178 



ELEMENTS OF GENERAL SCIENCE 



Q189. Decreasing friction by the use of rollers. 

 Everyone knows that if he were to attempt to 

 slide the boat along the board he would find a 

 great deal of resistance. Even if the boards were 

 horizontal instead of sloping up the beach we 

 should find that it would take a great deal of 

 force to move the boat. We also 

 know that this resistance, which would 

 be found if any attempt were made 

 to slide the boat over the boards, is 

 found whenever one surface is moved 

 in contact with another surface, and 

 that this resistance is called friction. 

 If the surfaces are very smooth, as 

 in the case of an ice skate, the fric- 

 tion is very small, but it is not wholly 

 absent. Friction is due mainly to the 

 fact that surfaces are never perfectly 

 smooth and the little irregularities tend 

 to catch on each other and thereby 

 resist the force which is moving the 

 object. The friction on the board 

 would be so great that there could 

 be little hope of moving the boat. 

 When one object rolls over another, 

 there is also some friction, but rolling 

 friction is very much less than slid- 

 ing friction. The boy should therefore 

 place rollers under the boat in order 

 to reduce the friction as much as 

 possible, but he could not get rid of 

 friction entirely (fig. 90). 



We may speak about perfect machines 

 (machines without friction) and calculate just what results 

 might be obtained with them, but none such exist. 



FIG. 91. Pulleys and cords 



The pull on the cord is meas- 

 ured hy the spring balance. 

 The weight of the load is 

 divided between strands of 

 cord by which it is suspended . 

 Disregarding friction, the pull 

 on the spring balance will be 

 one sixth as great as the pull 

 exerted by the load 



