44 



PRACTICAL LESSONS IN SCIENCE. 



The third law is that action and reaction are equal and in op- 

 posite directions. If the hand strikes the wall, the wall reacts on 

 the hand and the hand is bruised ; the bullet that shatters the 

 bone is itself bruised and flattened. The fact of reaction seems 

 evident, but careful experiments are necessary to show that 

 action and reaction are equal. 



A constant force acting alone causes uniformly accelerated 

 motion, in a straight line, as the motion of a body falling by the 

 force of gravity. Experiments show that a falling body passes 

 over a space of about 16^ feet in the first second, over three 

 such spaces in the next second, over five the third, and so on. 

 The distance a body falls in a given number of seconds, equals 

 the square of the number of seconds multiplied by 16 T L feet. 

 The distance a body will fall in four seconds may be shown as 

 follows: 4 X 4 = 16 X 16^ = 257.25 feet. In the case of falling 

 bodies the motion is due to the action of the one force of gravity 

 retarded somewhat by the resistance of the air, but most of the 

 motions met with in nature are the result of the combined action 

 of several forces. 



If we represent the intensity and direction of forces by lines, we 

 can more easily solve some of the problems of motion. Suppose 



a body at A (see Fig. 1) is acted upon 

 by a force, whose intensity and direc- 

 tion is represented by the line AB, 

 sufficient to carry the body to B. At 

 B the body is acted upon by a force, 

 represented by the line BC, sufficient 

 to carry the body to C. The forces 

 AB and BC, acting separately, have 

 carried the body to C over the lines 

 AB and BC. But if the two forces 

 AB and AD, which equals BC, should act together on the body at 

 A, they would carry it to C, as before, but along the line AC, not 

 along the lines AB and BC, as in the former case. This illustrates 

 the truth of the second law of motion, and shows that one force 

 whose intensity and direction is represented by the line AC would 



C 



Fio.l 



