Whenever anything moves,
there's usually some form of friction trying to stop it.
Friction is sometimes useful, at other times it's a problem.
There are two main
types of friction:-
"Static" or "sliding" friction
This type of
friction occurs when dry surfaces rub together.
frictional force depends only on:-
type of surfaces
hard the surfaces are pressed together.
makes this lorry difficult to move, but it does help by giving the
man a good grip on the road.
this diagram, the weight of the block provides the force pressing
the surfaces together. Watch the animation carefully:
If this picture isn't moving, click
we push the block harder and harder, the frictional force will increase,
until it reaches a maximum (in this case, 2.5N).
If we push harder still, (say, 2.6N), the block will start
to move, because we're now pushing harder than the frictional force.
can calculate the maximum frictional force using
- F is the frictional force,
- m is the "coefficient of friction",
a number we use for those particular surfaces,
- N is the "Normal reaction", which is the force pressing
the surfaces together.
This type of
friction is what happens with liquids and gases
(In Physics, liquids and gases are both called "fluids".
They behave in similar ways.)
is also known as "drag". On aircraft it's also called
It depends on:-
thick the fluid is
shape of the object
speed of the object
thin, runny liquid has a low viscosity.
A "viscous" liquid is thick and gooey.
car designers want to reduce drag, so that the vehicle can go fast
without having to waste too much fuel.
To reduce drag,
we need a shape that the fluid can flow past easily and smoothly,
without any swirls (called "eddies"). This tends
to mean using long, pointed, "streamlined" shapes.
When an object
falls, it accelerates. As its speed increases, the air resistance
Eventually, the force from the air resistance
will equal the force from the weight of the object.
At that point, the speed will remain constant: the object has reached
its "terminal velocity" and can't fall any faster.
depends on the drag, so a streamlined shape will fall quickly, whilst
a parachute will fall slowly.
a free-fall parachutist, before the parachute opens, terminal velocity
is around 120 miles per hour. When the parachute opens, the terminal
velocity decreases to only a few miles per hour - a much better
way to hit the ground!
on the graph to find out more.
be a nuisance, because it changes kinetic energy into heat which
is usually wasted. Friction also tends to wear away at the surfaces,
We can reduce
friction by oiling ("lubricating") the surfaces. This
means that the surfaces no longer rub directly on each other, but
slide past on a layer of oil. It's now much easier to move them.
on a cushion of air, which reduces the drag dramatically compared
to the drag on the hull of a ship.
Thus hovercraft can easily achieve much higher speeds than ships.
tables in amusement arcades use the same principle. The puck goes
a long way when you hit it, because there's almost no friction to
slow it down.
In winter sports,
we need friction to be as low as possible so that we can achieve
Ice skaters actually move on a layer of water, and don't skate on
ice at all. When ice is subjected to high pressure
The narrow blades of the skates create a very high pressure and thus the skaters glide along on a layer of
water they've just melted. The water refreezes as soon as they've
This is called "regelation" (sounds
like something that happens to a football team, but it's spelt differently!)
of reducing friction include:-
- using "ball bearings" or
"roller bearings", where balls or rollers allow the
surface to move easily without actually touching each other
- using special
materials, for example, Teflon, which have a very low coefficient
of friction and thus slide easily (Teflon is used in "non-stick"
frying pans for this reason)
use friction to help us grip. This means that our shoes grip the
floor, so we don't fall over.
Right now you're using a mouse, which works because of friction
between the ball and the mouse mat.
If it wasn't for friction between the tyres and the road, driving
a car would be like trying to drive on an ice rink. This would make
cornering and stopping very difficult!
the force to accelerate, stop or change the direction of the car.
Ice and water on the road reduce this friction, and make is easier