1. Chapter six: Friction
Section A
Frictional Phenomena 1

2. Friction Usual Assumption:
Forces of action and reaction between contacting surfaces
act normal to the surface
 valid for interaction between smooth surfaces
 often involves only a relatively small error in solution
 in many cases ability of contacting surfaces to support
tangential forces is very important (Ex: Figure beside)

3. Types of Friction Dry Friction (Coulomb Friction)
occurs between unlubricated surfaces of two solids
Effects of dry friction acting on exterior surfaces of rigid
bodies
Fluid Friction
occurs when adjacent layers in a fluid (liquid or gas) move at
a different velocities. Fluid friction also depends on viscosity
of the fluid.  Fluid Mechanics
Internal Friction
occurs in all solid materials subjected to cyclic loading,
especially in those materials, which have low limits of
elasticity  Material Science

4. Mechanism of Dry Friction
• Block of weight W placed on horizontal surface. Forces acting on block are its weight and reaction of surface N.
• Small horizontal force P applied to block. For block to remain stationary, in equilibrium, a R horizontal component F of the surface reaction is required. F is a Static-Friction force.
As P increases, static-friction force F increases as well until it reaches a maximum value Fmax .
Fmax=µs*N
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5. Further increase in P causes the block to begin to move as F drops to a smaller Kinetic-Friction force Fk .
Fk=µk*N
μs is the Coefficient of Static Friction
μk is the Coefficient of Kinetic Friction

6. Friction Angles
Friction Angles: The direction of the resultant R measured from the direction of N.
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7. Types of friction problems
1. Impending motion is known to exist
• F = F max = μs N
(in the direction opposing to the impending motion).
• Equations of equilibrium can be used as usual.

8. 2. Relative motion is known to exist
• F = μk*N
(in the direction opposing to the motion).

9. 3. Motion condition is unknown
• Assume static equilibrium
• Use equilibrium equation to solve the friction force F
• Check the value of friction force F
• No friction, (Px = 0)
Equations of Equilibrium Valid
• No motion, (Px < Fm )
Equations of Equilibrium Valid

10. (a) F < μ s N
• Friction force necessary for equilibrium can be generated by the surface
• The body is in static equilibrium as assumed
(b) F = μ s N
• Impending motion is deduced
(c) F > μ s N
• Impossible! Assumption of equilibrium is not valid
• Motion occurs, F = μ k N