A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . s than the power, frictionbeing supposed zero, the weight moves a greater distancethan the power does in the operation of the apparatus. If we study the matter more closely, we shall find thefollowing rule, or law, suggested, though we cannot say thatit is proved by our preceding experiments: where P stands for the power applied;W weight lifted;Dp distance the power moves; Dw weight moves. APPLICATIONS OF THIS LAW (1) It is evident that this law can be readily applied to
A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . s than the power, frictionbeing supposed zero, the weight moves a greater distancethan the power does in the operation of the apparatus. If we study the matter more closely, we shall find thefollowing rule, or law, suggested, though we cannot say thatit is proved by our preceding experiments: where P stands for the power applied;W weight lifted;Dp distance the power moves; Dw weight moves. APPLICATIONS OF THIS LAW (1) It is evident that this law can be readily applied to a case likethat of Fig. 53. We can see at once that, if the pulley B wereIKted one inch while P remained stationary, there would be fourinches of loose string under B. To make the string taut again, P 88 PHYSICS. would have to rise four inches. In actual use P and B rise at thesame time, P moving four times as fast as B. (2) In Fig. 54, representing the rear wheel and gear of a bicycle, letthe diameter of the tire be 28 in.; small sprocket-wheel be 2$ in.; large sprocket-wheel be 5 in.; length of pedal radius be 6 FIG. 54. If the weight of the rider, 150 Ibs., rests entirely upon the pedalshown, in its present position, how great a weight acting in opposi-tion, as in the figure, will just neutralize the driving-power? The circumference of the circle described by the pedal is (see ) 2;rX6 inches, or Yliz inches. One revolution of the pedal-crank makes the tire revolve 2 times,which drives the bicycle forward 2X27rX 14 inches = 567T inches. The weight W will be lifted as fast as the bicycle moves forward. If Dp stands for the distance the power P moves downward fromits present position during any very short time, and if Dw standsfor the distance W moves upward during the same time, we have Dp : Dw : :Hence the law WXDW =PXDP gives or TF=^P=tVX 150 = + (Ibs.) EQUILIBRIUM OF FORCES: THE LEVER. 89 PROBLEMS AND QUESTIONS. (1) A large pair of shears is used to cut a wire.
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