NEWTON'S THREE LAWS OF MOTION 47 



not act ? Will an engine begin to run without water in 

 its boiler and heat under it ; or an electric motor without 

 its current of electricity? Can even an animal move 

 itself without the energy supplied by food and air ? In 

 every case we should find that, if we knew enough about 

 it, we could trace any motion to some outside cause. 



Nor is it any easier to find a moving body which stops 

 without force being used. Many bodies may seem to do 

 so; but is it not, after all, the force of gravity which 

 stops a rolling ball, a bullet, or other such moving 

 body? Unfortunately we cannot, upon earth, find ex- 

 amples of constant motion without the action of force, 

 because all motion (except downward) will be stopped 

 by gravity if not by other forces ; but doubtless many 

 stars and planets are in constant motion simply because 

 there is no force to stop them. 



After all, this law merely states that no body of itself 

 can alter its state of rest or motion, and that is not very 

 odd. It would be far more strange if things could start 

 or stop their own motion without force being exerted. 

 This helplessness of matter is called inertia. 



57. The Second Law. The first part of this law 

 ( 55) may easily be understood any moving body 

 will go in the same direction that the force takes. 

 Strike a nail with a hammer and the nail moves on as 

 the hammer was moving. If two or more forces act 

 upon a body at the same time, the effect of each force 

 appears in the resulting motion. 



Experiment 41. At the same instant strike a ball a (Fig. 30) 

 with two mallets in two directions, ab and ac. One blow alone 

 would carry it to the right as far as 6j the other alone would send 



