CHAPTER LEARNING OBJECTIVES
Upon completion of this chapter, you should be able to do the following:
l Define the term work when applied to mechanical power.
You know that machines help you to do work. What
is work? Work doesnt mean simply applying a force. If
that were so, you would have to consider that the sailor
in figure 7-1 is doing work. He is busy applying his
220-pound force on the seabag. However, no work is
Work in the mechanical sense, is done when a
resistance is overcome by a force acting through a
measurable distance. Now, if that sailor were to lift his
90-pound bag off the deck and put it on his bunk, he
would be doing work. He would be overcoming a
resistance by applying a force through a distance.
Notice that work involves two factors-force and
movement through a distance. You measure force in
pounds and distance in feet. Therefore, you measure
work in units called foot-pounds. You do 1 foot-pound
of work when you lift a 1-pound weight through a height
Figure 7-1.No work is being done.
of 1 foot, You also do 1 foot-pound of work when you
apply 1 pound of force on any object through a distance
of 1 foot. Writing this as a formula, it becomes
D I S T A N CE
Thus, if you lift a 90-pound bag through a vertical
distance of 5 feet, you will do
WORK = 90 X 5 = 450 ft-lb.
You should remember two points about work
1. In calculating the work done, you measure the
actual resistance being overcome. The resistance is not
necessarily the weight of the object you want to move.
To understand this more clearly, look at the job the sailor
in figure 7-2 is doing. He is pulling a 900-pound load of
supplies 200 feet along the dock. Does this mean that he
Figure 7-2.Working against friction.