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4.15 - Friction
• Two types of friction:
- Dry friction (Coulomb) and Fluid friction
• Characteristics of friction:
- Force that resists the movement of two contacting
surfaces that slide relative to one another
- Acts tangent to the surfaces at points of contact
between bodies
- Opposing possible or existing motion of the body
relative to points of contact
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Theory of dry friction
• Consider pulling block horizontally:
• Many microscopic irregularities
• Reactive forces ΔRn develop at each protuberances
• Each reactive force consist of frictional component ΔFn
and normal component ΔNn
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Theory of dry friction
• Effects of normal and frictional loadings are indicated by
their resultants N and F
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Object is stationary
Impending motion
Limiting static friction
force
On the verge of slipping
Fs = μsN
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Object is in motion
Kinetic frictional force
P > Fs
Fk = μkN
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Characteristics of dry friction
• Based on experimental evidence
• The frictional force acts tangent to the contacting surfaces
• The max static frictional force Fs is independent of the
area of contact
• The max static frictional force is greater than kinetic
frictional force
• When friction is present: F = P
• When on the verge of slipping: F = Fs = μsN
• When in motion: F = Fk = μkN
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Types of friction problems
• No apparent impending motion:
- Standard equilibrium situation
- Fs ≤ μsN (Check)
• Impending motion at all points:
- Fs = μsN
• Impending motion at some points:
- Many options are possible
- Must perform checks (even evaluate all options)
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Problem
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