Friction
Friction is the force of resistance to motion between two surfaces
moving over each other. Friction only occurs when movement occurs. Friction
will stop motion if the moving force is not strong enough to overcome
the contact between the surfaces.
Friction is caused by tiny imperfections or roughness on surfaces that
prevent or impede movement. Lubricants such as oil fill these gaps so
reducing the effects of friction.
Rub your fingers over your desk top, then over your mouse pad and a
few other objects around you. You will notice that your fingers glide
more easily over some surfaces than others. You experience friction
in everyday situations; sometimes it can be useful and sometimes it
can be a problem.
Friction working for
you
When
you stand up and walk friction is an essential aid. There is friction
between your foot or shoe and the ground. Sometimes the friction is
high, like walking on carpet, and sometimes it is low when stepping
on shiny tiles. Stand up and take a couple of paces, noticing what happens.
You will observe that as you raise one foot and move it forward the
rear foot grips the floor. By this method you are able to push against
the floor surface and move forward confidently.
Now move your computer mouse across the mouse pad. The mouse seems
to ride smoothly, but the ball underneath grips the pad and rolls. This
movement is then translated into a cursor moving across your screen.
Then there is your car. The tyres grip the road because of friction
between the two surfaces and the force of the road on the tyres accelerates
the vehicle forward. The wheels try to spin, but friction between the
two surfaces forces the car to go forward. You will have been very aware
of the importance of friction if you have travelled over ice, or oil,
or when it has rained after a long dry spell and there is tyre residue
on the road! Under the bonnet, fan belts transfer energy because of
friction between rubber and metal. Clutches work by plates coming together
under pressure, and friction between the plates making the gear box
drive the wheels. Of course, friction is put to good use in braking,
where a brake pad is pressed against the disc or drum, slowing the wheels.
Railway engines move by friction between the wheels and the rail. Conveyor
belts transport goods using friction between the rollers and the belt.
Velcro grips work because of the huge frictional forces between the
hooks and rings on the velcro. There are many more examples of the importance
of friction in everyday situations.
Friction working against you
Rub your hands together and notice that friction results in a temperature
increase. Friction is sometimes a problem because it can cause a great
deal of wear and tear. You have probably noticed this on clothes and
furniture where there is rubbing, and even the tyres on your car wear
because there is always a small amount of slip when turning corners
or sliding on slippery roads.
Car engines have closely fitted parts that move quickly. For example,
piston rings in a cylinder travel at several thousand revolutions a
minute. In this case, the engine would get very hot, and finally seize
up, because of friction. That's why it is essential to have oil in the
engine to get between the moving parts, reduce friction and lower the
overall engine temperature.
A similar situation occurs with metal hinges that squeak. The sound
is a caused by vibrations as friction stops the hinge from swinging
freely, and again it can be reduced by oil in the parts that touch.
Air resistance as friction
Whenever an object moves through air it experiences friction in the
form of air resistance. Air resistance can be both helpful and a hindrance.
Meteoroids burn up in the atmosphere due to friction between themselves
and the air around them. Thank goodness, because if they did not burn
up, the Earth would be continually peppered with rock from space!
Aeroplanes can be controlled in flight because of air
resistance. The balance between the thrust of the engines and drag from
air resistance allows the pilot to gain or reduce altitude and speed
as well as maneuver in the air. (See Forces
in mechanical flight)
On the down side, the temperature increase caused by friction on
space shuttles as they enter the atmosphere could cook the crew if left
unchecked. The shuttle is covered in smooth ceramic tiles which act
as heat insulators, as well as reducing the air resistance of the craft.
Air resistance causes friction and slows a car, for example,
by causing drag. Even when driving along a straight flat road, the car's
engine must be kept going to overcome the drag of the air to keep the
car moving at a constant speed.
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