# Question Video: Identifying a Measuring Instrument for Measuring Length Physics

Which of the following items can be used to measure lengths? [A] A protractor [B] A thermometer [C] A stopwatch [D] A balance [E] A ruler

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### Video Transcript

Which of the following items can be used to measure lengths? A) A protractor, B) A thermometer, C) A stopwatch, D) A balance, E) A ruler.

Now let’s go through each one of these items one by one and see what they actually are. So first of all, what’s a protractor? Well, a protractor is a tractor that’s so good at being a tractor that is managed to make money from it. It’s a professional tractor. Oh wait, no! That’s not right. No, a protractor is a semicircular device used to measure angles. For example, the angle between these two pink arrows can be measured using a protractor. And hence, a protractor is not used to measure lengths. Therefore, option A is not the correct answer to our question.

Option B then, a thermometer. Well, a thermometer is a device that’s used to measure temperature. It’s a thermometer. Hence, this is also not the answer to our question. Let’s look at option C, a stopwatch. A stopwatch is a device that’s used for timing things. For example, we can use a stopwatch to time how long it takes you to run 100 meters. Or how long it takes for a pendulum to oscillate back and forth as it’s swinging. Or how long it takes your friend to realise that it’s a video when they thought you were taking a selfie. So a stopwatch is not the answer that we’re looking for either.

Option D then, a balance. A balance can come in many different forms. But the way that we’ve drawn it here is the old-style mass balance. How this one works is that you place an object with a known mass on one side and the object whose mass you’re trying to measure on the other side. If the two balance exactly, then you know that the orange object has the same mass as the blue object. If they don’t, then you can add or take away orange objects, that is, objects with known mass, until both sides of the balance balance perfectly. Then, you know exactly what the mass of the blue object is. Nowadays of course, we can get digital balances as well. But they do the same thing. They measure the mass of an object. And hence, they are not used for measuring lengths, which means that the final answer must be the correct one, E, a ruler.

So how do we use a ruler to measure lengths? Surely, they’re used for running countries or something like that. Oh whoops! The kind of ruler that we’re talking about is usually a rectangular piece of plastic or wood, with markings at regular intervals along its length, so that the straight edge with the markings on it can be used to measure the distance between two points. Let’s say, for example, this point and this point here. We can work out what the distances between these two using the ruler.

And hence, out of options A to E, a ruler is used to measure lengths.
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