46 FORCES ACTING ON A SINGLE PARTICLE 



Reaction between Two Bodies 



40. A second way in which force can be applied to a particle is 

 by the pressure between the particle and the surface of a solid 

 body. Such a force is commonly spoken of as a reaction. 



A body standing on the floor of a room is acted on by its 

 weight acting downwards, but is kept at rest by the action of a 

 second force acting upwards from the floor; this is the reaction 

 between the body and the floor. Clearly, in order that the body 

 may rest in equilibrium, the reaction in this case must be equal to 

 the weight of the body and must act vertically. 



FRICTION 



41. Imagine a small body standing on a plane of which the 

 slope can be varied, such as the lid of a desk. If the plane is held 

 horizontally, the body can stand at rest as already described. Let 

 the plane be gradually tilted, and it will be found that as soon 

 as the tilting reaches a certain angle the body will begin to slide 

 down the plane. The angle at which sliding first occurs is found 

 to be different for different pairs of substances ; thus for wood 

 sliding on wood it may vary from 10 to 25, for iron on wood it 

 varies from 10 to 30, while for iron sliding on iron it is only 

 about 10 or 15. 



When two substances are such that this angle is zero, i.e. such 

 that one can only rest on the other when the surface of contact is 

 perfectly horizontal, then the contact between them is said to be 

 perfectly smooth. The nearest approximation to a perfectly smooth 

 contact which we experience in actual life is probably that of steel 

 on ice, as in skating. 



f" It is found that the angle to which a plane made of one sub- 



j stance has to be tilted before a second substance begins to slide 



< on it is independent both of the amount of the second substance 



I and of the area in contact. 7 Thus the angle depends only on the 



nature of the two substances in contact. 



a ~ 



