kilograms, all we have to do to compare them is to just look at these
numbers. We know that the mass of the box of
books then is heavier than the mass of the box of toys. It weighs more. 31 kilograms is heavier than 25
kilograms.
Now, we’re given a sentence that we
need to complete. The mass of the box of books is
what kilograms heavier than the mass of the box of toys. In other words, we know that the
books weigh more. But how much more? Let’s count backwards from 31
kilograms to 25 kilograms to see. We could make a jump of one to go
from 31 back to 30. And then we know to go from 30 to
25 is a jump of five. So we’ve made a jump of one and
then a jump of five. How many kilograms is that? It’s six, isn’t it? The difference between 25 kilograms
and 31 kilograms is six kilograms. And so we can say the mass of the
box of books is six kilograms heavier than the mass of the box of toys. The missing number is six.
Isabella weighed these boxes of
fruit. Finish these sentences to describe
the masses of the boxes. The mass of the box of strawberries
is what kilograms. The box of what is the
heaviest. And the box of bananas is what
kilograms lighter than the box of oranges.
This question is all about
comparing the mass of some boxes of fruit. And we don’t need to do any
weighing to do this because Isabella has done it for us. We can see that she’s weighed a box
of bananas, a box of strawberries, and a box of oranges. And each of her sets of scales are
balanced. So this means we just need to count
how many kilograms she’s put on the other end. And then we’ll know how heavy each
box is.
How heavy is the box of
bananas? This large weight on the end here
is labeled five kg. We know this stands for five
kilograms. Then we have one labeled two
kilograms and two more that are labeled one kilogram. Five and two make seven. And then if we add the other two
ones, we have a total of nine. The box of bananas weighs nine
kilograms altogether. Now, let’s look at the box of
strawberries. If we look at the weights that are
labeled again, we can see one that’s labeled with a two on it. And there’s a two kilograms on the
top, another two kilograms here, and a one-kilogram weight on the end.
If we count in twos three times, we
get a total of six. Two, four, six. And then if we add the one
kilogram, this is seven kilograms altogether. Finally, if we look at the box of
oranges, we have a five-kilogram weight and another five-kilogram weight and, on the
top, we have a one-kilogram weight. Five and five make 10, and if we
add one more, that’s 11. The box of oranges weighs 11
kilograms.
Did you notice that each of these
masses is in kilograms? And because we’ve weighed these
objects in the same unit of measurement, all we have to do to compare them is to
look at the numbers. Let’s go through the sentences now
and see if we can complete them. The mass of the box of strawberries
is what kilograms. We’ve already worked that out,
haven’t we? Three lots of two and then one more
is seven kilograms. The box of what is the
heaviest. As we’ve just said, because
everything is measured in kilograms, we just need to compare the numbers. Which is the largest number out of
nine, seven, or 11? It’s 11, isn’t it? So 11 kilograms weighs most, and we
can say the box of oranges is the heaviest.
Finally, the box of bananas is what
kilograms lighter than the box of oranges. Well, we know that the box of
bananas weighs nine kilograms. And the box of oranges weighs 11
kilograms. What’s the difference between these
two masses? Nine, 10, 11. That’s a jump of only two
kilograms, isn’t it? So we can say that the box of
bananas is two kilograms lighter than the box of oranges. We’ve used the pictures of the
weighing scales to compare the masses of these three boxes of fruit. And we found that the mass of the
box of strawberries is seven kilograms. The box of oranges is the
heaviest. And the box of bananas is two
kilograms lighter than the box of oranges.
The can of pineapple is what grams
heavier than the can of corn.
We’re given two pictures here that
can help us find the mass of two different cans of food. We’ve got a can of pineapple and
underneath it a can of corn. Now, it might be quite hard to read
the dials on these scales. But we can see a magnified version
next to them. And we can read these to find out
how heavy each can is. Let’s start with the can of
pineapple.
Can you see where the needle is
pointing? It’s here, isn’t it? Let’s read the scale. It begins at zero. Then we can see 100 grams is
labeled, followed by 200 grams, then 300 grams, and 400 grams. We can stop there because we can
see that the needle is pointing in between the numbers 300 and 400 on the scale. In fact, it’s halfway between,
isn’t it?
Now, do you remember what number is
halfway in between 300 and 400? It’s 350. Although we can’t see the number
350 on the scale, we’ve worked out where it is. And so we know that the mass of the
can of pineapple is 350 grams. And remember, this question is all
about comparing these two masses. So we’re going to have to look at
the second scales. It’s the same set of scales, so we
can see the numbers 100, 200, 300 grams, and so on. But where’s the needle pointing to
this time? We can see that it’s in between the
numbers 200 and 300. And halfway in between the numbers
200 and 300 is 250. The mass of the can of corn is 250
grams.
Now that we’ve measured both
masses, we can compare them. The can of pineapple is what grams
heavier than the can of corn. We know that it’s heavier because
350 is larger than 250. So we know that the can of
pineapple weighs more. But how much more? If we look at the ones digit in our
numbers, we can see that it doesn’t change if we compare them. And they both contain five tens
too, so there’s no change there. But if we compare the hundreds
digit, we can see that there’s a difference of one. 350 is 100 more than 250. And so we can say the can of
pineapple is 100 grams heavier than the can of corn. The missing number is 100.
What have we learned in this
video? We’ve learned how to use scales to
measure the mass of objects in grams or kilograms. We’ve also learned how to compare
the masses of two objects using these measurements.
