24 FORCE AND ENERGY 



(1) Muscular force, such as that exerted by draught 

 animals and man. 



(2) Gravity. 



(3) The force of the wind. 



(4) Resistance, such as that of air, water, the ground, 

 or the parts of machinery. We commonly call this 

 friction (cf. 206). 



(5) The force of expanding gases. 



Of course one body may exert force upon another and 

 yet not move it, as when you try to lift a weight too 

 heavy for you. 



25. Work and Energy. When we raise a hammer 

 into the air, we do work upon it, for we lift it against grav- 

 ity. We also do work when we throw a ball, or wind up 

 a watch. In the last case we produce motion against the 

 elastic force of the spring. Because we can do work, we 

 say we have energy. Energy is the capacity for doing 

 work. A body that can do work upon another body 

 possesses energy. 



The lifted hammer has energy because we have done 

 work upon it, and it, in its turn, can do work upon some 

 other body. If we let it fall, it can break a nutshell or 

 drive a nail into wood. While poised in the air, the 

 hammer has energy because of its position; while it is de- 

 scending it has energy of motion. In the same way, the 

 water of a waterfall has energy of position at the top of 

 the fall, but energy of motion as it descends. 



A bullet in a gun at the instant of discharge can be said to have 

 energy of position because of the compressed gas behind it. In its 

 flight it has energy of motion. Vv n^n it strikes a rock or a tree or other 



