Lesson Video: Comparing Masses in Grams or Kilograms Mathematics • 3rd Grade

In this video, we will learn how to use scales to measure the mass of objects in grams or kilograms and compare the masses of two objects.

14:39

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.

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