To understand the principles underlying the use of tools and machines, it is necessary chiefly to understand the differences between force, work, and energy. Force is that which tends to produce, to change, or to destroy the motion of a body. The force may be the strength of man or animal, or of steam, or electricity, etc. Tools and machines when stationary are in what is called a state of inertia. The overcoming of resistance through any distance, such as putting tools or parts of machines in motion, is called work. Work is done when a force produces or destroys motion.