Friction is the resistance one body shows to
another which is sliding over it. When a wheel turns stiffly its bearings, it
means that there is friction between the two surfaces. Friction is very strong
between rough metal surfaces and so lubricating oil is used to separate them.
If a drop of oil is put between the rough surfaces it soon spreads into a
smooth, thin film that separates them and so most of the fiction disappears.
There is friction in water and even in
travelling through air. The surfaces of an aircraft flying at very high speed
have to be of a streamlined form, polished and absolutely smooth to prevent
them getting too hot, and to reduce the waste of engine power. One may be able
to turn a well-oiled wheel quite easily, but if its bearings are rusty, there
may be so much friction that he can hardly move it, however hard he tries.
Normally a considerable part of the effort is
wasted in overcoming friction. We never obtain the full benefit that would
otherwise be obtainable if the wheels were well oiled. If the bearings of an
engine are not oiled. The turning parts may get so hot that they expand and
become too big for their bearings. They then lock tightly into the bearings and
the engine seizes up. Hence all the extra energy exerted to turn the engine is
lost fruitlessly because it is changed into heat.
One way to reduce the effects of friction is to
use roller or ball bearings. A ring of hard steel balls is inserted between a
wheel and its axle, so that there is no direct friction between the wheel and
the axle, and there is hardly any rubbing between metal surfaces at all.
But friction can be very useful in railway,
motor - car and bicycle brakes. The blocks of a brake are made of materials
that are both rough and tough and when they are pressed firmly against the rim
of a wheel they produce enough friction to bring the wheel to a stand still.
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