76 Common Science 



thrown out by centrifugal force. Milk is heavier than 

 cream, as you know from the fact that cream rises and 

 floats on top of the milk. So when milk is put into a 

 centrifugal separator, a machine that whirls it around 

 very rapidly, the milk is thrown to the outside harder 

 than the cream, and the cream therefore stays nearer 

 the middle. As the bowl of the machine whirls faster, 

 the milk is thrown so hard against the outside that it 

 flattens out and rises up the sides of the bowl. Thus 

 you have a large hollow cylinder of milk on the outside 

 against the wall of the bowl, while the whirling cream 

 forms a smaller cylinder inside the cylinder of milk. 

 By putting a spout on the machine so that it reaches 

 the inner cylinder, the cream can be drawn off, while 

 a spout not put in so far will draw off the milk. 



Why a spinning top stands on its point. When a 

 top spins, all the particles of wood of which the top is 

 made are thrown out and away from the center of the 

 top, or rather they tend to go out and away. And the 

 pull of these particles out from the center is stronger 

 than the pull of gravitation on the edges of the top to 

 make it tip over ; so it stands upright while it spins. 

 Spin a top and see how this is. 



Application 21. Explain how a motor cyclist can ride on 

 an almost perpendicular wall in a circular race track. Ex- 

 plain how the earth keeps away from the sun, which is 

 always powerfully pulling the earth toward it. 



Inference Exercise 

 Explain the following : 



91. As you tighten a screw it becomes harder to turn. 



92. There is a process for partly drying food by whirling it 



rapidly in a perforated cylinder. 



