Work and energyHow does energy move and create change? The answer starts with the physical idea of work. In physics, work is done by forces that cause objects to move. As shown in equation (9.1), the amount of work done on an object is the force multiplied by the distance the object moves in the direction of the force. If you use a force of one newton to lift a cup up a distance of one meter, you do one newton-meter (N m) of work on the cup. Read the text aloud Show Can a force do no work?
(9.1) W=Fd
W  = work (J)
F  = force (N)
d  = distance (m)
The meaning of the word “work” is different in physics from how we use the word in everyday life. You might complain that you work too hard on your homework or that you have to go to work early in the morning. In physics, work is the product of force exerted and distance moved, and it can have a precise value such as 10 N m. Work is done whenever energy is transferred from one object to another. Read the text aloud
Force, work, and energy
In the SI system, one joule (J) is defined as one newton-meter (N m). The unit of work—the joule—is also the basic unit of energy. Throughout physics and throughout all of science—including chemistry, biology, and earth sciences—you will find the joule as a standard unit of energy. It is not, however, the only unit of energy. Calories (food), British thermal units (heat), and kilowatt-hours (electricity) are other units of energy that are commonly used. Read the text aloud
Work is force times distance moved
Work provides an operational definition for energy: Energy is the ability to do work. If you have 10 J of energy, you have the ability to produce a force of 10 N for a distance of 1 m, or 1 N for 10 m. A system that has 1 J of energy can exert any combination of forces and distances whose product is equal to or less than 1 N m. We say “less than” because the amount of energy available limits the amount of work that can be done. No system can produce any work that exceeds its available energy. Read the text aloud Show Other ways to define energy
Tim pulls his little sister in a wagon with a constant force of 50 N over a distance of 100 m. How much work does Tim do?
  1. 5,000 J
  2. 2,500 J
  3. 150 J
  4. 50 J

Previous Page Next Page257