Many believe that energy, by its classical definition, is solely the ability to do work. In a sense, this is correct, but it becomes slightly more complicated. Energy is, in effect, nothing; it is intangible and can only be measured by the effects it causes. So one cannot say correctly that this vague aspect of the universe is a mere tool to accomplish a task.
The modern-day definition of energy, and the one that is believed to be totally correct, is as follows: energy is the rate of interaction of anything that has mass; this point will be elaborated on later. The rate of interaction simply means to what degree some object with energy is interrelating. For example, a ball, traveling at 10 kilometers per hour hits a person with some force; the rate of interaction of this ball with the person is relatively weak because the kinetic energy of the ball was not very high. Yet, if the same ball is traveling at 1000 kilometers per hour and hits the same person, the rate of interaction is much higher. In other words, since the kinetic energy was so high, the person felt much more pain from the 1000 km/h ball than from the 10km/h ball. This indicates that the higher the energy of an object, the higher the rate of interaction.
One may raise the interesting point, mentioned above, that how can energy be present only in something with mass, if light does not have mass? The answer is that, first, light does have mass, and second, that the definition for mass has changed. The classical definition of mass states: mass is a quantity or value of matter. Yet, this has turned out to be false. The reason is that mass, like energy, is also the rate of interaction. This implies that energy and mass are, in effect, relative. This, through a series of experiments, was proven to be true. For example, when a liter of gasoline combusts (do not try this at home!), it releases heat, smoke and other byproducts. Yet, it was noticed that when all of the tangible matter was accounted for after the explosion, it did not equal the original liter. This seemed to violate the law of conservation and matter and energy. The only possible conclusion was that energy and mass were relative. This had to be true in order to follow the above law. Einstein’s equation, E=mc2, shows this relationship, through the constant of c, meaning speed of light. This does not only make sense with equations and experiments, but in real-life applications. Anything with a mass of 1 kg has a higher rate of interaction than something with a mass of 1 g.
Einstein’s equation also points out that energy and momentum are relative as well. Momentum equals (mass x speed), and, in the above equation, so does energy. This makes sense because anything with a high speed and mass has a greater rate of interaction than something with a low speed and mass. The conclusion from this is that energy, mass and momentum are all relative terms that become equal through constants.