174 EXPERIMENTAL GENERAL SCIENCE 



course by the string, it takes a circular path. When a wheel 

 or other object is rotating, there is always present this tendency 

 for each particle to fly off into space. The particular force 

 which causes this is called the centrifugal force. Any force 

 which tends to pull the particles of a moving body toward the 

 center of a circle is called the centripetal force. In all cases 

 where a force acts, there is a reaction equal to the action. 

 A moving body may be stopped only when the resistance to 

 its progress is equal to its momentum. If the resistance is 

 ^greater than the momentum, as when we strike a moving ball 

 with a bat moving in the opposite direction, a new motion 

 may be set up which is due to the excess momentum from the 

 bat which is now imparted to the ball. 



144. Friction. The resistance which moving bodies meet 

 in rubbing past other bodies is called friction. This resist- 

 ance is due to th fact that surfaces which appear smooth are 

 never entirely so. Even polished surfaces, such as glass and 

 metal, appear minutely roughened when viewed with a micro- 

 scope. When two surfaces are in contact, their irregularities 

 fit into one another and thus develop resistance to the passage 

 of the one over the other. In practice, it has been found that 

 a given body will often move over a body of an entirely 

 different substance more readily than over another body like 

 itself. This is because different substances may have differ- 

 ent irregularities in them and therefore fail to completely 

 interlock. For this reason, the journals in which steel shafts 

 turn are often made of brass or babbitt metal. Owing to the 

 nature of the motion, rolling friction, as when a ball or wheel 

 rolls on a surface, develops less resistance than sliding friction 

 in which one surface simply slides over another. The effort 

 needed to overcome friction at starting is much greater than 

 is required to keep the body moving after once started. In 

 either case, however, the friction is proportional to the pres- 

 sure; the heavier the body to be moved, the greater the effort 



