ENERGY AND FORCE 43 



continuation of motion at any particular instant. When 

 mud is thrown from a rapidly rotating carriage wheel, 

 it does not follow the curve of the wheel, but flies off in 

 a straight line with the direction in which it was going 

 when it left the wheel. The same phenomenon is noticed 

 when a sling is rotated (Figure , 



36) and suddenly released. The $'*"" *"X 

 weighted sling goes in a straight 

 line which is tangent to the circle // 

 in which it was rotating. 



Centrifugal Force. The force 

 which caused the sling to pull so 

 hard on the hand is centrifugal \ /] 



force or center fleeing force. It "- : *v : ^ ,.S 



is the same force which causes FlG . 36 . -Centrifugal Force, 

 the water to stay in a rotating 



bucket. It is not a new force but a name given to this 

 particular manifestation of inertia. It is the force which 

 sometimes causes emery wheels to burst, which enables 

 us to separate the milk from the cream in the separator 

 by causing the milk to move farther out than the lighter 

 cream, and which makes the equatorial diameter of the 

 earth greater than the polar diameter. 



At the beginning of this chapter we said it would be 

 necessary to think of force and energy as having distinct 

 meanings. There are a number of forces with which we 

 are in almost daily contact : gravitational force ; muscu- 

 lar force, such as that exerted by man and beasts of 

 burden; the force of the wind; the force of expanding 

 gases ; and others which are not so common. 



Muscular force is a force which will always be used. 

 Even if beasts of burden should no longer be needed, 

 muscular force exerted by man will always be a neces- 



