Friction

# Approximately valid for dry surfaces f s max is

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• Keith_2012
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approximately valid for dry surfaces: f s max is independent of the contact area between the object and the surface f s max is proportional to the normal force exerted on the object The constant of proportionality depends upon the object and surface materials n f f f F f s s s s to surface arallel applied, p s μ = - = max max where but : Summary The proportionality constant μ s (dimensionless) is called the “coefficient of static friction.” F applied = f s F applied > f smax F applied f s = 0 What happens when the applied force gets past the threshold?

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Physics 102 Friction Slide 4 Department of Physics, University of Arizona Kinetic Friction ( v ≠ 0) v F applied f k When the box is already sliding along the floor, the surface exerts a force of kinetic friction f k. approximately valid for dry sliding motion: f k proportional to the normal force exerted on the object The constant of proportionality depends upon the object and surface materials f k always directed opposite to v ) of direction to (opposite : Summary v n f k k μ = The proportionality constant μ k (dimensionless) is called the “coefficient of kinetic friction.” In almost all cases, μ k < μ s , so f k < f s max (easier to keep box moving than to start it moving).

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