149. The importance of friction in mechanics arises from its universal
presence. We often recognize it as a destroyer or impeder of motion,
as a waster of our energy, and as a source of loss or inconvenience.
But, on the other hand, friction is often indirectly the means of
producing motion, and of this we have a splendid example in the
locomotive engine. The engine being very heavy, the wheels are pressed
closely to the rails; there is friction enough to prevent the wheels
slipping, consequently when the engines force the wheels to turn round
they must roll onwards. The coefficient of friction of wrought iron
upon wrought iron is about 0·2. Suppose a locomotive weigh 30 tons,
and the share of this weight borne by the driving wheels be 10 tons,
the friction between the driving wheels and the rails is 2 tons. This
is the greatest force the engine can exert on a level line. A force of
10 lbs. for every ton weight of the train is known to be sufficient
to sustain the motion, consequently the engine we have supposed should
draw along the level a load of 448 tons.
150. But we need not invoke the steam-engine to show the use of
friction. We could not exist without it. In the first place we could
not move about, for walking is only possible on account of the friction
between the soles of our boots and the ground; nor if we were once in
motion could we stop without coming into collision with some other
object, or grasping something to hold on by. Objects could only be
handled with difficulty, nails would not remain in wood, and screws
would be equally useless. Buildings could hardly be erected, nay, even
hills and mountains would gradually disappear, and finally dry land
would be immersed beneath the level of the sea. Friction is, so far
as we are concerned, quite as essential a law of nature as the law of
gravitation. We must not seek to evade it in our mechanical discussions
because it makes them a little more difficult. Friction obeys laws;
its action is not vague or uncertain. When inconvenient it can be
diminished, when useful it can be increased; and in our lectures on the
mechanical powers, to which we now proceed, we shall have opportunities
of describing machines which have been devised in obedience to its laws.
LECTURE VI.
_THE PULLEY._
Introduction.—Friction between a Rope and an
Iron Bar.—The use of the Pulley.—Large and
Small Pulleys.—The Law of Friction in the
Pulley.—Wheels.—Energy.