# SI units

True or false? SI units are derived from each other, and they are connected.

## SI units

I'm invited to a party. I need a suit. No problem. I'll start by measuring your height. You are -- five feet tall.

Hmm -- No -- you are six and a half feet tall. - Five! - Six and a half! A long time ago, people used their bodies to measure. They measured length in inches -- Cubits -- And feet. But it was impractical. People are different sizes, and it was difficult to know how long a cubit actually was.

They tried to agree on the length of a cubit, and made sticks for measuring. But the sticks were often different from one town to another, from one country to another. It was still an impractical system. They needed a common international standard. In the 18th century, some Frenchmen had an idea.

They took the distance from the north pole to the equator. Then they divided that distance by ten million, and called the result one metre. Still today, that is the basic unit for length. One meter. When we measure area, we count the number of metres, times metres, or square metres.

Volume is measured in metres, times metres, times metres. Cubic meters. This standard for measuring is called the SI system, from the French Système International. International system. -- The units are called SI units. There are several more SI units.

When the the basic unit for mass was defined, they started with the unit for volume. They filled a cubic decimeter with water, and decided that the water in that container had the mass one kilogram. That is the basic unit for mass. Kilogram. A third basic unit, that you are familiar with, is used to measure time.

Seconds. Speed equals distance divided by time. Hence the SI unit for speed is the same as the SI unit for distance: meters... ... divided by the SI unit for time: seconds. Meters per second.

We say that the SI-unit for speed is derived from the basic units for distance and time. And this is the handy thing about SI units: They are derived from each other - they are connected. If you calculate with values measured in the correct SI units, the result is an answer in the correct SI units. So SI units are practical when calculating. Therefore, they are always used in a scientific context, like when studying Physics and Chemistry.

In everyday life, though, it can vary. If you drive a car, the velocity is often measured in kilometers per hour, but that is not an SI unit. So in the physics lab, you have to measure velocity in the SI unit meters per second. Get it? You're better off using this one instead.

Hmm -- you are -- one meter and -- Seventy centimetres.