60 ELEMENTARY GENERAL SCIENCE ciiAr. 



from which equation the third quantity can be obtained when- 

 ever we know the other two : 



Number of _ Number of Number of units 

 units of force ~ units of mass of acceleration. 

 F = ma (1) 



."'. a = - (2) 



in 



.'. m = ?- ...(3) 



The second equation can be expressed in words by saying that 

 the number of units of acceleration produced in the velocity of a 

 moving body is equal to the number of units of force acting upon 

 it, divided by the number of units of mass on which it acts. 



Similarly, the third expression means that the number of 

 units of mass in a moving body can be calculated by dividing the 

 number of units of force acting upon it by the number of units 

 of acceleration produced in it. 



The second equation tells us, moreover, that if the acceleration 

 produced in amoving body remains the same, or is uniform, that 

 the value of the force, or the number of units of force it contains, 

 must be the same throughout, or what is the same thing, the 

 force is uniform. 



The most important facts with reference to motion and force 

 have been described in the foregoing parts of this chapter from 

 the point of view of momentum. At this point the first law of 

 motion may be usefully introduced. 



Newton's First Law of Motion. Every body will continue 

 in its state of rest or of uniform motion in a straight line, 

 except in so far as it is compelled by impressed force to 

 change that state. 



This statement is called a law of motion, and it will be well to 

 make clear at once that a natural law is only an expression of 

 what has been found to always be the rule ; it is merely setting 

 down the result of experience ; and the idea of the word in its 

 legal sense must be carefully excluded from the mind. Indeed, 

 in nearly every case, it is better to substitute, at all events in one's 

 mind, the word tide. It is only a statement that certain things 

 always seem to take place ; it tells us nothing about why they do 

 so ; nor is the idea of compulsion included at all, for oftentimes 

 so-called "laws" have been formulated which have turned out 



