CHAPTER V. 



FORCES AND MOTION. 



THE forces which cause all the variety of motions in nature are 

 but different forms of attraction and repulsion. A constant force 

 is one that acts continually, as the force of gravity, while an 

 impulsive force is one that acts for a time, then ceases, as the 

 blow of a hammer, or the stroke of a bat that drives the ball. 



A unit of time is a measured portion of time, as a second, a 

 minute, or an hour, and velocity is the distance passed over by 

 a body in a unit of time, as the train moves 40 miles an hour. 

 The velocity of a body may be uniform, increasing, decreasing, 

 or variable. 



Space is the whole distance passed over by a body while in 

 motion. It equals velocity multiplied by the units of time, as 

 the velocity of a train is 40 miles an hour, units of time are 10 

 hours, then space equals 400 miles, the distance passed over by 

 the train. Motion will be uniform when the velocity of the body 

 is the same in successive units of time, and will be accelerated 

 when the velocity increases in successive units of time, and re- 

 tarded when the velocity decreases. Accelerated and retarded 

 motions may be uniform or variable. 



The first law of motion is that a body at rest will remain at 

 rest, or if in motion, will continue in motion, unless acted upon 

 by some force to change its condition. This is practically the 

 law of inertia. The second law is that a given force will produce 

 the same amount of motion whether it acts upon a body at rest 

 or upon a body in motion. This law is illustrated by the motion 

 of a boat in crossing a river; the steam drives the boat across 

 the river while the current carries it down the river, and under 

 the action of both forces the boat moves across the stream in a 

 diagonal course. 



(48) 



