Work and Energy | Research & Encyclopedia Articles

Work, in general, is an activity in which one exerts strength or faculties to do or perform something. In physics, work is defined as a force on an object multiplied by the displacement of the object. Measuring force in newtons (N) and displacement in meters (m) yields newtons times meters (Nm) as the units of work. To honor James Prescott Joule, who did pioneering work in the science of thermodynamics, a newton-meter is called a joule (J). A person pushing on a box with a force of 200 N (about 50 pounds) and moving the box 1 m would do 200 J of work. Energy is capacity, or ability, for doing work. Work and energy are both measured in joules.

After being outside on a cold day, a person's hand warms when shaking hands with someone who has spent time in a warm room. Heat flows from the warm hand to the cool hand to produce the warming. Rather than shaking hands, the person could briskly rub his hands together. In this case, warming is a result of doing work, not due to a flow of heat. Heat is energy in transit and, accordingly, is measured in joules. It takes 4,200 joules of heat to raise the temperature of 1 kilogram of water 1 degree Celsius. The same temperature rise would occur if 4,200 joules of mechanical work were done on the water using a food mixer, for example.

The concepts of heat and work are very important for understanding the roles of automobiles, airplanes, trucks, trains, and electric power plants that are essential for a modern industrial society. Examining the technology, one always sees the involvement of an engine of some sort. Most automobiles employ a gasoline engine, trucks and trains use diesel engines, and an electric power plant uses a steam turbine to drive an electric generator. There are significant differences in the size and complexity of the engines involved, but they all convert heat to useful work.

The word kinetic implies motion. An object in motion has a capacity for doing work and has kinetic energy. Forces acting on an object do work when the object moves and the kinetic energy changes. When pulling on a sled to start it moving, force and displacement are in the same direction. Work done by this force is taken to be positive. While the sled is moving, frictional forces on the runners pull backwards. The force and displacement are in opposite directions and the work is taken to be negative. The net amount of work is the algebraic sum of the work due to all forces acting on an object. For example, if pulling on the sled produced 100 J of work and friction produced –50 J the net amount of work is 100 J – 50 J = +50 J.

A very important principle called the work-energy principle reads as follows: net amount of work is equal to the change in kinetic energy. Emphasis is placed on net and change. If the net work is positive, this means the net force and displacement have the same direction, and the kinetic energy increases. Increasing the kinetic energy of a car requires positive work. Conversely, decreasing the kinetic energy requires negative work. When a braking system brings a car to rest, forces must act in a direction opposite to the direction the car is moving. If a car travels down a highway at constant speed, its kinetic energy does not change. This means the net work on the car is zero. Air always pushes against the car and does negative work. To keep the speed from changing, forces that propel the car forward must do positive work of equal magnitude. The faster the car travels, the greater the negative work done by the air. As a result, forces driving the car forward must do more work. This means more consumption of gasoline. This is the basic reason why the fuel economy of a car is sensitive to speed.

Joule, James Prescott.

Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.

Serway, R. A. (1998). Principles of Physics, 2nd ed. Fort Worth, TX: Saunders College Publishing.

This is the complete article, containing 714 words (approx. 2 pages at 300 words per page).