We say that work is done by a force if that force acts on an object which moves, or undergoes a displacement (a vector change in position). In particular, consider an object which undergoes a displacement x in a time t while a constant force F acts on it.
We say that the force F does an amount of work WF on the moving object during the displacement x. This work is given by
WF = x Fx = x (F cosq) ,
where Fx is the component of the constant force F in the direction of the displacement x . (We have to use calculus if we want to treat the case of a force which is not constant.) Thus, to find the work done by a constant force, we need only be able to find the component of a force! (We’re already good at this!)
In addition, we say that the power, P, delivered by the force F in doing the work WF in the time t is given by
The unit of work must be the unit of force times the unit of distance. In the MKS system, this combination of units is called the joule:
1 N m = 1 joule = 1 J .
The joule is the MKS unit of work and energy. Likewise, the MKS unit of power is called the watt:
1 W = 1 watt = 1 J/s .