In contrast to chemical, nuclear and most other energy forms, work and heat are not properties of matter, or of any particular 'system' (e.g., substance or body). They represent energy in transit, or energy transferred between substances or bodies.
Were it not for work and heat, energy - as a property of matter - would be very much analogous to another elusive property, our health: we don't notice it, or we don't appreciate it, until we lose it! Indeed, until energy is converted into work or heat, it remains pretty much unnoticed and unappreciated, even though it is a fundamental property of matter.
In the case of work, this transfer of energy is accomplished by the application of a force over a distance. One such force is the force of gravity, which is the product of the mass of a substance and its acceleration due to gravity (which is a constant, of the order of 10 meters per square second).
Here is a simple calculation of work, which illustrates why it is so profitable to harness the energy of falling water.
Heat is transferred between substances whenever they have different thermal energies, as manifested by differences in their temperature. Here also is a simple calculation of heat. It will be used in our discussion of residential comfort. We shall illustrate there how large does a hot water tank have to be to store enough energy for a viable solar heating system in our home.
It wasn't until mid-nineteenth century that we figured out the relationship between work and heat. In fact, it is only when the rule governing this relationship was discovered that it became possible to fully understand the concept of energy. This rule is called the First Law of Thermodynamics. The crucial experimental observation here was that of the Englishman James Prescott Joule (1818-1889), son of a Manchester brewer.
Review the historical development of the concepts of heat, work and energy.
Return to the general discussion of energy.