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Aim: How can we calculate static and kinetic friction? A 3.5 kilogram box is being pulled to the right with a force of 15 newtons. The box is moving at constant velocity. A 3.5 kilogram box is being pulled to the right with a force of 15 newtons. The box is moving at constant velocity. A) Sketch a free body diagram. A) Sketch a free body diagram. B) Identify the net force. B) Identify the net force. C) Determine the frictional force. C) Determine the frictional force.

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Activity Slide block without text book on table. Slide block without text book on table. Slide block with text book on table. Slide block with text book on table. Slide block on sand paper with text book. Slide block on sand paper with text book.

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Friction A force that causes surfaces to STICK TOGETHER At the MICROSCOPIC level, most surfaces are very BUMPY and IRREGULAR Ways to minimize friction SMOOTH SURFACESLUBRICATION

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Friction

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How can we describe frictional force? FRICTION How can we determine the direction of the frictional force? How can we determine the direction of the frictional force? Acts opposite to direction of motion. Acts opposite to direction of motion. Two types of friction: Two types of friction: 1. Static friction (force needed to put an object in motion) 1. Static friction (force needed to put an object in motion) 2. Kinetic friction (force needed to keep an object moving at constant speed). 2. Kinetic friction (force needed to keep an object moving at constant speed).

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Coefficient of Friction (μ) How much materials STICK TOGETHER How much materials STICK TOGETHER DIMENSIONLESS (no units) DIMENSIONLESS (no units) The HIGHER the coefficient, the MORE two surfaces STICK TOGETHER The HIGHER the coefficient, the MORE two surfaces STICK TOGETHER LOWER if surfaces are SLIDING past each other LOWER if surfaces are SLIDING past each other

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What else does frictional force depend on…… 1. Coefficient of friction. 1. Coefficient of friction. 2. The normal force of the object to be moved. 2. The normal force of the object to be moved.

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Coefficients of Friction [Examine table of coefficients of friction in reference table] [Examine table of coefficients of friction in reference table] Coefficient of friction relates to the roughness of the two surfaces in contact. Coefficient of friction relates to the roughness of the two surfaces in contact. Note: Coefficient of friction has no units. Note: Coefficient of friction has no units. Note: Static friction is always greater than kinetic friction between two surfaces – it is harder to start something moving than to keep it moving. Note: Static friction is always greater than kinetic friction between two surfaces – it is harder to start something moving than to keep it moving.

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Friction formula: Friction formula: F f = µF N, where F f = µF N, where F f = force of friction F f = force of friction µ = “coefficient of friction” and µ = “coefficient of friction” and F N = “normal force” acting on the object to be moved. F N = “normal force” acting on the object to be moved. F g =mg F N =mg Only for Horizontal surfaces F g = F N

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Does friction depend on area? Slide the block on your table. Read the scale (Newtons). Slide the block on your table. Read the scale (Newtons). The force of friction does not depend on the surface area of the surfaces. The force of friction does not depend on the surface area of the surfaces.

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Example: How much force must be applied to begin moving a stationary 5 kg wooden box on a wooden floor? How much force must be applied to begin moving a stationary 5 kg wooden box on a wooden floor? F f = µF N = µmg F f = µF N = µmg = (0.42) (5 kg) (9.8 m/s 2 ) = (0.42) (5 kg) (9.8 m/s 2 ) = 20.6 Newtons = 20.6 Newtons

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Aim: How can we solve for kinetic and static friction? 1) Explain the difference between static and kinetic friction. 1) Explain the difference between static and kinetic friction. 2) A student pushes a stationary 500 newton steel box on a level steel surface. The student exerts a 15 newton horizontal force on the box. 2) A student pushes a stationary 500 newton steel box on a level steel surface. The student exerts a 15 newton horizontal force on the box. A) Does the box move? What is the static frictional force? A) Does the box move? What is the static frictional force? B) Determine the minimum amount of force needed to move the box. B) Determine the minimum amount of force needed to move the box.

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Example 2: How much force does it take to keep a 1000 kg car moving at constant speed on a dry asphalt road? How much force does it take to keep a 1000 kg car moving at constant speed on a dry asphalt road? F f = µF N = µmg F f = µF N = µmg = (0.67) (1000 kg) (9.8 m/s 2 ) = (0.67) (1000 kg) (9.8 m/s 2 ) = 6,566 Newtons = 6,566 Newtons

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The coefficient of kinetic friction between a 640.-newton crate and a level warehouse floor is 0.25. Calculate the magnitude of the horizontal force required to move the crate across the floor at constant speed.

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Example 3: Calculate the coefficient of friction between a 60 newton block of wood and sandpaper if it takes 12 Newtons of force to move the wood on the sandpaper? Calculate the coefficient of friction between a 60 newton block of wood and sandpaper if it takes 12 Newtons of force to move the wood on the sandpaper? F f = µF N F f = µF N

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Example 4: What is the coefficient of friction of a tire on a level road if an 1,100kg car decelerates at a rate of 3 m/s 2 once the driver takes her foot off the gas pedal? What is the coefficient of friction of a tire on a level road if an 1,100kg car decelerates at a rate of 3 m/s 2 once the driver takes her foot off the gas pedal?

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Summary Describe friction Describe friction How are static and kinetic friction similar?...how are they different? How are static and kinetic friction similar?...how are they different? Describe what frictional force depends on. Describe what frictional force depends on.

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