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Basic Concepts

Several ideas are key to understanding how machines make work easier or peoples' efforts more powerful. Among them are the following:
  • Energy is a fundamental property of everything in our universe. Some would say it is the fundamental property. One way to think about energy is to see it as distributed between these forms:
    • Kinetic Energy -- This is the energy of motion. Kinetic energy increases as an object's mass increases,

      but it increases even more because of the object's speed.

      Both speeding bullets and locomotives have lots of kinetic energy.

    • Potential Energy -- This is the energy objects have because of their position in a force field. Gravity, electricity and magnetism, and nuclear forces create such fields. Moving an object against these forces increases its energy as it changes position. For example, lifting a book from the floor to a desktop increases its potential energy due to gravity.
    In many cases we design systems to change one type of energy into another. Hydroelectric generators tranform the gravitational potential energy of water held behind a dam into the kinetic energy of electrons flowing through wires.
  • Work changes the energy of objects and systems. Work may change either the kinetic energy or the potential energy of an object or both. Often we use machines to do work. In many cases we think of work as the application of a force through a distance as in the example above of lifting a book from the floor to the desktop. The force applied was equal to the weight of (the force of gravity on) the book. The distance was the height of the desk.
    Here Force X Distance = Weight X Height = Change in Potential Energy = Work.
  • Power is how fast work is done or energy is used. Power lets us compare different ways of doing work to see which is quicker. For instance, a car engine with high horsepower will accelerate a car (change its kinetic energy) from 0 to 60 mph in less time than one with less horsepower.
  • Efficiency measures how much of the energy we put into an engine or other system comes out as useful work. In every case some of the input energy is lost to heating the surroundings; nothing can be 100% efficient. What we call the "Laws of Thermodynamics" describe this relationship and limit people's quest for perpetual motion machines and "free" energy.


Last updated Tuesday, March 12, 2002
© 2002 All rights reserved   Tom Stork
Background adapted from "Design for a flying machine," a drawing by Leonardo da Vinci