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297Chap8

# 297Chap8 - Laws of dry or Coulomb friction"The physical...

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Laws of dry or Coulomb friction "The physical cause of the resistance offered by friction to the motion of surfaces which slide on each other can be explained either by supposing an interlocking of the roughness of the surfaces, which cannot be sepa- rated unless they yield or break or are lifted over the tops of on another, or by supposing the the molecules of the surfaces of the two planes in contact are so close together that they dev elop a cohesion which must be over- come to produce the motion; experiment alone can enable us to determine the reality of these different causes." --Coulomb, The ´orie des Machines Simples , (1779)

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CE 297 2 Laws of dry or Coulomb friction (cont) No motion For P small, the body is in equilibrium. N = V + W F = P V W N F P Impending motion If P is increased, there is no motion until P = F m . N = V + W F = F m = P V W N F m P
CE 297 3 Laws of dry or Coulomb friction (cont) Motion For P > F m , the body moves. N = W + V F = F k < F m The unbalanced force P - F k = ma causes the body to accelerate with accleration a . a = P - F k m V N P W ma = F k

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CE 297 4 Laws of dry or Coulomb friction (cont) Coefficients of friction Experimentally, F m = μ s N F k = μ k N where μ s = coefficient of static friction μ k = coefficient of kinetic friction Since F k < F m , then μ k < μ s .
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297Chap8 - Laws of dry or Coulomb friction"The physical...

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