In this topic you will learn about different kinds of energy and how they can be used to do work.

The height of an object above the ground gives it energy that is stored in its position. This type of energy is called potential energy. Any time an object is raised above the ground, the potential energy it gains is called gravitational potential energy. The greater the object's height and weight, the more gravitational potential energy it gains. You can calculate the amount of gravitational potential energy in an object by multiplying its weight by its height. If the weight is measured in newtons and the height is measured in meters, the gravitational potential energy is measured in units of joules.

Potential energy can also be stored in an elastic (springy) object that is stretched or squashed and held that way. When you release the object, its stored energy turns into motion as it springs back to its old shape.

A moving object has energy due to its motion. This energy is called kinetic energy. The kinetic energy of a moving object depends on its speed and its mass-the greater the mass or the higher the speed, the greater the kinetic energy.

The food we eat provides an example of energy changes. Food contains potential energy stored in various kinds of chemicals. Our bodies change the food energy into other forms such as kinetic energy for moving about, electrical energy for our nerves to carry signals, or thermal (heat) energy to keep our bodies warm.

Energy can take many forms. Atomic energy is found in radioactive material. Foods have chemical energy. Electrical energy powers household appliances. Light energy reaches us from the Sun. The moving parts of a bicycle are examples of mechanical energy. The vibrations of a stereo speaker are examples of sound energy. Heat (or thermal) energy may be associated with hot water in a radiator. Under the proper conditions, any of these forms of energy can be changed from one form to another. The chemical energy of a fuel cell can be converted to electrical energy.

Scientists define work as applying a force to an object to move it through a distance. The amount of work is calculated by multiplying the distance by the force. If the force is expressed in newtons and the distance is expressed in meters, the resulting units for the work are joules. Work is done on an object when energy is added to it. Work done is equal to the kinetic energy used.

Scientists believe that the total amount of energy in the universe remains constant. Energy may change in form, but it cannot be created or destroyed. The requirement that the total energy of the universe remain constant is called the law of conservation of energy. This law also applies to systems smaller than the universe as long as no energy is allowed to enter or leave the system.