Transcript Lecture 34 sect 8.1.ppt

ME 221 Statics
Lecture #34
Sections 8.1 – 8.2
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Homework #11
Chapter 6 problems:
– 2, 3, 6 & 7 – Method of Joints
– 32, 36, 47 & 53 – Method of Sections
– 68 & 75
Due Today
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Homework #12
Chapter 8 problems:
– 1, 2, 4, 8 & 15
Due Monday, December 1
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Quiz #7
Today
Analysis of Structures
Method of Joints or
Method of Sections
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No Class
Wednesday, November 26
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No Class
Friday, December 5
Mechanical Engineering Design Conference
MSU Union 9am to noon Demonstrations
1pm to 2pm Awards in Ballroom
ME371 Mechanical Design I
ME412 Heat Transfer Lab
ME456 Mechatronics Systems Lab
ME471 Mechanical Design II
ME481 Mech Engineering Design Projects
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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 (friction force)
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Friction Angle
• The Coefficient of Static Friction (ms) is defined
as the tangent of the maximum angle a body may
be inclined before slip occurs
ms = tan max = ƒmax / N
• The angle max is called the
friction angle
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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
f
– Static coefficient, ms
– Kinematic coefficient, mk
W
msN
mkN
f=P
P
f
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N
P
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Common Static Friction Coefficients
Steel on steel
Rubber on concrete
Rubber on ice
Metal on wood
Teflon on Teflon0.04
Tires on gravel
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0.75
0.5-0.9
0.05-0.30
0.2-0.6
0.5
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Example Problem
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Quiz #7
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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
• Examples: 8.71, 8.72, 8.80, 8.45
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