198 LECTURES. 



(40.) That which tends to produce change, or prevent motion, is 

 called a force, 



Or wliatever causes a body to exist under a given condition, or 

 whatever changes any of its relations, is called Si force. 



The muscular exertion of animals, the unbending of a bow, the 

 impulse of a moving body, are instances of active force. The resist- 

 ance of a rope which suspends a body, of a table which supports a 

 weight, are examples offerees which tend to prevent motion. 



Our idea of force is derived from the muscular effort required to 

 produce the motion of a mass of matter. 



The original meaning of the word was muscle or tendon. [Whe- 

 well.] It becomes metaphorical when applied in any other case, and 

 we must not, therefore, imagine that force is always connected with 

 labor or difficulty. 



Force which is capable of doing work_, that is of transforming mat- 

 ter is called power or energy. [See mechanics.] 



(41.) In all cases of the change of the state of a body in reference 

 to rest or motion, we can attribute this change to an extraneous force. 



The spontaneous motions of animals arc ascribed to vitality. 



The fall of a stone to the action of the earth. 



Two kinds offeree, Impulsive and Incessant; an incessant force may 

 be either Accelerating or Eetarding. Examples. 



(42.) Force is measured by its effects. 



We usually call that a double force which produces a double velocity 

 in the same mass, or 



/ : F : : v : y 



We also call that a double force which produces the same velocity 

 in double masses, or 



/ : F : : m : M 



When the velocities and masses are both unequal, the forces are 

 measured by the product of the velocities into the masses, or 



/ : F' : : mv : MV 

 The force proportional to the product of the mass into the velocity 

 is called the quantity of motion or momentum. 

 An incessant force is measured by the relation 



V 

 FT=V or F=— . (2.) 



T 

 (43.) It must be observed that the relations here given are the re- 

 sults of experience. We know nothing of force but by its effects, and 

 in some cases we are obliged to adopt the relation 



/ : F : : 'ys : V2 



Force is also sometimes measured by pressure. 



The laws of force and motion will be fully developed under the head 

 of Mechanics. 



(44.) Illustrations of the foregoing principles. 



Tendency of bodies to remain at rest. Wood split by the inertia 

 of an iron wedge. 



Tendency of bodies to continue in motion. The inertia in this re- 



