Work, energy and power
There are a number of diff erent forms of energy, including heat, light, sound, electricity, chemical energy and various forms of mechanical energy. The total amount of energy in the universe is constant; it cannot be created or destroyed, it can only be transformed from one form to another. All interactions in nature are the result of transformation of energy from one form to another. For example, the combustion of oil, gas or coal produces heat that can be used to produce electricity that can be used to produce heat in a toaster, heat and light in a light bulb, heat, light and sound in a television or mechanical energy in the form of movement in a model train. Living organisms consume nutrients in order to produce chemical energy to maintain all of the life processes. The majority of the energy produced from nutrients is used to produce mechanical energy in the form of movement of the body segments. Transformation of energy into mechanical energy is referred to as work. All forms of energy are equivalent in their capacity to do work, i.e. to bring about the transfer of energy from one body to another through the action of a force or forces that deform or change the position or speed of movement of the bodies. Power is the rate of transformation of energy from one form to another. Mechanical power is the rate at which energy is transformed in the form of work. The purpose of this chapter is to describe the relationships between work, mechanical energy and mechanical power in human movement. Before reading this chapter, the reader should read the sections in Chapter 6 concerning energy and the diff erent forms of mechanical energy.