p140w07_ct_07

p140w07_ct_07 - Physics 140 Winter 2007 Lecture#7 Dave Winn...

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Unformatted text preview: Physics 140: Winter 2007 Lecture #7 January 25, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy Force laws for friction • Two parts to the contact force – Normal force perpendicular to interface – Frictional force along interface • Both due to interactions between atoms in the surfaces • Last time we saw two kinds of friction: – Kinetic: active force, F f = μ k F N – Static: passive force, limited to F f ≤ μ s F N • Today, look at these more, and examine fluid friction Static example: truck accelerating with a box on the flatbed a What is the maximum acceleration the truck can use if the box is not to slip off the back? F friction F N W=mg g a g m F a m F s box s N s box static friction μ μ μ ≤ = ≤ = The maximum acceleration is typically a fraction of “g”. How can one make this bigger? Higher μ s , or larger F N . An example: truck accelerating with a box on the flatbed a What if the truck accelerates faster than this limit, so that the box begins to slip across the flatbed? F friction F N W=mg g a g m F a m F k box k N k box kinetic friction μ μ μ = = = = The box does accelerate to the right, but with an acceleration less than that of the truck: a box = μ k g < a truck . Eventually the box falls behind the truck and falls to the ground. Friction acting on a slope θ W N F friction ( ) ( ) cos sin = − = Σ = − = Σ ⊥ θ θ W N F F W F friction along θ 90- θ θ A l o n g ⊥ ( ) ( ) ( ) ( ) θ θ θ θ cos cos sin sin mg W N mg W F friction = = = = Friction on a slope θ W N F friction θ 90- θ θ A l o n g ⊥ What’s the maximum angle before the climber slips?...
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This note was uploaded on 04/04/2008 for the course PHYSICS 140 taught by Professor Evrard during the Fall '07 term at University of Michigan.

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p140w07_ct_07 - Physics 140 Winter 2007 Lecture#7 Dave Winn...

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