523. A body unacted upon by force will continue for ever at rest, or
for ever moving uniformly in a straight line. This is asserted by the
first law of motion (Art. 485). It is usual to say that _Inertia_ is
a property of all matter, by which it is meant that matter cannot of
itself change its state of rest or of motion. Force is accordingly
required for this purpose. In the present chapter we shall discuss some
important mechanical considerations connected with the application
of force in changing the state of a body from rest or in altering
its velocity when in motion. In the next chapter we shall study the
application of force in compelling a body to swerve from its motion in
a straight line.
524. We have in earlier lectures been concerned with the application
of force either to raise a weight or to overcome friction. We have now
to consider the application of force to a body, not for the purpose of
raising it, nor for pushing it along against a frictional resistance,
but merely for the purpose of generating a velocity. Unfortunately
there is a practical difficulty in the way of making the experiments
precisely in the manner we should wish. We want to get a mass isolated
both from gravitation and from friction, but this is just what we
cannot do—that is, we cannot do it perfectly. We have, however, a
simple appliance which will be sufficiently isolated for our present
purpose. Here is a heavy weight of iron, about 25 lbs., suspended by
an iron wire from the ceiling about 32 feet above the floor (see Fig.
82). This weight may be moved to and fro by the hand. It is quite free
from friction, for we need not at present remember the small resistance
which the air offers. We may also regard the gravity of the weight as
neutralized by the sustaining force of the wire, and accordingly as the
body now hangs at rest it may for our purposes be regarded as a body
unacted upon by any force.
525. To give this ball a horizontal velocity I feel that I must apply
force to it. This will be manifest to you all when I apply the force
through the medium of an india-rubber spring. If I pull the spring
sharply you notice how much it stretches; you see therefore that the
body will not move quickly unless a considerable force is applied to
it. It thus follows that merely to generate motion in this mass force
has been required.
526. So, too, when the body is in motion as it now is I cannot stop
it without the exertion of force. See how the spring is stretched and
how strong a pull has thus been exerted to deprive the body of motion.
Notice also that while a small force applied sufficiently long will
always restore the body to rest, yet that to produce rest quickly a
large force will be required.
527. It is an universal law of nature that action and reaction are
equal and opposite. Hence when any agent acts to set a body in motion,
or to modify its motion in any way, the body reacts on the agent, and
this force has been called the _Kinetic reaction_.
528. For example. When a railway train starts, the locomotive applies
force to the carriages, and the speed generated during one second is
added to that produced during the next, and the pace improves. The
kinetic reaction of the train retards the engine from attaining the
speed it would acquire if free from the train.