MATTER AND FORCE 39 



and see if it moves of itself. What is a reasonable inference 

 concerning the ability of a body to move itself? When a 

 body is found moving, what may be assumed as cause of its 

 motion ? [In the study of Physics phenomena due to any 

 " life " in matter are not considered. Bodies are supposed 

 to be lifeless.] 



Hold a pencil out at arm's length, and then let go of it. 

 When does it stop falling? Infer if it could have stopped 

 itself when falling, all other bodies being out of the way. 

 The tendency of a body when in motion to continue moving, 

 and the inability of a body to set itself in motion, constitute 

 together the characteristic of all matter known as inertia. 

 Formulate a definition for this property. 



4. Stretch a rubber eraser ; then bend it, and then twist it. 

 What holds the particles (molecules) of the eraser together, 

 resisting their separation? Are the parts held together 

 alike in nature, or unlike? Define cohesion. What force 

 was applied in these cases to change the form of the eraser ? 

 Where a body subjected to any force has its form changed 

 rather than its position, i.e., where it resists being moved, 

 it is said to be under strain. Where the body resists being 

 pulled apart it is under tension; when twisted, it suffers 

 torsion; when bent, flexion. When the parts of a body 

 are pressed together it is said to be under a strain of com- 

 pression. Formulate definitions for these four forms of 

 strain. 



The force which causes any of these forms of strain in a 

 body is spoken of as a stress, regardless of what force it may 

 be. Stress and strain are thus correlative terms, and the 

 use of one of them implies the other. 



