FrictionIntroduction & Dry Friction

ENGR B36 - StaticsPat Aderhold11/19/2014

News & Notes

Around the World in 40 Minutes!12:15-12:55 p.m.

November 20, 2014Performing Arts Center (PAC)

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News & Notes

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

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 337

• 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

The Takeaway

The Caveats• Difference between static and kinetic• Equivalence only at maximum friction force

• 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

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 nightEngineers’ 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

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346

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.

Practice Problem 6/5

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346

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.

Practice Problem 6/3

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346

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?

Practice Problem 6/9

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 347

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.

Practice Problem 6/20

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 346

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.

For Next Class• Sample Problems quite good

=> read and understand them• Read through Articles 6/4 & 6/5 • Practice Problems 6/27, 32 & 34