Suppose a constant force F acts on a body while the object moves over a distance d. Both the
force F and the displacement d are vectors who are not necessarily pointing in the same direction
(see Figure 7.1). The work done by the force F on the object as it undergoes a displacement d is
defined as
The work done by the force F is zero if:
* d = 0: displacement equal to zero
* [phi] = 90deg.: force perpendicular to displacement
Figure 7.2. Positive or Negative Work.
The work done by the force F can be positive or negative, depending on [phi]. For example,
suppose we have an object moving with constant velocity. At time t = 0 s, a force F is applied. If
F is the only force acting on the body, the object will either increase or decrease its speed
depending on whether or not the velocity v and the force F are pointing in the same direction (see
Figure 7.2). If (
F
*
v
) > 0, the speed of the object will increase and the work done by the force on
the object is positive. If (
F
*
v
) < 0, the speed of the object will decrease and the work done by
the force on the object is negative. If (
F
*
v
) = 0 we are dealing with centripetal motion and the
speed of the object remains constant. Note that for the friction force (
F
*
v
) < 0 (always) and the
speed of the object is always reduced !
Per definition, work is a scalar. The unit of work is the Joule (J). From the definition of the work
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 Fall '09
 Force, Work, Angle of inclination

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