Summary

a distance S1 of 6 x 2nR, or 94.2 feet. Then, by substitution: ~60 =600x7.85 F1 X 94.2 F1 = 600 x 7.85 = 83.3 pounds. 94.2 x 0.60 Because this machine is only 60-percent efficient, you have to put 94.2 x 83.3, or 7,847 foot-pounds, of work into it to get 4,710 foot-pounds of work out of it. The difference (7,847 – 4,710 = 3,137 foot-pounds) is used to overcome friction within the machine. SUMMARY 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. 7-6