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By the end of this section, you will be able to:
  • Represent the work done by any force
  • Evaluate the work done for various forces

In physics, work    represents a type of energy. Work is done when a force acts on something that undergoes a displacement from one position to another. Forces can vary as a function of position, and displacements can be along various paths between two points. We first define the increment of work dW done by a force F acting through an infinitesimal displacement d r as the dot product of these two vectors:

d W = F · d r = | F | | d r | cos θ .

Then, we can add up the contributions for infinitesimal displacements, along a path between two positions, to get the total work.

Work done by a force

The work done by a force is the integral of the force with respect to displacement along the path of the displacement:

W A B = path A B F · d r .

The vectors involved in the definition of the work done by a force acting on a particle are illustrated in [link] .

A curved path connecting two points, A and B, is shown. The vector d r is a small displacement tangent to the path. The force F is a vector at the location of the displacement d r, at an angle theta to d r.
Vectors used to define work. The force acting on a particle and its infinitesimal displacement are shown at one point along the path between A and B . The infinitesimal work is the dot product of these two vectors; the total work is the integral of the dot product along the path.

We choose to express the dot product in terms of the magnitudes of the vectors and the cosine of the angle between them, because the meaning of the dot product for work can be put into words more directly in terms of magnitudes and angles. We could equally well have expressed the dot product in terms of the various components introduced in Vectors . In two dimensions, these were the x - and y -components in Cartesian coordinates, or the r - and φ -components in polar coordinates; in three dimensions, it was just x -, y -, and z -components. Which choice is more convenient depends on the situation. In words, you can express [link] for the work done by a force acting over a displacement as a product of one component acting parallel to the other component. From the properties of vectors, it doesn’t matter if you take the component of the force parallel to the displacement or the component of the displacement parallel to the force—you get the same result either way.

Recall that the magnitude of a force times the cosine of the angle the force makes with a given direction is the component of the force in the given direction. The components of a vector can be positive, negative, or zero, depending on whether the angle between the vector and the component-direction is between 0 ° and 90 ° or 90 ° and 180 ° , or is equal to 90 ° . As a result, the work done by a force can be positive, negative, or zero, depending on whether the force is generally in the direction of the displacement, generally opposite to the displacement, or perpendicular to the displacement. The maximum work is done by a given force when it is along the direction of the displacement ( cos θ = ± 1 ), and zero work is done when the force is perpendicular to the displacement ( cos θ = 0 ).

The units of work are units of force multiplied by units of length, which in the SI system is newtons times meters, N · m. This combination is called a joule , for historical reasons that we will mention later, and is abbreviated as J. In the English system, still used in the United States, the unit of force is the pound (lb) and the unit of distance is the foot (ft), so the unit of work is the foot-pound ( ft · lb ) .

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
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hey , can you please explain oxidation reaction & redox ?
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hey , can you please explain oxidation reaction and redox ?
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for grade 12 or grade 11?
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use fnet method. how many obects are being calculated ?
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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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