**Energy** is the final chapter in our terminology saga. We'll need everything we've learned up to this point to explain energy.

If power is like the strength of a weightlifter, energy is like his endurance. Energy is a *measure of how long we can sustain the output of power*, or how much work we can do. Power is the rate at which we do the work. One common unit of energy is the kilowatt-hour (kWh). You learned in the last section that a kW is a unit of power. If we are using one kW of power, a kWh of energy will last one hour. If we use 10 kW of power, we will use up the kWh in just six minutes.

There are two kinds of energy: **potential** and **kinetic**.

### Potential Energy

**Potential energy** is *waiting to be converted into power*. Gasoline in a fuel tank, food in your stomach, a compressed spring, and a weight hanging from a tree are all examples of potential energy.

The human body is a type of energy-conversion device. It converts food into power, which can be used to do work. A car engine converts gasoline into power, which can also be used to do work. A pendulum clock is a device that uses the energy stored in hanging weights to do work.

When you lift an object higher, it gains potential energy. The higher you lift it, and the heavier it is, the more energy it gains. For example, if you lift a bowling ball 1 inch, and drop it on the roof of your car, it won't do much damage (please, don't try this). But if you lift the ball 100 feet and drop it on your car, it will put a huge dent in the roof. The same ball dropped from a greater height has much more energy. So, by increasing the height of an object, you increase its potential energy.

Let's go back to our experiment in which we ran up the stairs and found out how much power we used. There is another way to look at how we calculated our power: We calculated how much potential energy our body gained when we raised it up to a certain height. This amount of energy was the work we did by running up the stairs (force * distance, or our weight * the height of the stairs). We then calculated how long it took to do this work, and that's how we found out the power. Remember that power is the rate at which we do work.

The formula to calculate the potential energy (PE) you gain when you increase your height is:

**PE = Force * Distance**

In this case, the force is equal to your weight, which is your mass (m) * the acceleration of gravity (g), and the distance is equal to your height (h) change. So the formula can be written:

**PE = mgh**