In this presentation you will:

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• In this presentation you will
• Explore the concepts of friction and the
  coefficient of friction.

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Friction affects our everyday lives. It can
explain why we make some things of particular
substances and why they have particular shapes.
In this presentation you will find out why most
vehicle wheels are made of rubber, why high speed
vehicles have aerodynamic profiles and why space
shuttles need to be covered with special thermal
protective tiles.
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What is Friction?
Friction is the force that opposes the motion of
two bodies in contact. You may clearly observe
this effect every time you drag something heavy.
You will also notice friction if you rub the
palms of your hands rapidly and repeatedly
against each other, since friction generates
thermal energy (heat).
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What is Friction?
When an object sits on a slope without slipping,
there is a force that maintains that object on
the top of the slope.
The forces acting on the object are balanced.
The diagram shows the forces acting on the weight
sitting on the slope.
These forces are the weight W, the normal force
N of the weight and the frictional force F.
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What is Friction?
Friction is caused when two materials that are
not completely smooth are rubbed together.
Even very smooth surfaces can be rough at a
microscopic level so there will always be a
degree of friction.
Of course, the rougher the surface, the greater
the friction.
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Question
What is friction?
A) The heat of an object.
B) A force that makes an object slip.
C) An object in motion.
D) A force that opposes motion.
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Types of Friction
There are two main types of friction static and
dynamic.
Static friction is the resistance of a stationary
object to start moving.
Dynamic friction is the resistance of a moving
object to continue its motion.
You may have noticed how hard it is to start
moving a fully loaded shopping cart, but once it
is moving it seems easier to keep in motion.
This occurs because static friction is normally
higher than dynamic friction.
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Factors Affecting Friction
There are three facts to remember about friction
Frictional force opposes the motion (or attempted
motion).
Friction is not related to the area of contact.
weight
Friction is directly proportional to the weight
of the object.
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Question
If a brick is flipped over to lie on one of its
smaller sides, how would the friction compare
with the friction on one of the larger sides?
A) Since the contact area is smaller, the
friction will be smaller too.
B) Friction will be the same since friction is
not related to area of contact.
C) Friction will increase due to the same mass
lying on a smaller area.
D) It will depend upon the material that the
brick is made of.
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Coefficient of Friction
The coefficient of friction is a constant value
that determines the level of friction between two
bodies.
This coefficient depends on the materials used.
For example, the coefficient of friction of
rubber on tarmac is much higher that the
coefficient of friction of ice on metal.
This coefficient is defined by the formula
where µ is the coefficient of friction, F is the
force of friction and N is the normal or
perpendicular force.
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Question
What is the coefficient of friction between two
materials?
A) It is the sliding coefficient between those
two materials.
B) It is the normal reaction N of an object made
of those two materials.
C) It is the angle at which these two materials
would begin to slide.
D) It is a constant that determines the level of
friction between those two materials.
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Coefficient of Friction on a Horizontal Plane
Applying the previous formula, the calculation of
the coefficient of friction on a horizontal
plane, where normal force N equals the weight of
the object, is very simple.
where F is the force of friction in newtons, the
mass will be in kg and 9.8 is the acceleration
due to gravity g in m/s2.
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Question
How is the coefficient of friction between two
materials calculated?
A) Divide the force of friction by the normal
force of the object.
B) The coefficient of friction equals the mass of
the object.
C) Measure the angle of the slope only.
D) Multiply mass of the object by gravity only.
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Angle of Friction
On an inclined plane of angle ? at which the
block is just about to slip, the three forces
acting on the block are

• The normal force N of the surface.

• Its weight W.

• The limiting frictional force F.

The forces are in equilibrium.
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Angle of Friction
If mg (W) is resolved into components
mg sin? along the surface.
mg cos? perpendicular to the surface.
Therefore N is defined by mg cos? and the value
of the friction force F applied to the object can
be calculated by mg sin?.
Hence µ can be found by measuring ?, called the
angle of friction.
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Friction of Air
Air itself creates friction. This is why vehicles
moving at high speeds are designed
aerodynamically so the friction with the air is
reduced and energy can be saved.
Since friction generates heat, vehicles that
operate under huge levels of friction (therefore,
very high heat), need to be built with protection
against heat.
This explains why the bottom face of the space
shuttle is covered with ceramic tiles. These
insulate the vehicle from heat when it re-enters
the atmosphere.
Friction against air is also what burns
meteoroids when they enter Earths atmosphere and
we get to see them in a clear night sky as
shooting stars.
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Benefits of Friction
Fiction is not always to be avoided. Materials
that generate a high level of friction such as
rubber, are extensively used in the tire industry
where friction is essential for motion and
control.
When a tire rotates on a road, the friction
between the tire and the road helps the tire grip
and transforms the energy from the engine to the
motion of the car. If the tire was rotating on
ice, the friction would be low and the tire would
just spin. It would not grip.
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Summary
After completing this presentation you should be
able to

• Show knowledge and understanding of friction
  between solids and different types of friction.

• Show knowledge and understanding about the laws
  related to friction.

• Show knowledge on how to calculate the
  coefficient of friction between different
  materials.

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