34 MECHANICS. 



CENTRIFUGAL FORCE. 



All bodies, when in motion, have a tendency to move 

 forward in a straight line. A stone thrown into the air 

 is gradually bent from this straight course into a curve 

 by the attraction of the earth. When a ball is shot 

 from a gun, the force being greater, it flies in a longer 

 and straighter curve. A famihar example also occurs, 

 while driving a wagon rapidly, in attempting to turn 

 suddenly to the right or left ; the tendency of the load 

 to move straight on will sometimes cause its overtlu-ow. 

 An observance of this principle would prevent the error 

 which some commit by making sharp turns or angles in 

 ditches and water-courses ; the onward tendency of the 

 water drives it against the bank, checks its course, and 

 wears away the earth. By giving the ditch a curve, 

 the water is but slightly impeded, and a much larger 

 quantity will escape through a channel of any given size. 



When a grindstone is turned rapidly, the water upon 

 its surface is thrown off by this tendency to move in 

 straight lines. In the same way, a weight fastened to 

 a cord, whirled by the hand, wiU keep the cord stretch- 

 ed during the revolution. The same principle causes 

 a stone, when it leaves a sHng, to fly off" in a line. This 

 tendency to fly off from a revolving centre is called 

 centrifugal force — the word centrifugal meaning fly- 

 ing from the centre. Large grindstones, driven with 

 great velocity by machinery, are sometimes split asun- 

 der by centrifugal force. 



The most sublime examples of centrifugal force oc- 

 cur in the motion of the earth and planets, which will 

 be more fully explained on a future page. 



