Video: Ordering the Masses of Three Objects by Using Scales

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

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Video Transcript

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.

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