Force of Friction Friction acts to oppose motion between two surfaces in contact Friction acts to oppose motion between two surfaces in contact F f F f Dependant on Dependant on –Surfaces –Normal Force F

Causes of friction: When two surfaces touch, they temporarily bond. In order to move the object one must break this bond. When objects are moving past each other, there is still an electrostatic attraction at the atomic level and this is the weaker kinetic friction. When two surfaces touch, they temporarily bond. In order to move the object one must break this bond. When objects are moving past each other, there is still an electrostatic attraction at the atomic level and this is the weaker kinetic friction.

Static friction: Static friction: –opposing force exerted by a surface when there is no relative motion Kinetic friction: Kinetic friction: –opposing force exerted by a surface when there is relative motion F f kinetic = μ k F N F f kinetic = μ k F N μ is a constant dependant on the surfaces in contact. μ is a constant dependant on the surfaces in contact. Some common coefficients are in table 4-2, pg 97 Some common coefficients are in table 4-2, pg 97 0 ≤ F f static ≤ μ s F N or 0 ≤ F f static ≤ μ s F N or F f kinetic = μ k F N

Example You push a 25 kg wooden box on a wooden floor with a constant velocity of 1.00m/s. How much force do you exert on the box? μ k = 0.2 You push a 25 kg wooden box on a wooden floor with a constant velocity of 1.00m/s. How much force do you exert on the box? μ k = 0.2 Constant Velocity ΣF = 0 ΣF x = 0 = F a – F f F a = F f m=25kg v=1.00m/s μ k = 0.2 F a = ? F f = μ k F N F N = F g =mg So substituting: F f = μ k mg F f = 0.2 (25kg) (9.80m/s 2 ) F f = 5N F a = 49N