System Approximate Coefficients of Static and Kinetic Friction Static Friction /4 Kinetic Friction pk Rubber on dry concrete 1.0 Rubber on wet concrete 0.5-0.7 0.3-0.5 Wood on wood 0.5 0.3 Waxed wood on wet snow 0.14 0.1 Metal on wood 0.5 0.3 Steel on steel (dry) 0.6 0.3 Steel on steel (oiled) 0.05 0.03 Teflon on steel 0.04 0.04 Bone lubricated by synovial fluid 0.016 0.015 0.7 Shoes on wood 0.7 Shoes on ice 0.1 0.05 Ice on ice 0.1 0.03 Steel on ice 0.4 0.02 The coefficients describe the dependence of friction on the materials of the two surfaces in contact. We also need to consider the normal force. The direction of friction is always opposite that of motion, parallel to the surface between objects, and perpendicular to the normal force. For example, if the crate you try to push (with a force parallel to the floor) has a mass of 100 kg, then the normal force is equal to its weight, w = mg = (100 kg) (0.80 m/s2) = 980 N, perpendicular to the floor. If the coefficient of static friction is 0.45, you would have to exert a force parallel to the floor greater than f. (max) = 1&N = (0.45) (980 N) = 440 N to move the crate. Once there is motion, friction is less and the coefficient of kinetic friction might be 0.30, so that a force of only f = \N = (0.30) (980 N) = 290 N keeps it moving at a constant speed. If the floor is lubricated, both coefficients are considerably less than they would be without lubrication. Coefficient of friction is a unitless quantity with a magnitude usually between 0 and 1.0. The actual value depends on the two surfaces that are in contact.