Work and Force.

Energy is a fundamental property of matter. In functional terms, it is a measure of how much work a system (such as a car engine or a human body) can perform. According to the first law of thermodynamics, the total energy of the universe is constant: it can be neither created nor destroyed. Instead, energy is transformed or transferred. When energy is transformed or transferred, work is being done. In physics, work does not need to be useful or desirable to be considered work. When the energy from a heavy wind blows over a tree, the tree is doing work.