Friction A retarding force that resists the relative movement of - - PowerPoint PPT Presentation

Friction

 A retarding force that resists the relative movement

• f two bodies in contact with each other

 Always acts in oppose direction of motion – actual motion

• r impending motion

 Always acts parallel or tangent to surface(s) in contact  Magnitude is mainly dependent on surface roughness

(other contributing factors: temperature, molecular adhesion, electrostatic attraction, lubrication, relative velocities)

Coefficient of Static Friction on Dry Surfaces

Materials Coefficient of Static Friction or μ

Metal on metal 0.15 – 0.60 Metal on wood 0.20 – 0.60 Metal on stone 0.30 – 0.70 Metal on leather 0.30 – 0.60 Wood on wood 0.25 – 0.50 Wood on leather 0.25 – 0.50 Stone on stone 0.40 – 0.70 Earth on earth 0.20 – 1.00 Rubber on concrete 0.60 – 0.90

Friction

 Two types of friction

 Dry or Coulomb friction  Static – surfaces are at rest with respect to each other  Kinetic/dynamic – surfaces are moving with respect to

each other

 Fluid Friction  Viscosity, friction developed between layers of a fluid

moving at different velocities

We’ll limit our discussion to dry friction

Friction Theory

 Consider a block on a horizontal surface

 No Frictional resistance  N is referred to as the Normal force, perpendicular to the

contact surface W N

Friction Theory

 Next an external force is applied

 If no friction, block would immediately move  If force is applied, F = P, for equilibrium  At some point, P > F, motion occurs

W N P F

Friction Theory

Ratio of F to N is called the Coefficient of Static Friction or μ

N F

s 



Where

μs = coefficient of static friction (unitless)

F – Max or limiting frictional force resistance (lb, k, N) N – Normal force, perpendicular to contact surface (lb, k, N)

Friction Theory

Stated another way: Fs = μs N (for impending motion) Fk = μk N (for sustaining motion, kinetic) In general: F ≤ μ N