# Everyday Physics: Estimating Values for Energy

We live in a world of energy. You store energy in your body by eating food. Your car transforms the energy in gasoline into motion. Even your phone has a battery that stores energy. But what about the actual numbers? How can we compare the numerical values for energy in different cases? Let’s go over some of my favorite calculations.

# What is a Joule?

There are many different units for energy, but the best is the Joule. There’s no question about this — the Joule is the one you should use. In terms of fundamental units, we have the following:

But still, that’s just a number. Here’s an example that can help you relate to the numerical values. Take a textbook and put it on the floor. Now lift the textbook up and put it on a table. It would look like this.

The textbook will probably have a mass somewhere around 1 kilogram and the table is probably about 1 meter above the ground. Since the gravitational field is 9.8 Newtons per kilogram, this gives a total work of something on the order of 10 Joules. Lifting the book required you to use 10 Joules of energy. There — now you know about Joules.