CHAPTEE V 

 STATICS OF RIGID BODIES 



ElGIDITY 



62. If we press a lump of wet clay or of soft putty with the 

 finger, we find that a dent is left in the clay or putty ; the force 

 applied to the substance by the finger has caused it to change its 

 shape. If we press a mass of jelly with the finger, we do not find 

 any dent left in the jelly, but we notice that so long as the force 

 is applied the shape of the jelly is altered, although it returns to 

 its original shape as soon as the pressure is removed. 



On the other hand, if we press a lead bullet or an ivory billiard 

 ball with the finger, we do not notice any change of shape either 

 while the pressure is applied or after. In ordinary language we 

 say that the lead and ivory are harder than the clay and putty; 

 in scientific language we say that they are more rigid. 



63. A perfectly rigid body would be one which showed no 

 change of shape under any force, no matter how great this force 

 might be. A bullet and a billiard ball are not perfectly rigid, 

 billiard ball is pressed out of shape during the interval of collisioi 

 with a second billiard ball, but regains its shape immediately, 

 while a lead bullet is pressed permanently out of shape by striking 

 a target. A perfectly rigid body does not exist in nature, althougl 

 such bodies as a billiard ball or a bullet may be regarded as per- 

 fectly rigid, so long as the forces which act upon them are n< 

 too great. 



We can give a mathematical definition of a perfectly rigid bod] 

 as follows: 



A body is perfectly rigid when the distance between any pail 

 of particles in it remains unaltered, no matter what forces act 

 the body. 



90 



