When you hear the word work or energy, what comes to mind? Is raking the yard or playing baseball work? Most people would answer that raking the yard is work, but you are actually using energy with both activities. Energy makes everything happen. You live in an ocean of energy. It takes energy to drive a car, throw a football, or play a game. It even takes energy for our bodies to sleep. Just exactly what is energy?

Energy is the ability to do work. Energy comes in many different forms: heat, light, mechanical, electrical, chemical, and nuclear. All these forms of energy are stored in different ways in the energy sources that we use everyday. These sources can be divided into two groups: renewable and nonrenewable energy.

Renewable energy is an energy source that we can use over and over again. These sources can be replenished in a short time. There are five renewable sources that we use most often. They include hydropower (water), solar, wind, geothermal, and biomass. Except for geothermal energy, all these sources ultimately come from the Sun.

A nonrenewable energy source is one that we use up. It cannot be re-created in a short period of time. Millions of years ago, the heat from the Earth's core and pressure from rock and soil on the remains of dead plants and animals (fossils) formed fossil fuels such as oil, natural gas, and coal. These fossil fuels did not form quickly, and once they are gone, we cannot re-create them. Another source of nonrenewable energy comes from uranium, an element in the Earth¿s crust. Scientists split uranium atoms to release energy through a process called nuclear fission.

There are two types of energy: potential (stored) and kinetic (motion) energy. Potential energy is energy that an object has because of its position. For example, if you are standing on the edge of a diving board 1 meter above the water, you have potential energy. Your potential energy is determined by your position above the water. If you were standing on a diving board 3 meters above the water, you would have three times the potential energy you would if the board were 1 meter above the water. Energy that is dependent upon height above a reference point, such as the surface of the Earth, is called gravitational potential energy (GPE).

Weight also affects the amount of GPE an object has. The old saying, "The bigger they are, the harder they fall," is an observation of the effect of weight on GPE. The relationship between GPE, weight, and height can be expressed by the following formula:

From this formula you can see that the greater the weight, the greater the GPE. Also, the higher the position above a surface (height), the greater the GPE.

Kinetic energy, the last type of energy, is energy in motion. Any object that moves has kinetic energy. The faster an object moves, the more kinetic energy it has; consequently, kinetic energy is directly related to the velocity of an object. You have more kinetic energy when you run than when you walk. Kinetic energy is also dependent on the amount of mass in an object. The relationship between kinetic energy, mass, and velocity is