174 ELEMENTS OF GENERAL SCIENCE 



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 

 board, 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 a skate which 

 slides on ice, the friction is very small, but it is not wholly 

 wanting. 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 objects. 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 fric- 

 tion, but rolling friction is very much less than sliding fric- 

 tion. The boy would therefore place rollers under the boat 

 in order to reduce the friction as much as possible. He 

 could not get rid of friction entirely. 



We may speak about perfect machines (machines without 

 friction) and calculate just what results might be obtained 

 from such machines, but we must always remember that no 

 frictionless machine can be constructed. 



193. Some facts about pulleys. Questions must certainly 

 arise regarding the advantage of using rope and pulleys 

 either in moving a boat or for other purposes. Anyone who 

 has observed the use of pulleys, as when a building is being 

 moved through the street or when a piano is being moved to 

 an upper floor of a house, has probably noted that the load 

 is moved more easily by their use. It is also noted that the 

 load is not moved rapidly. Ability to move a greater load is 

 an advantage, but the slow motion is often a disadvantage. 

 It is important for practical use of pulleys that one should 

 know precisely the relation between the force exerted and the 

 load moved, as well as understand the accompanying rate of 



