## Units

This brings up the issue of units. In your math class, numbers by themselves are fine to work with, but in science a number without units is pretty useless. Try this problem: a train of length 2, weighing 200, travels from Denver to Santa Fe at 15. How long did the trip take?

What? 2 kilometers, 200 tons, 15 miles per hour?

That's right, we have no clue unless we put units on those numbers.

Okay, but what in the world is ?

 The way you build a unit system is to have a fundamental set of units - things like length, mass, time, and temperature, to name a few. In the standard system of units used by scientists, we measure those quantities in meters [m], kilograms [kg], seconds [s], and Kelvin [K] respectively.

I see that the s stands for seconds, but where did that J come from, Dr. Mahan?

That J stands for Joule, in honor of James Joule who examined the relationship between heat and energy in the 19th Century. The Joule is the SI unit for measuring energy. A joule is related to the fundamental units in that a joule is 1 kg m2/s2. By no means is that obvious, but it is similar to measuring area in m2 (distance times distance) or speed in m/s (distance divided by time).

So is a Joule second. What would that be? That's not quite energy, not quite time.

The fancy name for what a Joule represents is angular momentum, but all that you need to know now is that it involves things that spin, like bicycle wheels and tops.

Weird. And that has something to do with photons?

Yep, there's a lot we haven't told you about light.

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