Frictionandmotion
4
●Friction: pros and cons
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As you have learned, moving objects often slow down because there is
a force acting on them. The force is acting in the opposite direction
to the way the objects are moving. This force is called friction.
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Friction is a force that tends to stop two things from sliding over each
other. Sometimes friction is useful:
Car tyres can push against the surface of the road, so the car can
move forward.
● Brake blocks can squeeze against a bicycle wheel rim, so that the
bike slows down.
● Shoes can grip the floor, so that you don’t slip when you try to walk.
●
However, sometimes friction is a nuisance:
It slows moving things down, and extra force is needed to keep
them moving.
● It can heat things up and can damage the moving surfaces.
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●
Friction can be
useful!
Friction can be a
nuisance!
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Engine: Friction generates
thermal energy and wears
engine parts out.
Steering wheel and driving gloves: These
make it easier to hold on and steer the car.
Brake pads and discs: Friction
between them will slow the car down.
Tyres and ground:
Friction means:
• the engine can push car forward;
• grip means the car will stick to the ground
and will stop the car when braking; and
• the car can be made to change direction.
Brakes, tyres, roads and friction
Driving a car or motorbike safely would be impossible without friction.
Especially important is the friction between the tyres and the road.
When the driver or rider puts on the brakes, the wheels stop turning
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Friction: pros and cons
and the vehicle will slow down as long as the tyres can grip the road.
The grip between the tyre and the road depends on friction. This
friction can be affected by:
The type of road surface. Some surfaces are specially roughened to
provide good friction and safe braking.
● Whether the road is wet or not. Water acts as a lubricant (see How
to reduce friction, later in this chapter) between the tyre and the
road and makes braking more difficult.
● The condition of the tyres. Worn tyres are smooth and so friction
with the road is reduced, especially in wet conditions.
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●
This diagram shows how braking is affected by the condition of the
road. Assume a driver of a car travelling at 80 km per hour (50 mph)
wants to stop his car.
16 m
Dry
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The car travels this distance while
he thinks about braking.
On dry roads:
Total stopping distance
= 51 metres
35 m
m
Wet
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16
The car travels this
distance with the
brakes on before it stops.
70
m
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On wet roads:
Total stopping distance
= 86 metres
Speed and braking
Even when a car has perfect tyres and brakes and the road is dry,
stopping a car can be very difficult. The faster a car is travelling, the
longer it will take for it to stop. This is because the car will travel
further while the driver reacts to the situation, and also because the
friction must act for longer to stop the car moving forward.
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The UK Highway Code shows the distances required for braking at
different speeds. At 60 mph (96.6 km per hour) under perfect conditions,
stopping the car still takes nearly 20 times the length of the car.
20 mph
6m
6m
= 12 m (40 ft) 3 car lengths
30 mph
9m
14 m
40 mph
24 m
12 m
= 36 m (120 ft) 9 car lengths
50 mph
15 m
38 m
= 53 m (175 ft) 13 car lengths
60 mph
55 m
18 m
= 73 m (240 ft) 18 car lengths
70 mph
21 m
75 m
Standard car length = 4 m
Thinking time
■■
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= 96 m (315 ft) 24 car lengths
Key:
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= 23 m (75 ft) 6 car lengths
Speed affects braking distance
Braking distance
How to reduce friction and stop damage to surfaces
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Friction is caused by tiny bumps between surfaces. These bumps are just
like tiny pieces of sandpaper and stop the surfaces from moving. They also
damage the surface by scraping pieces of material away and by heating
the surfaces. The heating effect is caused by the particles in the surface
being forced to vibrate. As they vibrate, they will have more kinetic energy
and will get hotter. The heat can be enough to melt one or both of the
surfaces and can stop the materials from carrying out their job properly.
Friction, and the damage it can cause, can be reduced by:
Smoothing off the surfaces. A smooth surface has less friction than
a rough one.
● Adding a substance that keeps the surfaces slightly apart. This kind
of substance is called a lubricant. Good examples are grease or oil.
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Friction and motion
●
4
Friction is always
more between
rough surfaces.
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So, reduce friction by
Smoothing out
the surfaces;
Adding oil(A
lubricant) to
keep the rough
surfaces apart.
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Air resistance and thermal energy
■■ This motorbike would
meet a lot of air resistance.
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The type of friction described in the example of the space shuffle on
the previous page is called air resistance (also called drag). This is
a kind of friction between a moving object and the air. Drag acts on
you as you cycle along the road. The amount of drag can be reduced
by making the object more streamlined. A streamlined object is
shaped so that the air can flow smoothly past it. This reduces the air
resistance so that the object can move more quickly through the air.
