50 MOTION AND FORCE 



SECTION II 

 SOME EFFECTS OF NEWTON'S LAWS 



59. Inertia. The tendency of a body to remain in 

 its state of rest or motion has been called inertia ( 56). 

 Owing to its inertia, a body acted upon by force gener- 

 ally starts slowly, increasing its speed as long as the 

 same amount of force acts. We have often received a 

 heavy jarring as a car started violently from a state of 

 rest ; this is because the back of the seat runs into us 

 before the body has begun to move. 



Experiment 42. Balance a visiting card on the end of the 

 finger and place a coin upon it, directly above the finger tip. 

 With the other hand suddenly snap the card away edgewise. 

 After a bit of practice, this may be done so as to leave the coin 

 upon the finger. Why does not the coin move off with the card? 

 Slowly push the card off the finger. Note and explain any dif- 

 ference in the behavior of the coin. 



60. Momentum Inertia also causes moving masses 



to continue in motion. But as all moving bodies on 

 earth are verjr soon acted upon by at least one force 

 tending to stop them, it is clear that the ability of any 

 body to keep on moving will depend upon its ability to 

 overcome opposing forces. And this in turn depends' 

 upon what may be called its " quantity of motion " 

 or momentum. A thrown ball, for example, is set in 

 motion by force exerted upon it at the hand; once 

 out of the hand, its progress depends upon its momen- 

 tum, that is, the quantity of motion given to it by 

 the arm. 



