friction - f k 5 How do we find s and k ? s and k are...

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1 Frictional Forces Friction: a difficult force to quantify. - we base our analysis on observation and experiment. Example: What’s missing from this picture? P v = 0
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2 P v = 0 f s P v = 0 friction 1. before the time at which contact is broken, f s < μ s n { where μ s is the coefficient of static friction. } Units of μ s :
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3 P v = 0 f s 2. At the instant just before contact is broken, f s has its largest magnitude: f s (max) = μ s n If the object is not moving ( v = 0 ), then f s μ s n This means f s takes on all values from zero to μ s n.
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4 3. After contact is broken the object slides across the table under the influence of a smaller force, the force of kinetic friction f k , where f k ≈ constant (why?) and f k = μ k n ( In general, f k < f s ) { where μ k is the coefficient of kinetic friction. } * It is not correct to say “friction is opposite the direction of motion”. * The friction vector points in a direction that opposes the relative motion of the two surfaces in contact. Origin of friction? P v
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Unformatted text preview: f k 5 How do we find s and k ? s and k are empirical constants. You can find s and k between your book cover and your calculator by find the steepest angle for which the calculator does not slip. 6 Coefficients of Friction Materials Coeff. of Static Friction s Coeff. of Kinetic Friction k Steel on Steel 0.74 0.57 Aluminum on Steel 0.61 0.47 Copper on Steel 0.53 0.36 Rubber on Concrete 1.0 0.8 Wood on Wood 0.25-0.5 0.2 Glass on Glass 0.94 0.4 Waxed wood on Wet snow 0.14 0.1 Waxed wood on Dry snow - 0.04 Metal on Metal (lubricated) 0.15 0.06 Ice on Ice 0.1 0.03 Teflon on Teflon 0.04 0.04 Synovial joints in humans 0.01 0.003 7 A crate of eggs is being carried on a truck traveling at 15 m/s. If the coefficient of static friction between the crate and truck bed is 0.32, and the coefficient of kinetic friction between the crate and truck bed is 0.14, find the minimum stopping distance for the truck so that the crate does not slide....
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friction - f k 5 How do we find s and k ? s and k are...

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