3.3 The Law of Conservation of Energy
Energy is defined as the capacity to do work. An object which has the capacity to accelerate an object (including itself) possesses energy. The SI unit for energy is the joule (J). Remember that the unit for work was also the joule. In fact, energy and work are closely related.
The Law of Conservation of Energy, derivable from Newtons Laws, states that, in a closed system, energy is neither created nor destroyed. Rather, energy is a fixed quantity that can be converted from one form to another. For example, a car at the top of a hill possesses potential energy, the energy of position. As the car rolls down the hill, it loses potential energy. This "lost" energy is really converted to kinetic energy, the energy of motion, and heat (a form of energy), some of which is transfered to the surrounding environment. In sections 3.4 and 3.5, we will calculate the transfered quantities of energy using the idea that work and energy are interchangable.