ENERGY AND WORK
Energy, Work, and Power
Work is commonly thought of as any activity requiring exertion. However, the definition in
physics is much more specific. Work is done by a force acting on a moving object if the object
has some component of motion in the direction of the force. Work can be done BY a person,
a machine, or an object by applying a force and causing something to move. More specifically,
work is done by a force acting on a moving object if the object has some component of motion
in the direction of the force. Work can be done ON an object by applying a force that causes
it to move. For example, if you push on a box (apply a force) and it moves three feet, work has
been performed BY you to the box, while work has been performed ON the box. If you push
on the box and it does not move, then work, by our definition, has not been accomplished.
Work can be defined mathematically by Equation 5-3.
W = F x d
work done in ft-lbf
force applied to the object in lbf
distance the object is moved (in ft.) with the force applied
You push a large box for three minutes. During that time, you exert a constant
force of 200 lbf to the box, but it does not move. How much work has been
F x d
200 lbf x 0 ft
0 ft-lbf work done
Remember that if no movement is achieved, no work has been accomplished. Even if you feel
fatigued, no work has been done. Work can be thought of as what has been accomplished. If
nothing is accomplished, then no work has been done.
You push the same box as mentioned above. You apply a horizontal force of
200 lbf to the box, and the box moves five feet horizontally. How much work
have you done?
F x d
200 lbf x 5 ft
In this case, work can be described as work done by the person pushing the box or work
performed on the box. In either case, the amount of work is the same.