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What’s Stopping You?
Friction
Physics 11
Humour again..
What is friction?

The force that opposes the relative motion of two
surfaces and acts along the surfaces.
 Frictional
forces inhibit relative motion between two
objects in contact with each other
 CONTACT
FORCE
Expanded…


When two surfaces are at rest and in contact, the
surface atoms interact to from relatively strong
attractive forces. When you push one object, static
friction “pushes back” with exactly the same
magnitude as an applied force until the applied
force is great enough to break the attractive forces
between the surface atoms.
Once the object is in motion, new “bonds”
continually form and brake over and over.
Two types of friction:
1.
Static Friction:

2.
This value represents the relative force necessary to
make an object move
Kinetic Friction:

This value represents the relative force necessary to
keep an object moving at a constant rate
Fs  Fk
Fs
Max
Fk
Fs  Fk
Static
Region
Kinetic Region
Force Causing the Object to Move
What affects the force of friction?

Type of surfaces in contact
 Ex:
Rubber on rubber verse rubber on ice
 Coefficient of friction (µ)

Magnitude of the normal force
 “the
surface pushes back against the weight of the object”
 Ex: Which is easier to move a box full of bricks or a box full
of t-shirts?
Coefficients of friction:



“stickiness value”
Known values for
certain surfaces,
determined
experimentally
Units = none!
Normal Force


The normal force is sometimes—but certainly not
always—equal in magnitude and opposite in
direction to weight (Fg)
When an object lies on a level, non-accelerating
surface (rest or constant motion):
Fg =FN
Frictional Forces Occur When Materials are in
Contact:
N
F
fs
M1
Surfaces in
Contact
W
F = Force Causing Motion (Pull on Scale)
Fs = Force of Static Friction (Resists Motion)
N = Force Normal Holds Surfaces in Contact
W = Weight of Object ( Mass x Gravity)
Calculating force of friction:

Direction of Ff is always opposite to the direction of
motion
Ff = µ FN
 For
both static and kinetic friction
Ex 1:A glass of water is being pushed along a glass
table. If the glass has a mass of 0.675 kg, calculate the
force of friction.

Fg = mg
= (0.675 kg)(9.81)
= 6.62 N
 Fg
= FN = 6.62 N
 FF =
µFN

Look up µ in table on pg 140
= (0.40)(6.62)
= 2.65N
Ex 2: A group of people are pushing a large wooden trunk
across a wood floor at a constant velocity and you are
exerting a force of 625.0 N, what is the mass of the block?
 Constant
 FF =
velocity  Ff = Fapp
µFN

Look up µ in table on pg 140
(625) = (0.20)FN
625 = FN
0.20
3125 N = FN
 FN
= Fg = 3125 N
Continued…
 Fg
= mg
(3125) = m (9.81)
3125 = m
9.81
319 kg = m
Practice:



Pg 144 # 5, 6
Pg 151 # 30 - 33
Friction and Forces Sheet