6. ME221 Lecture 34 6
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
8. ME221 Lecture 34 8
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)
9. ME221 Lecture 34 9
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
10. ME221 Lecture 34 10
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, ms
– Kinematic coefficient, mk mkN
msN
P
f
P
N
f
W
f=P
11. ME221 Lecture 34 11
Common Static Friction Coefficients
Steel on steel 0.75
Rubber on concrete 0.5-0.9
Rubber on ice 0.05-0.30
Metal on wood 0.2-0.6
Teflon on Teflon0.04
Tires on gravel 0.5
14. ME221 Lecture 34 14
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
