Units | Movement and forces | Middle school physics | Khan Academy

• [Illustrator] Did you know that communication is actually one of the most important things in science? As we discover cool things, we need to be able to share them with others. And when we're talking about data and measurements with other scientists, we need to make sure we're on the same page.

So how do we do that? Well, one of the ways is to use units. We use units whenever we talk about things like position, where an object is located, how long it is, its mass, how much matter it's made up of or its motion. How is that object moving? You probably hear units every day.

For example, you've grown, let's say, an inch and a half in the past year, or that tree over there is 25 feet tall. And maybe you went swimming in a 25 meter pool. And we're just gonna pretend that the pool is a rectangle because, as you can tell from my tree, my artistic skills are not that great.

Anyway, this brings up a super important point about why we use units. I just used three examples of length measurements with three different units: inches, feet, and meters. Imagine if I didn't attach a unit to any of these measurements. You grew one and a half, what? Meters? Whoa, one and a half hands. Well, whose hands? Your hands or my hands? Woof, well, pretend those are hands.

Units let us know how much of a quantity there is. So a meter is always used to measure length, and we know exactly how long a meter is. That way, when we say something is two meters long, no one has to guess at how big that is. Any measurement or data point always needs to have a unit, or else it's just a meaningless number.

To avoid any confusion, in science we use what are called SI units. SI units are the International System. Could there be any more letters in this word system used by scientists all over the world? We'll use meters to describe position or length, kilograms for mass, and if we're talking about the motion of something, meters per second.

And while this is the agreed upon scientific unit system, you should be aware that other systems do exist, which means things can very easily get very confusing if you forget your units. And you might be thinking, "Oh, come on, who mixes up units?" Well, it happens more often than you think; even rocket scientists have done it.

I mean, a Mars Orbiter actually crashed due to a mix up in units. Now seriously, that actually happened, look it up and remember to use your units.