Sal explains how to get a sense of the size of meters and centimeters and applies it to measuring objects. Created by Sal Khan.
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- [Instructor] In this video, I'm going to talk about a unit of length known as the meter, which you might have heard of before. It's really probably the most used unit of length in the world. And so the natural question is how long is a meter? Well, one way to get a rough sense of a meter is imagine a reasonably tall basketball player. So a good bit taller than your average man. So a good bit taller than me, I'm about average height. So that's your basketball player there. He's got his basketball. He might be roughly two meters tall. So this would be two meters, one meter, and this lowercase m is short for meter. And then two meters. Another way to think about a meter is if you took a long step, if you're, let's say, seven or eight years old, and if you were to take a long step, you might travel about a meter. So that's your long step, right over there. It might be close to a meter, roughly, it's not necessarily going to be exactly a meter, but it might be close. Now a related unit of length is known as the centimeter, and you might already recognize that it has the word meter in it, and it also has the word centi. Now you might already know that the word cent, 100 cents make $1. You might know the word century, that's 100 years. So you might say, is this 100 meters? You'd be kind of close, but kind of far off. It's not 100 meters, it's not bigger than a meter. It's actually a lot smaller than a meter. A centimeter, 100 of them will equal one meter. So one way to think about it is if I write it in shorthand 100 and you normally write it as lowercase c and m. So 100 centimeters are equal to one meter. So on these pictures right over here, a centimeter would be much smaller than a meter right over, even what I just drew there would be a lot less than a centimeter. And one way to think about a centimeter, it might be if you stick your finger out, your finger, depending on how large your finger is, might be about a centimeter wide. As a grown average-sized man, my finger is actually about two centimeters wide. If I take my index finger and if I were to measure it, I would approximate it as about two centimeters. So 100 centimeters make up a meter, which hopefully makes a little bit of sense to you. Now given what we've just learned about meters and centimeters, let's think about what is better to measure with, depending on the size of the thing. So if I was looking at a let's say fairly tall of some kind, what would I wanna measure that with? Would I be more likely to measure that in meters or would I be more likely to measure that in the centimeters? Pause this video and think about that. Well, a fairly tall tree is going to be a lot taller than even this basketball player, so normally, people are likely to measure this in meters. A reasonably tall tree could be 10 meters or 20 meters, or even in certain cases 30 meters or taller. Now what if I were instead measuring the length of a mouse, so my best drawing of a mouse here, draw its little paws, kind of looks like a fish with a tail. But let's say that this is a mouse, would I measure this in meters or centimeters? Well, a mouse is much smaller in length than a tree is tall, so you would likely measure this in centimeters. A small mouse might be maybe five or 10 centimeters in length, right over here. Now let's compare another way or decide another way to think about whether we're measuring in centimeters or meters. Let's say that I went to the local gymnasium, and there's a pool there. There's a pool, that's the water in the pool. And I were to measure the length. And I know it is 25-something, it's either 25 meters or 25 centimeters. Remember, this is the pool at the gym. Pause this video, do you think this would be 25 centimeters or 25 meters? Well, as you can imagine, the length of a pool, this tall basketball player, many of them could lie down and to end to get to this length of this pool. So this would be a 25 meter pool. A 25 centimeter pool, that would actually probably be about the size of a cell phone or maybe smaller or a little bit larger, depending on the cell phone that you're talking about. So for a pool, you'd be talking in terms of meters. What if we had a building of some kind, so this is a building right over here, and let's say it is a four storey building. And someone were to say that it is 15 something tall, would this be 15 meters tall or 15 centimeters tall? Well, 15 centimeters, that might be a Lego building. A real building would be 15 meters in this scenario. Now what if I were to say the length of my thumb, the length of my thumb, right over here. And I know it is five-something, this length right over here, it is five-something, it's either five meters or five centimeters, which one do you think it is? Well, if it was five meters, I would be a very, very, very large giant. My thumb would be twice as tall or more than twice as tall as a fairly tall basketball player. So this would be five centimeters right over here. So hopefully that gives you a sense of meters and centimeters. And on Khan Academy, there's a lot more practice. So you can get a little bit more sense of estimating with these, or thinking about when one unit or the other is more appropriate.