 One thing that is fairly easy to measure is temperature. An increase in the temperature of something means its thermal energy is increased. So within the SI system the unit of energy is called the joule, and one joule is the amount of energy required to heat 0.24 grams of water by one degree centigrade. Now this definition probably seems a little bit arbitrary, but it makes the equations for other types of energy look neater. For example, that kinetic energy of an object is one-half mv squared, when mass is measured in kilograms and velocity is measured in meters per second with energy in joules. If the joule was defined differently then the factor of one-half would be different. But with this definition we see that the joule is a derived quantity that in terms of basic units is kilograms times meters squared divided by second squared. Now when we talk about energy and use it to analyze a physical system we are almost always studying a change in state. So when we say that kinetic energy is one-half mv squared we really mean that it took that much energy to change the object from being still to moving with velocity v. This reference to a change in state is particularly important when talking about potential energy. It is meaningless to say that an object has x joules of potential energy. Potential energy must always be given relative to some reference state. For objects near the surface of the earth changing the height of an object corresponds to a change in gravitational potential energy proportional to the object's weight as follows. Remember little g here is acceleration due to gravity at the earth's surface. Another common type of potential energy related to motion is the elastic energy in a spring. The reference state here is the spring in its relaxed state with no forces being exerted on it. Then if delta x is the change in length from this rest length the corresponding increase in potential energy is delta es equals one-half k delta x squared. k is called the spring constant and it quantifies how stiff the spring is. So now you've heard about a variety of different types of energy and how these are quantified.