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Friction
Objectives
• After the lesson, you should be able to:
– What is friction
– Describe the effects of friction on motion
Frictional Force
How does friction affect motion?
• Friction always opposes motion between two surfaces
in contact.
• The diagram below shows a microscopic view of two
surfaces in contact.
Frictional Force
• Friction is a force that opposes motion. SI unit N
• Friction makes our daily life possible.
• The presence of friction reduces the net force
acting on an object.
3N
5N
2N
2N
5N
3N
5N
Frictional Force
• A girl pushes a box of mass 5kg along a
frictionless surface with constant force of 10N.
– What is the acceleration of the box?
• F = ma,
a = F/m = 10N/5kg = 2ms-2
– If the surface has a friction of 4N, what will be the
acceleration of box?
• Net force on box = 10 – 4 = 6N
• a = F/m = 6N/5kg = 1.2ms-2
Constant velocity 0.2ms-1
Frictional Force
50N
20kg
Frictional
force
• A boy pushes a box of mass 20kg across the floor
at a constant velocity of 0.2ms-1 by exerting a
horizontal force of 50N.
– What is the resultant force acting on the box?
Since velocity is constant, zero acceleration. Resultant
force is zero.
– What is the frictional force acting of the box?
For resultant to be zero, frictional force on box is equal
and opposite to applied force, i.e. 50N
Constant velocity 0.2ms-1
Frictional Force
50N
20kg
Frictional
force
• Now, instead of 50N, the boy exerts 80N on the
box.
– What is the new resultant force acting on the box?
The frictional force on box is still 50N.
New resultant force = 80 – 50 = 30N
– What is the acceleration of the box?
a = F/m = 30N/20kg = 1.5ms-2
Acceleration a
80N
20kg
Frictional
force
Which Direction is Friction?
Frictional Force
•
•
•
•
Without friction life would not be possible
Daily activities involve friction
Contact force
Space has no friction
Frictional Force
• How tyres work?
Frictional Force
• Advantages
– Make daily activities
possible
– Earth’ atmosphere use
friction to protect us from
meteorites
force of foot
pushes the
ground
backwards
force of ground
pushes the foot
forward
Frictional Force
Friction can be a problem.
Three reasons for friction being a nuisance:
• rubbing of surfaces wears away materials.
• friction slows down moving objects.
• friction produces unwanted heat.
friction slows down this cyclist
friction has worn out
the rubber on the top
tyre
Frictional Force
• But what if the meteorites are bigger?
Summary
• Friction is a force that opposes motion.
• The presence of friction reduces the net
force acting on an object.
• Frictional force is fixed for a particular
object.
Frictional Force
• Ways to reduce friction include:
– Wheels
– Ball Bearings
– Lubricants and polishing surfaces
– Air cushion e.g. hovercraft and magnetic
levitation in trains
– Make body streamlined (reduced air
resistance)
methods to reduce friction
Methods to reduce
friction
Examples
Moving parts are made
as smooth as possible
The piston and cylinders
of an engine made of
highly polished steel or
aluminium
Materials with very low
frictional resistance are
used
Slippery graphite used
for contact points in
motors and dynamos
Ball and roller bearings
are placed between the
moving parts
In the hubs of bicycle
wheels
Surfaces are separated
by a lubricant
Engine oil and grease
used to lubricate
engines
Surfaces are separated
by air cushion
A hovercraft travel over
rough land, swamp or
sea
friction
Friction and Its Effects
Identifying forces and free body diagrams
• Block diagrams with arrows used to
represent forces acting on a body are
called free body diagrams.
• These diagrams help you to solve
problems on forces in Physics.
Friction and Its Effects
Key Ideas
• Friction opposes motion between two surfaces in
contact.
• A non-zero resultant frictional force will cause a
moving object to slow down to a complete stop.
• There are both positive and negative effects of
friction in our daily lives.
• Free body diagrams are drawn to help us identify
forces acting on a body or system of bodies, in order
to solve problems.
Friction and Its Effects
Test Yourself 3.4
1. Can you lean against a wall if frictional forces are absent?
Answer: No. When you lean against the wall, the
frictional force between your body and the wall, as well as
the frictional force between your feet and the floor, keep
you from slipping.
2. How can you reduce the effect of friction on the motion
of a body?
Answer: Using wheels, ball bearings and lubrication.
Friction and Its Effects
Test Yourself 3.4
3. A force F of 50 N is needed to keep a trolley of mass
60 kg moving at a uniform velocity of 2 m s-1. What is
the frictional force f on the trolley?
a=0
F = 50 N
friction f = ?
Answer:
Since the trolley is moving at uniform velocity, this means
that acceleration a = 0. The resultant force is thus zero.
Hence the pushing force F is equal to the friction force f.
 f = F = 50 N
Friction and Its Effects
Test Yourself 3.4
4. A feather and a stone are released simultaneously from the same
height. Explain why the feather will fall more slowly to the ground
than the stone even though the acceleration due to free fall is the
same for both
f the feather and the stone.
air
stone
W
Answer: The air resistance fair
experienced by the stone is
relatively much smaller compared
to its weight W. Hence the stone
has an acceleration close to g,
the acceleration due to gravity.
fair
W
Answer: The feather experienced
an air resistance fair that is quite
comparable to its own weight W.
Hence the feather’s acceleration
is very low.