Movement through water
●●Air resistance is a kind of friction
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When an object moves through the air, it will make the particles of
the air move about. This can be very serious. The space shuttle, for
example, generated an enormous amount of thermal energy when it
re-entered the Earth’s atmosphere. Some of this thermal energy was
transferred back to the spacecraft, making it glow ‘white hot’. The
shuttle would have been severely damaged by this massive amount
of thermal energy if it did not have protection from the special tiles
that covered it. This heating effect, together with damage to the tile
covering (which happened on take-off), caused the disastrous breakup of the space shuttle Columbia in 2003, resulting in the deaths of
all seven crew members.
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■■ This motorbike is
streamlined to reduce wind
resistance. The same amount
of force will make this one go
faster than the one above.
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Surface of spaceship
Air
■■ Vibrating particles generate thermal energy. The particles
on the surface of the shuttle are fixed in place, so no
thermal energy is generated by vibration.
■■ The space shuttle (NASA 1981–2011) was covered in
thermal-resistant tiles made of silica. These protected
it from the thermal energy generated by friction as it
re-entered the atmosphere from space. The tiles became
heated until they were white hot (see at the nose at
upper right).
●●Movement through water
As well as air, water also provides resistance to the motion of objects
and living things through it.
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The streamlined shapes that help to reduce air resistance also help
in movement through water.
The evolution (see Biology, Chapter 9) of living organisms that live
in water has provided many excellent examples of streamlining. Some
of these are so effective that humans have copied them.
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Shark: Streamlining
reduces drag in water.
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Submarine
Torpedo
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Because friction is a force, it too can be measured using a
force meter. The way that this is done is shown in the diagram.
Measurements of friction between different kinds of surface must be
done as a fair test.
Mass can be added: This means
the trolley and the test material
don’t slide so easily with the result that
the force meter can not measure the pull.
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Friction and motion
Test material: This can
be wrapped around the
trolley. Good materials
to test are carpet,
nylon and sandpaper.
Force meter: As you pull on the force meter,
the trolley will begin to slide. You can read
the force needed to make the trolley move.
The force on the scale will be measured
in newtons (N).
Remember – if you want reliable results:
make it a fair test – only change one factor (variable) at
a time
● repeat the experiment and calculate an average of your
results.
●
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Surface: This can be a smooth piece
of wood or metal. Formica (like a
kitchen work surface) is also suitable,
as is glass.
Wooden trolley: A hook means it
can be pulled by the force meter.
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The aim of this experiment is to investigate friction.
Your group will be provided with a friction sledge or block of wood (as shown
on the previous page), some weights, a force meter and a set of surfaces
(materials) that you can attach to the bottom of the sledge. The surfaces
are polythene, J cloth, sandpaper, carpet, denim cloth (or any other materials
that your teacher has provided you with).
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Your task is to investigate how the surface on the bottom of the sledge
affects the force that is needed to pull the sledge across the lab bench.
Movement through water
WorkingScientifically
Investigation: Measuring friction
1 Write down the independent (input) variable, the dependent (outcome) variable.
2 Write down any other things that could affect the outcome. These are the
control variables.
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Before you begin to take readings from the apparatus you must do some
preliminary work to find out what values of controlled variables to use. See
how to attach the surfaces to the sledge and find out which surface is the
most slippery and which is the least slippery.
Now find out what is a suitable weight to put on your sledge. You want the
slipperiest surface to need enough force for you to be able to measure it.
Check that you are using the best forcemeter for the job. You will need to be
able to measure the force that is needed to pull the sledge across the bench
at a steady speed. Practice pulling the sledge steadily. Create a suitable table
using the headings below, and record the value of the force used.
Force needed to pull sledge steadily in newtons
Reading 1
Reading 2
Average
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Surface
Now test the least slippery surface under the same conditions. You may need
to use a different forcemeter. Record the force.
Test each of the other surfaces in turn. Make sure that you are always using
the forcemeter that is most appropriate for the task and record the value of
the force in the table.
Repeat your tests on each surface making sure you record your results. Then
work out the average value of the force needed.
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3 Represent your results in a bar chart.
4 (a) Which was the most difficult surface for the sledge to move on?
(b) Represent the surfaces in a drawing to explain your findings.
Exercise 4.1: Friction
1 State the names of two friction forces that would slow down a bicycle.
2 State two things that always happen when friction takes place.
3 Which of the following statements does not describe air resistance?
(a) It is also called drag.
(c) It helps streamlined cars to move quickly.
(b) It is a kind of friction.
(d) It produces thermal energy.
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