p140w07_ct_07 - Physics 140: Winter 2007 Lecture #7 January...

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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 = k F N Static: passive force, limited to F f 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 = = The maximum acceleration is typically a fraction of g. How can one make this bigger? Higher 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 = = = = The box does accelerate to the right, but with an acceleration less than that of the truck: a box = k g < a truck . Eventually the box falls behind the truck and falls to the ground. Friction acting on a slope W N F friction ( ) ( ) cos sin = = = = W N F F W F friction along 90- A l o n g ( ) ( ) ( ) ( ) cos cos sin sin mg W N mg W F friction = = = = Friction on a slope W N F friction 90- A l o n g Whats the maximum angle before the climber slips?...
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p140w07_ct_07 - Physics 140: Winter 2007 Lecture #7 January...

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