28 



FORCE AND ENERGY 



6. What work was done upon the water of a waterfall to give it 

 energy? 



7. What device is used to prevent a railway train from leaving the 

 track when rounding a curve? Is any similar device used in a gym- 

 nasium? On an automobile or motorcycle race course? 



30. Cohesion and Adhesion. If we hold a sheet of 

 glass face down against the surface of some water, and 

 then pull the glass away from the water, we must use 

 more force than is needed to lift the glass against gravity 

 alone. Another force is being exerted on the glass. The 

 under side of the glass will be wet. This shows that in 

 pulling the glass away we did not separate glass from 

 water, but water from water. We therefore did work 



(cf. 25) against the force that 

 holds the water together. The 

 force that is exerted between 

 the particles of matter is called 

 cohesion. 



We can find the amount of cohesion, 

 in the case of water, by attaching a 

 piece of glass by means of strings to 

 one arm of a balance (Fig. 22), allowing 

 the glass to touch the water, and then 

 adding weights to the other side of the 

 balance until we tear the glass away. 



Of course we must subtract the weight of the glass from the total 



weight to get the ''breaking weight" of the water. 



When cohesion is exerted between different substances, 

 we call it adhesion. Thus cohesion holds water together, 

 and cohesion holds glass together, but adhesion holds 



FIG. 22. 



Measuring the Force Needed to 

 Tear Water from Water. 



