# Building an energy description

You build an energy description assuming that energy is conserved, but also that it's often a tricky quantity to calculate.

### taking care, this is a long way from the lived-in world

Be guided by the reason for building your description. What's the question you want to answer? As energy only tells you what cannot happen, this limits the kind of questions you can answer.

### Four kinds of questions

Energy descriptions are good for figuring out four kinds of things.

### Could it happen?

Possible or not? The description sets up calculations so you can decide.

### It happens: calculate a value

The description sets up a calculation so you can figure out the value of a quantity.

### What are the possible ways of doing it?

The description sets up calculations so you can explore trade-offs.

### How well did we do it?

The description sets up calculations to find how much of the energy ends up where you intended. That's called the efficiency.