MOTION. 4I 



But you may say a body will stop itself. Your ball on the ground, or 

 even upon ice, will eventually come to a stop. We fire a bullet, and it will 

 stop in time. We reply it does not stop of itself The resistance of the Air 

 and Friction tend to bring the body in motion to a state of rest. In the case 

 of a bullet gravity brings it down. 



There is no need to insist upon the resistance offered by the air even 

 when it is not rushing violently past to fill up a vacuum beyond us, and 

 called a breeze, or high wind. But we may say something of Friction. 



Friction is derived from the Latin frico, to rub, and expresses the 

 resistance to motion which arises from uneven surfaces. It is a passive 

 resistance, and depends upon the force which keeps the bodies together. 

 Thus a train running upon a smooth iron rail would never be able to proceed 

 but for friction, which gives the necessary purchase or grip to the wheel and 

 rail in contact. 



No surface is perfectly smooth, for we must push a body upon the 

 smoothest surface we possess. Friction tends to resist motion always, and is 

 the cause of a great loss of power in mechanics, though it is employed to 

 stop motion by certain appliances, such as " breaks " and " drags," for sliding 

 friction is greater than rolling friction. But without friction most structures 

 would fall to pieces, and all forward motion would cease. So though it is 

 an inconvenient force to overcome, we could not do without it. 



If a body is set in motion, we see that the tendency of it is to go on for 

 ever. Such, indeed, is the case with the stars ; but so long as we are within 

 the influence of the earth's attraction, we cannot expect such a result. We 

 know now what motion is ; we must also, to understand it perfectly, consider 

 its direction and its velocity. 



The line which indicates the way from the starting point to the end is 

 the direction of the object in motion, and the rate it moves at its velocity. 

 The latter is calculated at so many miles an hour, as a train ; or so many 

 feet in a second if the object be a shot, or other very rapidly-moving body. 

 In equal velocity the same distance is traversed in the same time ; and so if 

 a train run a mile in a minute, we know it will travel sixty miles in an hour, 

 and is therefore during that minute going at the rate of sixty miles an hour. 

 We have already spoken of the velocity of a stone falling from a cliff as 

 sixteen feet in a second, and a stone thrown into the air to rise sixteen feet 

 will be a second in going up, and a second in descending. But the velocity 

 will be accelerated in the descent after the first second of time, and retarded 

 in the upward cast by gravity. So we have two terms accelerated and 

 retarded velocity used to express an increased or decreased force of attrac- 

 tion. 



Perpetual motion has often been sought, but never discovered, nor will 

 it ever be till the elixir of life has been found. It is quite impossible to 

 construct any machine that will work without friction ; if any work be done 

 energy will be expended and transformed into other energy, so the total 

 must be diminished by so much as was employed to transform the remainder. 



