The person holding the briefcase in Figure 7.2(b) does no work, for example. To examine what the definition of work means, let us consider the other situations shown in Figure 7.2.

Here the work done on the briefcase by the generator is negative, removing energy from the briefcase, because F F and d d are in opposite directions. (e) When the briefcase is lowered, energy is transferred out of the briefcase and into an electric generator. Energy is transferred to the briefcase and could in turn be used to do work. (d) Work is done on the briefcase by carrying it up stairs at constant speed, because there is necessarily a component of force F F in the direction of the motion. (c) The person moving the briefcase horizontally at a constant speed does no work on it, and transfers no energy to it. No energy is transferred to or from the briefcase. (b) A person holding a briefcase does no work on it, because there is no displacement. Note that F cos θ F cos θ is the component of the force in the direction of motion. (a) The work done by the force F F on this lawn mower is Fd cos θ Fd cos θ.

For one-way motion in one dimension, this is expressed in equation form asįigure 7.2 Examples of work. The scientific definition of work reveals its relationship to energy-whenever work is done, energy is transferred.įor work, in the scientific sense, to be done, a force must be exerted and there must be displacement in the direction of the force.įormally, the work done on a system by a constant force is defined to be the product of the component of the force in the direction of motion times the distance through which the force acts.

Certain things we think of as hard work, such as writing an exam or carrying a heavy load on level ground, are not work as defined by a scientist. The scientific definition of work differs in some ways from its everyday meaning.

Explain how relative directions of force and displacement determine whether the work done is positive, negative, or zero.

Explain how an object must be displaced for a force on it to do work.

By the end of this section, you will be able to:

Print conductor 7.1