EFFICIENCY

To make it easier to explain machine operations, we have neglected the effect of friction on machines up to this point. Friction happens every time two surfaces move against one another. The work used in overcoming the frictional resistance does not appear in the work output. Therefore, its obvious that you have to put more work into a machine than you get out of it. Thus, no machine is 100 percent efficient.

Take the jack in figure 7-6, for example. The chances are good that a 2-pound force exerted on the handle wouldnt do the job at all. You would need a pull of at least 10 pounds. This shows that only 2 pounds out of the 10 pounds, or 20 percent of the effort, is employed to do the job. The remaining 8 pounds of effort was is in overcoming the friction in the jack. Thus, the jack has an efficiency of only 20 percent. Most jacks are inefficient. However, even with this inefficiency, it is possible to deliver a huge push with a small amount of effort.

A simple way to calculate the efficiency of a machine is to divide the output by the input and convert it to a percentage:

Now go back to the block-and-tackle problem illustrated in figure 7-5. Its likely that instead of being able to lift the load with a 120-pound pull, the sailor would have to use a 160-pound pull through the 100 feet. You can calculate the efficiency of the rig by the following method:

and, by substitution,

Theoretically, with the mechanical advantage of 12 developed by the cable winch in figure 6-11, you can lift a 600-pound load with a 50-pound push on the handle. If the machine has an efficiency of 60 percent, how big a push would you actually have to apply? Actually, 50 + 0.60 = 83.3 pounds. You can check this yourself in the following manner:

Here are some of the important points you should remember about friction, work and efficiency:

You do work when you apply a force against a resistance and move the resistance.

Since force is measured in pounds and distance is measured in feet, we measure work in foot-pounds. One foot-pound of work is the result of a 1-pound force, acting against a resistance through a distance of 1 foot.

Machines help you to do work by making it possible to move a large resistance through a small distance by the application of a small force through a large distance.

Since friction is present in all machines, more work must be done on the machine than the machine actually does on the load.

You can find the efficiency of any machine by dividing the output by the input.

Friction is the resistance that one surface offers to movement over a second surface.

Friction between two surfaces depends upon the nature of the materials and the size of the forces pushing them together.