SCIENCE OF COMMON THINGS. 23 



Impossibility of perpetual motion. Centrifugal force. 



132 Is it possible to construct any machine, w arrangement of matter^ 

 which will perpetually continue in motion ? 



It is not / because the operations of gravity, the re- 

 sistance of the medium through which the body moves , 

 or the friction of the surfaces upon which the lody 

 rests, will, in a given time, destroy and terminate all 

 motion. In addition to this, all materials which we 

 employ in construction will, in the course of time, wear 

 out by use, or decay by natural agencies. 



133 Do we know of any instances of perpetual motion in nature 1 



Yes the various planetary bodies belonging to the 

 solar system have been moving with undiminished 

 velocity for ages past ; and, unless prevented by the 

 agency which governs all nature, will continue to 

 move in the same manner for ages to come. 



134 Why are horses obliged to make a much greater exertion to start a 

 carriage than afterwards to keep it in motion ? 



Because when a carriage is once put in motion upon 

 a level road, with a determinate speed, the only force 

 necessary to sustain the motion is that which is suffi- 

 cient to overcome the friction of the road / but, at 

 starting, a greater expenditure of force is necessary, 

 inasmuch as not only the friction is to be overcome, 

 but the force with which the vehicle is intended to 

 move must be communicated to it. 



135 What is centrifugal force? 



It is that force which causes a revolving body to fly 

 from a centre. 



136 Why does a stone, discharged from a sling, move forwards, when 

 the cord which retained it is loosened f 



Because of the centrifugal force it- has acquired by 

 the whirling of the sling previous to the discharge. 



137 Why do grindstones or wheels, in rapid motion, not unfrequenfty 

 'break and fly to pieces with great violence ? 



Because the centrifugal force, generated by the rapid 

 revolving motion, overcomes the cohesion of the parti- 

 cles, and thus causes them to separate and fly from the 

 centre. 



