7JV POPULAR PHYSICS. 27 



23. Suppose in a wheel and axle the P 2O Ibs., the W 

 24O Ibs., and the diameter of wheel = 4 ft. Required the 

 circumference of the axle. 



20 Ibs. : 240 Ibs. : : x : 48 in. 

 x = 4 in. (diameter of axle). 

 4 in. x 3.1416 = 12.56 in. (circumference). 



24. Hequired , in a tvheel and axle, the diameter of the 

 wheel, the diameter of the axle being 1O inches, P 1OO 

 Ibs. and W = 1 ton. 



100 Ibs. : 2,000 Ibs. : : 10 in. : x = 200 in. = 16f ft. 



25. Why is the rim of a fly -wheel made so Jieavy ? 



The largest momentum possible is desired. The velocity of 

 a particle is proportional to its radius in rotation. Hence the 

 largest part of the mass of the wheel is fixed at the rim. 



26. Describe the hammer, when used in drawing a 

 nail, as a bent lever, i.e., one in which the bar is not 

 straight. 



If a lever is bent, or if, when it is straight, the bar is not 

 at right angles to the lines of action of the P and the W, it is 

 necessary to distinguish between the arms of a lever and the 

 arms of the P and the "W, regarded as forces which have mo- 

 ments around the F. In the latter sense, the arms are the per- 

 pendiculars, dropped from the F to the lines of action of the 

 P and the W. 



27 Describe the four levers shown in Fig. 46, when 

 both the load of hay and the weight are considered, re- 

 spectively, as the W and the P. 



This is so fully answered in the text that no further expla- 

 nation seems necessary. The pupil should be required to as- 

 sume values for the W and for each of the lever arms, and 

 compute the weight of the wagon and hay together. Call his 

 attention to the fact that half the weight of the wagon and 

 hay is transmitted at the point P, and the other half at P' ; 

 also that the vertical rod at the lift serves only as a connector, 

 and not as a lever. 



