Bell Ringer 12.1.15
Is it harder to move a car at the beginning or to
keep moving it once it has already started
moving? Justify your answer with reasoning.
Bell Ringer 12.2.15

A trunk weighing 325N slides down a 20
degree ramp after a force if 211N is applied
to the right. What is the acceleration of the
trunk?
Static and Kinetic Friction
Toys for Tots



In the spirit of giving….
Please bring a toy to donate…
You can receive 5 Extra points on your test
Friday…..
Instructional Objectives

We will complete a lab investigation to determine the
mathematical relationship between FNorm & Friction.
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I will write a complete lab report.
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Due Friday 12.4.15
Agenda 12.2.15
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
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Complete Lab- Table
Lab analysis- plot 2 graphs, find slope
Conclusions
H/W: You are responsible for completing a formal
lab report write up that will be due in your
notebook in the green bin on Friday.
Follow the lab report format. If you need further
directions be sure to get it prior to Friday.
Essential Questions
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
What is Friction?
What are the two types of friction?
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

What is Static friction ?
What is Kinetic friction?
How does static friction differ from kinetic
friction?
What is Friction?

The Force that
opposes the motion
of two surfaces

High for dry and
rough surfaces
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Low for smooth and
wet surfaces
Free Body Diagram
Normal
Force FN
Applied
Force F
Friction
Force Ffrict
Gravity
Force Fg
Fg = mg
FN = F g
ffrict = F
Static Friction
The Force of Static
Friction keeps a
stationary object at
rest!
f s  FN   s
FN
F
Ffrict(s)
Fg
 s  coefficient of static friction
Kinetic Friction
Once the Force of Static
Friction is overcome, the
Force of Kinetic Friction
is what slows down a
moving object!
FN
Motion
F
fk
Fg
f k  FN   k
 k  coefficient of kinetic friction
Examples of 2 Types of Friction
To initiate motion of the box
the man must overcome the
Force of Static Friction
I better be
safe Ump!!
Upon sliding, the baseball
player will come to a
complete stop due to the
Force of Kinetic Friction
Which type is it Static or Kinetic Friction?
Static & Kinetic Friction Coefficients
Material
Coefficient of
Static Friction S
Coefficient of
Kinetic Friction S
Rubber on Glass
2.0+
2.0
Rubber on Concrete
1.0
0.8
Steel on Steel
0.74
0.57
Wood on Wood
0.25 – 0.5
0.2
Metal on Metal
0.15
0.06
Ice on Ice
0.1
0.03
Synovial Joints in
Humans
0.01
0.003
Static Friction Lab Procedure
•
•
•
Connect the force detector to a block of
wood
Pull gently on the force sensor unit the
block begins to move
Move the block slowly with constant force
Static Friction Lab Procedure
Step1: practice pulling block at a constant rate. 2-3 times
Step2: Choose 6 different masses. Record mass in kg from
smallest to largest
Step3: For each mass find the normal force(FNorm=Fgrav=W= m*g)
Step4: Choose an ideal point for static friction& coefficient friction
for each mass trial. Record in the table
•
Let’s go through the equipment instructions.
Ideal Result
Static Friction Lab Objective: to determine
the mathematical relationship between
FNorm & Friction
Data Analysis:
•
•
Table
•
plot 2 graphs: Ffrict (static) vs. F Norm & F
vs. F Norm
•
Find Slope of line
frict
(kinetic)
Data Table
Trial
1
2
3
4
5
6
Mass(kg)
Fnorm=mg
Ffrict
FFrict
(static)
(kinetic)
Plot the graphs & Find the slope

Ffrict (static)

F Norm
F frict (kinetic)

F Norm
Conclusion

Discuss your results, what type of
relationship did you find?What did you notice
about static friction compared to kinetic
friction?Reasoning for your answer. What
errors were made?Recommendations?
Application Analysis
An empty cart is being
rolled across a
warehouse floor. If
the cart was filled, the
force of kinetic friction
between the cart and
the floor would

1.
2.
3.
Decrease
Increase
Remain the same
Application Analysis
Sand is often placed on
an icy road because the
sand:

1.
2.
3.
4.
Decreases the coefficient
of friction between the
tires of a car and the road
Increases the coefficient
of friction between the
tires of a car and the road
Decrease the gravitational
force on a car
Increases the normal force
of a car on the road
Questions?
Kinetic Friction Lab Procedure
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•
•
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Line up the block with the motion detector
Push the block gently toward the motion detector
so that it comes to a stop approximately 1 foot
away from it
From the velocity curve determine the deceleration
of the block
Calculate the Force of Kinetic Friction
Determine the coefficient of Kinetic Friction
Wooden block
Push