ME221
ME 221 Statics
Lecture #35
Sections 8.1 – 8.3
Lecture 35 1

Homework #12
Chapter 8 problems:
– 1, 2, 4, 8 & 15
Due Monday, December 1
ME221 Lecture 35 2

No Class
Wednesday, November 26
ME221 Lecture 35 3

Quiz #8
Monday, December 1
Friction problem
ME221 Lecture 35 4

ME221
Final Exam
Section 1 (3:00pm)
Monday, 12/8, 3:00 – 5:00pm
Section 2 (12:40pm)
Wednesday, 12/10, 12:45 – 2:45pm
See University policy for multiple exams
Lecture 35 5

Chapter 8 - Friction
• Overview
– Types
• Coulomb friction (dry friction)
– Static and kinematic
• Fluid friction
– Wedges
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Friction Angle
• A body on an incline, acted upon by gravity alone, will slip at an angle related to the coefficient of friction
• From the FBD:
N – W cos α = 0 &
ƒ – W sin α = 0
• Such that:
ƒ = N tan α
ME221 Lecture 35

7

Friction Angle
• The Coefficient of Static Friction ( m s
) is defined as the tangent of the maximum angle a body may be inclined before slip occurs m s
= tan
 max
= ƒ max
/ N
• The angle  max friction angle is called the

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Coulomb Friction (dry friction)
• Friction force (ƒ) is proportional to the normal force ( N ) and opposes motion
• Coefficient of friction, m
, is the proportionality constant
– Static coefficient, m s
– Kinematic coefficient, m k
W f m m s
N k
N f=P
P f
P
N
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Common Static Friction Coefficients
Steel on steel 0.75
Rubber on concrete 0.5-0.9
Rubber on ice
Metal on wood
0.05-0.30
0.2-0.6
Teflon on Teflon0.04
Tires on gravel 0.5
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Friction: Wedges
• Wedges are a simple means for lifting
• Again, friction force opposes the motion
• Disassembled FBDs are essential for solving wedge problems
– Apply equilibrium equations to disassembled FBDs
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Example Problems
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