FORCES AND MOTIONS IN DAILY LIFE 



151 



earth. The earth is not a perfect sphere. Because of 

 the centrifugal force resulting from the spin of the 

 earth on its axis, it bulges at the equator and is 

 flattened at the poles. Hence the poles are a little 

 closer to the center of the earth than the equator, and 

 the force of gravity is consequently greater at the 



Courtesy Bureau or Standards, L '. S. Deft, of Commerce 

 FIG. 238. STANDARD KILOGRAM: 



poles. The farther away from the center of the earth a 

 place on the surface is, the less is the attraction of the 

 force of gravity and therefore the less substances 

 will weigh. However, this difference is very slight 

 and for all practical purposes of weighing may be 

 neglected. 



Exercise. It is not possible to detect the difference in 



a. Spring balance 



b. Household spring balance 



c. Grocer's scales 



FIG. 239. SCALES IN DAILY LIFE 



d. Bath scales 



e. Truck scales 



f. Platform scales 



the gravitational pull between the equator and the poles 

 with a platform scale and a set of weights. It may be 

 detected, however, with a spring scale. Explain why this 

 would be true. Record in your notebook. 



What are the systems for measuring weight? Sev- 

 eral systems of weighing are used in this country, 



KIG. 240. INKKTIA IN OUR DAILY LIFE 



two of which are important for general knowledge. 

 These are the commonly used English system, and 

 the metric system used mostly in scientific work. The 

 units in the English system which are used most are 

 the ounce, the pound, and the ton ; in the metric 

 system they are the gram, the kilogram, and the 

 metric ton. 



The metric system of weights is better adapted for 

 scientific work because it is a decimal system. The 

 fundamental unit, the gram, is defined as the weight of 

 one cubic centimeter of water at 4 degrees centigrade. 

 The gram is a very small unit amounting to about 

 1/28 of an English ounce. There are 454 grams in a 



