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PublishHomer Holland Modified over 9 years ago
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Exploring Frictional Forces Friction Friction and Newton’s Laws Static and Kinetic Friction Coefficient of Friction Practice
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Friction The analysis of friction is very important to practical situations since friction is present everywhere. Friction is a good and bad thing. –Good when our tires meet the pavement –Bad when reducing efficiency and/or producing heat
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Friction Friction always opposes motion (or F net ) –Kinetic friction –Static friction –Air resistance
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Friction and Newton’s Laws Friction is simply a force included and accounted for in a FBD. Applies to situations where: –F net = 0 –F net = ma
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Two types – Static and Kinetic Static friction – stationary scenarios Kinetic friction – moving scenarios The magnitudes of friction depend on –the surfaces in contact with each other (ie. shoe and floor) –the normal force
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Static Friction The magnitude of the force needed to start a stationary object moving is the maximum static friction, F S max
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Kinetic friction Kinetic friction acts on any object that is in motion The magnitude of the force needed to keep the object moving at a constant velocity is the kinetic friction, F K.
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Coefficients of Friction The coefficient of friction is a ratio of the magnitude of the force of friction between two surfaces to the normal force between those surfaces.
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Coefficients of Friction Determining μ S and μ K for given substances is done through experimentation. i.e.
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Practice 1.A crate of fish of mass 18.0 kg rests on the floor of a parked delivery truck. The coefficients of friction between the crate and the floor are μ S = 0.450 and μ K = 0.410. The local value of gravitational acceleration is 9.80 m/s 2. What are the force of friction and the acceleration if: a)a horizontal force of 75.0 N [E] is applied to the crate? b)if a horizontal force of 95.0 N [E] is applied?
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Practice 2.A musician applies a horizontal force of 17 N [W] to an instrument case of mass 5.1 kg. The case slides across a table with an acceleration of 0.39 m/s 2 [W]. What is the coefficient of kinetic friction between the case and the table?
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Practice Solutions 1. a)No movement because the F app < F s b)a = 1.26 m/s 2 2. μ K = 0.30
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