Coefficient of Friction

Approximate Coefficients of Friction Rubber on concrete (dry) Rubber on concrete (wet) Rubber on asphalt (dry) Rubber on asphalt (wet) Rubber on ice Waxed ski on snow Wood on wood Steel on steel Copper on steel Tenon on Teflon Kinetic o.6S 038 0.67 053 0.15 030 057 036 0.04 Static 0.90 o. 85 0.14 0.74 053

  • Ratio of the frictional force and the normal force provides the coefficient of friction

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Kinetic or Static

Sled sliding down a snowy hill Refrigerator at rest that you want to move Car with tires rolling freely? 5-b4tc Car skidding across pavement?

Calculating the Force of Friction

  • The force of friction depends only upon the nature of the surface in contact (μ) and magnitude of the normal force FN

  • Combine with Newton's Second Las and FBDs to solve more involved problems

2007 Free Response Question 1

Mech. 1. A block of mass m is pulled along a rough horizontal surface by a constant applied force of magnitude Fl that acts at an angle 9 to the horizontal, as indicated above. The acceleration of the block is al . Express all algebraic answers in terms of m, Fl , 9 , al , and fundamental constants. (a) On the figure below, draw and label a free-body diagram showing all the forces on the block. (b) Derive an expression for the normal force exerted by the surface on the block. (c) Derive an expression for the coefficient of kinetic friction g between the block and the surface. (d) On the axes below, sketch graphs of the speed v and displacement x of the block as functions of time t if the block started from rest at x = 0 and t = 0.

(e) If the applied force is large enough, the block will lose contact with the surface. Derive an expression for the magnitude of the greatest acceleration amax that the block can have and still maintain contact with the ground.

Fur, z fisiMô-*3 șOȘ F, 9 c) Fot-rx:

fcos a = Fcose é) Hg cose _L Sìve

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