LESSON 1.9 - FRICTION
Objectives
On completion of this lesson, you should be able to:
Define: Friction,
Coefficient of friction, Viscosity.
Describe: The difference between static
friction and dynamic friction.
Explain: Why friction occurs between two
surfaces. Explain why friction occurs inside liquids and
gases.
Calculate: The sizes of frictional
forces. Calculate coefficients of friction between
surfaces.
ACTIVITIES
- Inclined plane:
Place different objects on a board and tip the
board until the objects start to slide. Each
object should start to slide when it reaches the
same angle each time. The angle is an indication
of the coefficient of static friction.
- Temperature and
Friction: Measure the increase in temperature
as a jar of sand is shaken. Measure the increase
in temperature as dough is kneaded.
Friction: The force
that resists or tends to resist relative motion of
surfaces, materials or particles in contact with each
other.
Static friction: A
certain force is needed to initiate relative motion
between surfaces or parts of a material. The force that
opposes initial movement is called static friction.
Dynamic friction: Less
force is usually needed to maintain relative movement
than to initiate movement. The force that opposes
constant relative movement between surfaces or parts of a
material is known as the dynamic friction.
Coefficient of friction:
For a particular pair of surfaces, the coefficient of
friction is the ratio of the sliding force over the force
acting perpendicular to the surfaces in contact.
Fluid: A material
that can flow: usually a liquid or a gas.
Viscosity: An
indicator of the tendency to oppose relative motion of
the particles/molecules that make up a fluid. The greater
the viscosity the slower the fluid tends to flow under
the influence of a specific force.
Friction
Friction is a force that opposes the motion between two
surfaces or materials that are in contact. Friction
opposes motion. There are many different types of
friction. Friction occurs when solid objects are in
contact with each other. Friction also occurs between
solids and liquids and between solids and gases, and
between liquids and gases.
Static and dynamic
friction
There are two types of friction between solids: static
friction and dynamic friction. Static friction affects
objects before they start to move. Dynamic friction
affects objects after they have started to move.
Static friction is usually
slightly larger than dynamic friction.
The coefficient of
friction
In order to compare the friction between different pairs
of substances, we use a characteristic known as the
coefficient of friction. The coefficient of friction
relates the amount of frictional force to the amount of
force holding the two surfaces together. The force of
friction between two surfaces = the coefficient of
friction x the force between the two surfaces.
Theoretically, this means that the force of friction does
not depend on the area of contact.
Viscosity
Friction in liquids and gases is caused mainly by
viscosity. The viscosity value indicates how easily a
liquid or gas will flow. More viscous materials tend to
move more slowly. This makes them oppose movement of
substances through or past them. Molecules in liquids and
gases tend to attract each other or interfere with each
other. They are also slightly attracted to solid surfaces.
This affects their ability to get out of the way of
objects moving through them.
. .
. . . . . . . . . . . . .. .. . . . . Friction force
Coefficient of friction =
-------------------------------------
. . . . . . . . . . . . . . . . . . . Force between surfaces
Example 1.9.1 Coefficient of friction
A force of 1.5 N is needed to pull a wooden block across
the surface of a table. If the weight of the block is 0.3
N, what is the coefficient of friction between the
surface of the block and the surface of the table?
Solution
The coefficient of friction is the sliding force divided
by the force between the surfaces.
The sliding force = 1.5 N.
The force between the surfaces is 0.3 N
Coefficient of friction
= 1.5 / 0.3 = 5 (The
coefficient has no units)
Example 1.9.2
Coefficient of friction
If a force equivalent to the weight of 100 g is needed to
pull a block of wood across a flat piece of glass and the
block of wood has a mass of 120 g, the coefficient of
friction is calculated as follows:
Determine the force from
the weight reading: Friction force = (100 / 1000)kg x 9.81
N/kg = 0.981 N
Force between surfaces = (120
/ 1000) x 9.81 = 1.18 N
Coefficient of friction
= 0.981 / 1.18 = 0.83
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Review
Questions
- If a person is
pulling a sled on a dry, flat, horizontal surface
in a southerly direction, in what direction does
friction on the sled’s runners act?
- If the sled were
stopped on a dry horizontal surface, would it be
more difficult to get the sled moving than to
keep it moving? If so why?
- Why is it more
difficult to stir a cup full of honey than a cup
full of water?
- When a motor vehicle
is traveling at high speed, a large amount of the
energy used is to overcome air resistance. Is it
true that much of this energy is used to deal
with drag which is caused by friction between the
air and the vehicle and friction between air
particles moving relative to each other?
- What is the
coefficient of friction between two surfaces if
the friction force is 900 N when the force
pushing the two surfaces together is 2000 N?
- The coefficient of
friction between the runners of a sled and the
ground is 25. What force will be needed to pull
the sled if it has a mass of 500 kg?
- List the parts of a
motor vehicle that use friction to ensure that
the vehicle operates safely and reliably.
- How is energy usually
converted when frictional forces are applied to
moving objects?
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ACTIVITIES
- Inclined plane:
Place different objects on a board and tip the
board until the objects start to slide. Each
object should start to slide when it reaches the
same angle each time. The angle is an indication
of the coefficient of static friction.
- Temperature and
Friction: Measure the increase in temperature
as a jar of sand is shaken. Measure the increase
in temperature as dough is kneaded.
HANDS-ON
HOMEWORK
Select one or more of the recommended
activities for Lesson 1.10, collect the items needed and test the
procedure before demonstrating the activity during the
next theory lesson.
Lesson
1.9 Friction
- In a Northerly
direction.
- Yes. Static friction
between solid surfaces s greater than dynamic
friction.
- Honey has a greater
viscosity than water.
- Yes.
- 900N / 2000N = 0.45
- The weight of 500 kg
= 9.81 x 500 = 4905 N.
The friction force would be = 25 x 4905 = 122625
N
- Brakes, clutch,
tires, nuts and bolts – almost every part of
a car relies on friction. Most parts are held
together by friction. The steering wheel works
because of friction between the hands of the
driver and the wheel.
- Friction almost
always results in an increase in temperature at
the surfaces or particles in contact.
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