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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 1 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 1 Theory of dry friction • Effects of normal and frictional loadings are indicated by their resultants N and F 1 Object is stationary Impending motion Limiting static friction force On the verge of slipping Fs = μsN 1 Object is in motion Kinetic frictional force P > Fs Fk = μkN 1 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 1 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) 1 Problem 1