156 



USING MACHINES 



efficiency a percentage obtained by dividing the work 

 got out of a machine by the work put into it. 



work out 



Efficiency = : 



work in 



energy the capacity of a body to do work. A raised 

 hammer has capacity for doing work and therefore 

 possesses energy. 



foot pound a unit for measuring work. The number 

 of foot pounds of work is found by multiplying the 

 force in pounds by the distance moved in feet. If a 

 two-pound weight is lifted four feet, then eight foot 

 pounds of work have been done. 



force a push or a pull on a body. 



gw a wheel with teeth around the outside. It is a 

 type of machine. 



WtiuTuMiiiW having to do with machines. 



fulcrum the point about which a lever turns. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 108. What is the principle of the lever 

 and how does the lever give an advantage in doing 

 work? 



Drive a nail through the midpoint of a yardstick or a 

 piece of wood from thirty-five to forty inches in length 

 which is marked off in inches. The nail should project on 

 each side of the stick to form an axle when it is balanced 

 between two wood blocks as shown in Figure 250. 



FIG. 250 



Part 1. Either brass weights, as shown in the drawing, 

 or small bags of sand which have been carefully made the 

 same weight as those made of brass may be used. Place a 

 one-pound weight on the right side of the lever bar at a 

 distance of ten inches from the nail or fulcrum at the cen- 

 ter of the stick. On the other side of the fulcrum place 

 another one-pound weight at such a point that the lever 

 will exactly balance. Observe the distance of each weight 

 from the fulcrum and record your results. Remove the 

 weights from the bar. 



Part 2. Place a one-pound weight on the left side, six 

 inches from the fulcrum, and a half-pound weight on the 

 right side at such a point that it will balance the lever 

 bar. Again observe the distance of each weight from the 

 fulcrum and record in your notebook. Remove the weights 

 from the bar. 



Part 3. Place a one-pound weight on the left side of the 

 fulcrum twelve inches away from it and a half-pound weight 

 on the same side eight inches from the fulcrum. On the 

 right side of the fulcrum place a one-pound weight at such 

 a distance that the bar will balance. Carefully observe all 

 the weights and their distances from the fulcrum. 



1 See workbook, p. 55. 



Perform the simple calculations indicated below after 

 you have recorded your results. 

 Part 1. 



Weight on left of fulcrum = pounds. 



Distance of weight from fulcrum = inches. 



Weight X distance = 



Weight on right of fulcrum = . pounds. 



Distance of weight from fulcrum = inches. 



Weight X distance = . .. 



Part 2. 



Weight on left of fulcrum = pounds. 



Distance of weight from fulcrum = inches. 



Weight X distance = 



Weight on right of fulcrum = pounds. 



Distance of weight from fulcrum = inches. 



Weight X distance = 



Part 3. 



pounds. 

 _ inches. 



pounds. 

 . inches. 



First weight on left of fulcrum = _ 



Distance of weight from fulcrum = 

 Weight X distance = 



Second weight on left of fulcrum = . 



Distance of weight from fulcrum = 

 Weight X distance = 



Sum of products = . 



Weight on right of fulcrum = pounds. 



Distance of weight from fulcrum = inches. 



Weight X distance = 



In this experiment, when a lever is in balance, the product 



f~thc on one side of the fulcrum by its distance from 



the must be equal to the product of the weight and 



its from the on the other side. When a weight is 



twice as great as another, the greater weight will be 



as far from the fulcrum as the smaller. If two or more 



weights are on either side of the fulcrum, the of the 



products of the weights and their distances from the ful- 

 crum on one side must equal the of the products on 



the other side if the lever is in balance. 



Problem. A boy weighing 100 pounds wishes to raise a 

 400-pound weight with a lever bar by placing the weight 

 on one end and standing on the other side of the fulcrum. 

 If the lever is ten feet long, where should he stand to gain 

 the needed advantage when the weight is one foot from 

 the fulcrum? 



Experiment 109. How do pulleys gain an advantage 

 and how may this be found? 



Read on page 161 the section "How may machines be 

 made more efficient?" be- 

 fore doing this experi- T 



ment. 



Part 1. Hang a single 

 pulley from a support as 

 shown in Figure 251 at A. 

 Thread a piece of strong 

 string through it. To one 

 end of the string attach a 

 weight of one or two 

 pounds and to the other a 

 ten-pound spring balance. 

 Raise the weight and read 

 on the balance the num- 

 ber of pounds of force re- 

 quired to raise the weight. 

 In raising the weight one 

 foot, how far did the effort FIG. 251 



