What is Energy?

What do all of the following have in common?

A stretched bungee rope.     A hot cup of tea.     A tank of petrol.     A man on a cliff.     A spinning flywheel.

At first sight they don't seem to have a great deal in common. We know that they all posses this thing we call energy but the next question is then obvious, what exactly is energy?

One thing they do have in common is the ability to do work. (For the moment lets just imagine that work is lifting a heavy box onto a shelf.)

This gives us a definition of energy. Energy is the ability, either directly or indirectly, to do work.

Energy, like work is measured in Joules (J) named after the English physicist James Prescott Joule. It is interesting to note that nearly all energy on Earth originates from the Sun.

Don't confuse types of energy with energy resources, i.e. wind and waves are not types of energy. They both posses kinetic energy which can be harnessed to generate electricity.

Energy is a conserved quantity. This means that energy cannot be created or destroyed in a closed system although it can be transferred from one form to another. The principle of conservation of energy is one of the most important and useful ideas in physics.