LESSONS IN PHYSICS. 45 



carry the body to the same point as the two forces AB and BC, 

 acting separately or together. The separate forces, represented 

 by the lines AB and BC or AD, are called components and the 

 single force, represented by the line AC, their equivalent, is called 

 the resultant. The process of finding the resultant of two or 

 more forces is called the composition of forces. A resultant may 

 be made up of, and equivalent to, more than two forces. Suppose 

 that when the body is at C it is acted on by a force represented 

 by CF sufficient to carry the body to F. The forces AB, BC and 

 EF, acting successively, have carried the body to F. In the 

 former case we found that the force represented by AC equaled 

 the forces represented by the lines AB and BC ; now if we com- 

 plete the parallelogram on AC and CF we find that a force 

 represented by AF will carry the body to the same point as the 

 forces represented by AC and CF, acting separately, and conse- 

 quently to the same point as all three forces represented by AB, 

 BC and CF would carry it if acting separately, or as AB, AD, 

 which equals BC, and AE, which equals CF, would carry it if 

 acting together. In this way a resultant may be found for any 

 number of component forces. 



If two or more parallel forces act on a body, at the same time, 

 and in the same direction, their resultant would be a force equal 

 to the sum of the component forces, and the motion would be 

 in the same direction. A span of horses drawing a wagon illus- 

 trates this principle. If two parallel forces act on a body at 

 different points, and in opposite directions, they will produce 

 rotary motion. No single force can replace two such forces. 



If we have a force represented by AF ( Fig. 1 ) , we may find other 

 forces that would produce the same effect. AC and CF together 

 would equal AF. AB and BC would equal AC, and AB and BC 

 and CF together equaling AF, reversing the process of composi- 

 tion offerees. This process of finding two or more forces that 

 are equivalent to one force, is called the resolution of forces. 



The study of motions in straight lines is interesting and im- 

 portant, but most of the motions in nature are along curved 

 lines ; so that the study of motion in curved lines will be as prac- 



