Common Science 



FIG. 24. Hockey is a fast game because there is little friction between the skates 



and the ice. 



friction between the wheels of a car and the track they 

 are pushing against, or the wheels would whirl around 

 and around uselessly. 



But we can increase or decrease friction a great deal. 

 If we make things rough, there is more friction between 

 them than if they are smooth. If we press things 

 tightly together, there is more friction than if they 

 touch lightly. A nail in a loose hole comes out easily, 

 but in a tight hole it sticks ; the pressure has increased 

 the friction. A motorman in starting a trolley car 

 sometimes finds the track so smooth that the wheels 

 whirl around without pushing the car forward; he 

 pours some sand on the track to make it rougher, and 

 the car starts. When you put on new shoes, they are 

 so smooth on the bottom that they slip over the ground 



