Units
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 ?
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 m^{2}/s^{2}. By no means is that obvious, but it is
similar to measuring area in m^{2} (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.

