FORCES AND MOTION. | 7 
body moves, the longer it continues to move. We also know that when 
a body, a pendulum for example, is set in motion in a place from 
which the air has been removed, it continues to move for a very long 
time. It may therefore be inferred, that if friction and the resistance 
of the air could be removed altogether, a body would, if once set in 
motion, continue to move for ever. 
Forces and Motion. 
It has been shewn in the definition of Inertia that a body, whether at 
rest or in motion, cannot ‘self alter that state. In order to do so, 
some external cause is necessary. Such a cause is called a Force. Force 
is thus intimately connected with motion ; for, if a body be at rest, a 
force is necessary to set it in motion ; and, if it be in motion, a force is 
necessary to bring it to rest. The principles connecting force and 
motion have been expressed in three laws, called the Laws or Morton. 
First Law of Motion.—The first law of motion is simply a more precise 
definition of Inertia. A body will remain either in a state of rest, or in 
a state of uniform motion in aw straight line, unless compelled to change 
that state by some external force. 
The next consideration that naturally occurs is: How does this change 
of state depend on the force that produces it? The answer to this 
question is a statement of the second law of motion, which is as follows: _ 
Second Law of Motion —When a body is in motion under the influence 
of any number of forces, each force produces the same effect as 1t would if the 
other forces were not acting. 
Suppose a number of forces, 
P, Q, BR, and 8, to be acting 
on a body B; each of these 
forces, if it were acting: by 
itself, would move the body 
a certain distance, propor- 
tioned to its stréngth, and 
in its own direction. Let 
the lines BA, BC, BD, and 
BE represent the forces P, 
Q, R, and S in direction 
and magnitude, If P alone 
were acting on the body, it would be moved to A; if, then, Q were to act 
on it, it oui move along AF, parallel to BC, and to a distance from A 
equal to BC. Let AF be neal to BC; and if, when the body was at F, it 
were acted upon by R, it would move from F to G, FG being equal and 
parallel to BD; and so from G to H, GH being equal and parallel to BE, 
If acted on by the single forces in succession, then, the body would arrive 

Fig. 1. 
