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Unit Two: Dynamics
Part 3: Friction
What is the symbol for weight?
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What is the symbol for mass?
What type of frame of
reference???

You are standing in an elevator waiting for it
to go up 10 flights.
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You are standing in an elevator that is just
starting to move.

You are standing in an elevator going down at
a constant speed.
Friction – Quick Review

A contact force

Electromagnetic Force (between surface
atoms of objects touching)
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Always opposes motion
Friction Summary Slide
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There are 2 types of friction:
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Static Frictional Force
 When you start to move an object from rest
 Larger than Kinetic Frictional Force due to
Inertia
 Symbol: ųs
Kinetic Frictional Force
 Exists when the object is moving
 Symbol: ųK
Why?

Think about moving a large couch across the
floor. Which part is more difficult – starting
the movement or continuing the movement?
Why?

Starting the movement is more difficult
because … (can you complete this
sentence?)
Coefficient of Friction
Summary

“Stickiness value”
ų (symbol “mu”) has no units

Page 140, table 4.5
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Formula: Ff = ųFN
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*** NOTE: FN = - Fg in most of our grade 11
questions (as we typically deal with forces in 1
dimension only, no angles).
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Friction Example

During the winter, owners of pickup trucks
often place sandbags in the rear of their
vehicles. Calculate the increased static force
of friction between the rubber tires and wet
concrete resulting from the addition of 200.
kg of sandbags in the back of the truck.

Use the table of coefficients of friction on
page 140.
Friction Example 2

A horizontal force of 85N is required to pull a
child in a sled at constant speed over dry
snow to overcome the force of friction. The
child and sled have a combined mass of
52kg. Calculate the coefficient of kinetic
friction between the sled and the snow.
Friction Example 3

A 7.0kg box is being pushed horizontally at a
constant speed. If the coefficient of friction is
0.30, how much force is being used to push
the box? DRAW A FBD!
Example 4

You are pushing horizontally on a book
against a wall so that it does not slide down
the wall.


Draw a FBD for this situation. What condition is
necessary for the box to not slide down the wall?
If you lessen the horizontal push that you are
exerting, the box will start to slide down the wall.
Explain why this happens.
Practice Friction Problems

Page 144
Questions 5, 6, 7, 8

LAB!!!

Day 2

Go through errors!
Check Your Learning
A friend pushes a 625g textbook horizontally along a
table at a constant velocity with 3.50N of force.
a) Determine the normal force supporting the
textbook.
b) Calculate the force of friction between the book and
the bench.
 c) Calculate the coefficient of friction between the
book and the bench.
 d) Which coefficient of friction have you found:
static or kinetic? Explain.

Answers
Answers
Starter/Review
From your lab…

What things affect the coefficient of friction
(and therefore affect the amount of friction)?

Mass?
Surface Area?
Surface Type?



What do you think about the speed/velocity of
the object – does this affect friction?
Friction - Summary

The strength of friction depends on…



Surface materials (type of surface)
 Rough surface means more friction (ie, larger coefficient of
friction)
Magnitude of forces pressing surfaces together (mass)
 More mass means more friction
The strength of friction DOES NOT depend on…

Surface area
Velocity of object moving

See page 140, table 4.5 for a list!

Just some stuff to think
about…

Can I remember the following…

Explain Newton’s First law and give an
example of this law.
What is the net force on Mrs. Evans when
she is sitting down? Why?


What type of reference is this (inertial or noninertial)?


Predict the motion (direction, is it slowing
down or speeding up) that each object
would undergo based on the free-body
diagram provided:
a)
b)
Quiz Tomorrow

Friction (and Newton’s First Law since it
deals with friction)

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Friction – formula/questions
FBD (to determine normal force)
Coefficient of friction
How does Newton’s First law apply to friction?
What things affect friction?
Questions???