.. IV.] 



MECHANICS. 



77 



distances from the fulcrum g. We must now find out the relation which exists 



between the size of the forces p and P, and the lengths kg undfg. It will be seen 



_ / that the triangles c a e in Fig. 8, and afg are similar to each 



' * ' ' ' other, and therefore a c and a e are equal iofg and a g t and 



the lengths a c and a e are to each other as the forces p and P, 

 consequently we may say that these forces bear an inverse 

 ratio to the distances of their points of action from the 

 fulcrum g. We can now see that a lever does revolve round a 

 ** * fixed point, as at g in Fig. 9. 



QUESTIONS. 



45. T. What is meant by the ttatic mo- 

 ment of a f 



/'. It is the product obtained by mul- 

 tiplying the force by the arm of the lever, 

 :hat force which, acting at an arm of 

 t on the opposite side of the fulcrum, 

 ^reserve the state of equilibrium. 

 /'. Explain this more fully. 



suppose that the force to the 

 right of Fig. 9 is equal to 6, and the arm 

 of the lever equal to 4, the static n. 

 of the force will be 4 multiplied by 6, or 

 24 ; then if the force on the left hand is to 

 be in a state of equilibrium with the former, 

 the static moment of the two must be equal, 

 and the force acting on the left side of an 

 arm equal to 2 must have a weight of 12. 



/.What do you mean by a lever 

 of the second kind ? 



P. It i a lever which has the weight 

 placed between the fulcrum and the power. 



/If. 10. 



A fam < of this kind of lever is 



. the fulcniM. 



presses on the 

 lii-ing the barrow and 



Its load, wh ted l>v 



the two handles v- ui lifts, and 



in propor 1 . <n AS .s or shorten* 



handles, so is the power 

 *. [See Fnmtbpttc*.] 

 48. / familiar 



examples of a lever of the second class. 



P. The oar which urges a boat forward 

 is an excellent example ; the blade forced 

 against the water is the fulcrum, the boa; 

 the weight, and the man's hands the power. 

 The rudder of a ship acts in the same 

 manner. When we open a door, the hinge 

 is the fulcrum, the air the resistance, an. 

 the person opening it the power. If we 

 crack a nut we use two levers of the second 

 kind, the hinge which keeps them together 

 forming the fulcrum, the nut the resistance 

 or weight, and the hand the power. The 

 old sugar-chopper used by grocers is a very 



IV n. 



good example of a lever of this kind ; the 

 !" -ing the fulcrum, the sugar ihe 

 resistance or weight, and the handle the 

 When a crowbar is placed under- 

 neath a stone, and the end of the bar raised, 

 it becomes a lever of the second kind, the 

 end resting on the ground being t: 

 crniii. the M ,,| the 



upward movement of the man's hand the 

 power. Two men carrying a sedan-chair 

 t.irnung the 



moving power and fulcrum with rov, 

 each other, while the weight in the chair. 



49. T. What is meant by a one-armed 



/ It is a lever which has one end fixed, 

 and two forces acting in opposite dir< 

 same idr 



i give me an example of 

 this kind of lever t 



