. Practical physics. ever,namely B/E, is equal to l/V, that is, to the lever arm of tlieeffort divided hi/ the lever arm of the resistance. 131. General laws of the lever. If parallel forces are appliedat several points on a lever, as in Figs. 119 and 120, it will befound, in the particular cases illustrated, that for equilibrium 200 X 30 = 100 X 20 +100 x 40and 300x20 + 50x40 = 100 x 15 + 200 x ; that is, the sum of all tlie moments ivhich are tending to turnthe heam in one direction is equal to tlie sum of all the momentstending to turyi it in the opposite direction. 112 WORK AND MECHANI
. Practical physics. ever,namely B/E, is equal to l/V, that is, to the lever arm of tlieeffort divided hi/ the lever arm of the resistance. 131. General laws of the lever. If parallel forces are appliedat several points on a lever, as in Figs. 119 and 120, it will befound, in the particular cases illustrated, that for equilibrium 200 X 30 = 100 X 20 +100 x 40and 300x20 + 50x40 = 100 x 15 + 200 x ; that is, the sum of all tlie moments ivhich are tending to turnthe heam in one direction is equal to tlie sum of all the momentstending to turyi it in the opposite direction. 112 WORK AND MECHANICAL ENERGY . If, further, we support the levers of Figs. 119 and 120by sprmg balances attached at P, we shall find, after allowmgfor the weight of the stick, that the two forces mdicated bythe balances are respectively 200 + 100 + 100 = 400 ^and 300 + 100 + 200 - 50 = 550 ; that is, the sum ofall the forces acting in one direction on the lever is equal tothe sum of all the forces act-ing in the opposite direction. 50. Fig. 120 Condition of equilibrium of a bar acted upon by seAeral forces These two la^^?s may be combined as follows: If we thmkof the force exerted by the spring balance as the equilibrantof all the other forces acting on the lever, then we find that theresidtant of any number of parallel forces is their algehraic sum,and its point of application is the point aboid tchich the algehraicsum of the moments is zero. 132. The couple. There is one case, however, in which paral-lel forces can have no single force as their resultant, namely,the case represented in Fig. 121. Such a pair of equal y^and op>posite forces acting at different pioints on a lever iscalled a couple and can be neutralizedonly by another couple tending toproduce rotation in the oppositedirection. The moment of such a ^^ ^- ^- ^^^ ^°l^^^couple is evidently F^xoa+F^x oh=F^ X ah; that is, it isone of the forces times the total distance between tliem. Theforces applied to the steermg wheel o
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectphysics, bookyear1922
