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Friction
Introduction & Dry Friction
ENGR B36 - Statics
Pat Aderhold
11/19/2014
News & Notes
News & Notes
Around the World in 40 Minutes!
12:15-12:55 p.m.
November 20, 2014
Performing Arts Center (PAC)
Free food will be provided outside the theater after the presentation
• IEW is a joint initiative of the U.S. Department of State and the U.S. Department of Education (Visit www.iew.state.gov for
more information.)
• Celebrating IEW provides students, faculty and staff opportunities to:
 Celebrate the benefits of international education and exchange worldwide
 Prepare Americans for a global environment
• Develop a broader understanding of the world cultures and languages while creating an enduring friendship among
cultures.
Friction - The Basics
• Usually frowned upon...
 Losses in pipes
 Wearing of moving parts
 Drag on moving bodies
• ... But it’s got a bad rap
 Pumps and fans in your car (belt driven)
 Safe stops at red lights (brakes)
 Walk down hallway without skates (rubber soles)
Friction - The Basics
• It’s always there
 Sometimes not noticeable - “ideal”
 Sometimes a major effect on system - “real”
• Always tangential to contact surface
• Always opposes motion (or attempted motion)
Types of Friction
✓ Dry Friction -or- Coulomb Friction
• The one you usually think of
• “Dry” with or without WD-40
✗ Fluid Friction
• Will spend whole semester on this - Fluid Mechanics
• ~Sheets of molecules sliding over each other
✗ Internal Friction
• Accompanies plastic deformation
• Some coverage in - Mechanics: Properties of Materials
Macroscopic Happenings
• FBD from Physics I days
• More happening than see at first glance
Why does it behave this way?
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 337
Microscopic Happenings
•
•
•
•
Surface irregularities => contact forces
Different for every surface
Force opposing sliding - Friction
Force opposing falling - Normal
• When static F=P by definition
• At some point, friction overcome
• Lower when in motion because not
“settled in” to lowest energy position
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 337
The Takeaway
F = mN
The Caveats
• Difference between static and kinetic
• Equivalence only at maximum friction force
Fmax = ms N
Fs £ ms N
Fk = mk N
• F and µ “essentially independent” of area
Other Nomenclature
• Can use angle “α” to describe friction
 Gives orientation of resultant R
 Measure from N to F
 Max value labeled as φ
• Yields Cone of Friction
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 340
tan fs = ms
tan fk = mk
A Lesson in Physics and Engineering
• What is really happening at surfaces?




Molecular attraction?
Effect of hardness of constituent materials?
Localized temperature effects?
Surface oxides? contaminants? adsorbates?
• No “comprehensive theory”
Let the Physicists lose sleep at night
Engineers’ job is to USE this info*
*where it’s proven to work and ONLY where it’s proven to work
Problem Solving Approach
• Is it moving or not? Can you tell?
 Moving => Use µk & Fk goes opposite motion
 Static, but about to move => Fs = Fmax
 Static, but not about to move => balance forces
• On planes
 Set coordinate system with n and t to simplify
 N changes when you’re pulling up
Practice Problem 6/2
The 700 N force is applied to the 100 kg block, which is stationary before the
force is applied. Determine the magnitude and direction of the friction force F
exerted by the horizontal surface on the block.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346
Practice Problem 6/5
The magnitude of force P is slowly increased. Does the homogeneous box of
mass m slip or tip first? State the value of P which would cause each occurrence.
Neglect any effect of the size of the small feet.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346
Practice Problem 6/3
The designer of a ski resort wishes to have a portion of a beginner’s slope on
which the skier’s speed will remain fairly constant. Tests indicate the average
coefficients of friction between skis and snow to be µs = 0.10 and µk = 0.08. What
should be the slope of angle θ of the constant-speed section?
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346
Practice Problem 6/9
The light bar is used to support the 50-kg
block in its vertical guides. If the coefficient of
static friction is 0.30 at the upper end of the
bar and 0.40 at the lower end of the bar, find
the friction force acting at each end for x = 75
mm. Also find the maximum value of x for
which the bar will not slip.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 347
Practice Problem 6/20
The force P is applied to (a) the 60-lb block and (b) the 100-lb block. For each
case, determine the magnitude of P required to initiate motion.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346
For Next Class
• Sample Problems quite good
=> read and understand them
• Read through Articles 6/4 & 6/5
• Practice Problems 6/27, 32 & 34

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