statics ch 8 section 2

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

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

3. ME221 Lecture 35 3
No Class
Wednesday, November 26

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

5. ME221 Lecture 35 5
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

6. ME221 Lecture 35 6
Chapter 8 - Friction
• Overview
• Coulomb friction (dry friction)
– Static and kinematic
• Fluid friction
– Wedges
– Types

7. ME221 Lecture 35 7
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

8. ME221 Lecture 35 8
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

9. ME221 Lecture 35 9
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

10. ME221 Lecture 35 10
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

11. ME221 Lecture 35 11
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

12. ME221 Lecture 35 12
Example Problems