 # FRICTION A Force that opposes the motion of an object.

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FRICTION A Force that opposes the motion of an object.

There are two types of sliding friction: F s = Static friction – A frictional force that prevents motion between two objects. F k = Kinetic Friction – A frictional force that acts after the object is moving. F s >F k

µ = The coefficient of friction. A number that determines the force of friction relative to the perpendicular force (F ). It depends on the texture and composition of the two materials in contact. It is determined experimentally (no units) Rough surfaces have a large µ. Smooth surfaces have a small µ. 0 µ < 2 µ s > µ k

Coefficient of friction values: surfacesµsµs µkµk Rubber on concrete dry 1.2.85 Rubber on concrete wet 0.800.60 Wood on snow.05.03 Steel on steel.75.48 Steel on steel lubricated.12.07

Calculating the force of friction

Calculate the force of friction as a 1500kg car skids to a stop. (µ k =.85) F g = -15000N F =15000N F k = -12750N

What force would have to be applied to set a 20kg crate into motion (µ s = 0.60) F g = -200N F =200N F s = -120N 120N Static friction will match the applied force until it reaches 120N at which point it will break free. Then kinetic friction will act.

A 15kg box is pulled across the floor at a constant speed by a 80N force as shown in the diagram. What is the coefficient of friction? 80N 30º F g = -150N 80cos30 = 69.3 80sin30° = 40.0N F = 110N f = -69.3N

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