46 INERTIA. 



rapid motion which the paper now has, either a greater 

 force would have been required than that which friction 

 exerts in this case, or the mass of the body must be 

 diminished, If a piece of wood were glued to the papar, 

 or a flat piece of cork be placed upon it, that is, if either 

 the force which acts upon the body be increased, or its 

 mass diminished, it will readily follow the quicker 

 motion of the paper. 



A. body at rest offers resistance to being set in 

 motion ; this resistance is greater the greater the mass 

 of the body. Conversely, a moving body offers resist- 

 ance to being brought to rest; this resistance is also 

 greater the greater the mass of the body, but besides, it 

 is also greater the more rapid the motion. This property 

 of bodies, to resist changes of state, is called their inertia. 

 If inertia, or any other resistance, is overcome, mechani- 

 cal work is performed. To put a ball into motion, a 

 certain amount of work must be done. The ball takes 

 Tip the work, which becomes as it were stored or 

 accumulated in it, and enables the ball itself to perform 

 work, to overcome resistances. The work accumulated 

 in the ball is, when it rolls along, soon exhausted by 

 having continually to overcome the resistance of friction, 

 but if it should strike upon another ball, a great por- 

 tion of the accumulated work will be at once expended 

 in overcoming the inertia of the second ball, thai 

 is, in setting it in motion. The greater the violent 

 of the impact, or the greater the amount of accumulatec 

 work in the first ball, the greater will be the velocity 

 with which the second ball will begin to move. 



A few simple experiments will illustrate these facts. 

 Place a number of wooden draughtsmen upon each 



