76 ELEMENTARY GENERAL SCIENCE CHAP. 



EXPT. 62. Attach an additional weight to the string, which 

 is fastened to the centre of the rod, and also weights to the 

 ends, so that the rod is in equilibrium. Show that the pull 

 on the balance is equal to the difference of the forces which 

 act downwards and upwards respectively. 



Principle of Parallel Forces. We are now in a position to 

 state definitely the principle of parallel forces demonstrated by 

 the foregoing experiments. It may be expressed as follows : 

 " The resultant of a number of parallel forces is numerically 

 equal to the sum of those which act in one direction, less 

 the sum of those which act in the opposite direction." In 

 other words, we may say, the resultant is equal to the algebraic 

 sum of the forces 



If two equal parallel forces act in the same direction upon a 

 body, the total force will be obtained (as might be expected) by 

 adding the two individual forces together. In like manner, if 

 two unequal parallel forces act in opposite directions the net 

 effect will be found by subtracting the lesser of the two forces 

 from the greater, and the direction of the resultant will be that 

 of the greater force. 



Eesultant of Parallel Forces. It has been shown experi- 

 mentally that the resultant of a system of parallel forces is 

 equal in magnitude to the algebraic sum of the forces ; the 

 direction of the resultant is the same as that of the greater of 

 the parallel forces. This is illustrated by Expts. 61 and 62 ; 

 for while the forces due to the weights 011 the rod act vertically 

 downwards, the resultant, represented by the pull of the 

 spring balance, acts vertically upwards. The position which 

 the resultant occupies with reference to the component forces 

 can also be shown by the same experiments, or by the following 

 one. 



EXPT. 63. Using the arrangement described in Expt. 62, 

 suspend different weights from the rod and then move the 

 spring balance along the rod to a point about which the 

 weights counterpoise one another. Repeat the experiment 

 with the weights at unequal distances from the balancing 

 point, and observe in each case (a) the reading of the spring 

 balance, (6) the distance of each weight from the point at 

 which the spring balance is attached to the rod. Record as 

 below and compare columns 5 and 6. 



