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PH 201 Dr. Cecilia Vogel Lecture 10
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REVIEW Free Body Diagrams forces OUTLINE Friction examples kinetic, static, max static
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Friction and Drag Friction and drag are called “dissipative forces” Both oppose the relative motion of two things friction opposes two surfaces sliding against each other drag opposes a fluid flowing past an object OFTEN friction and drag slow things down but not always! You can use friction between your hand and paper to move paper across table Wind can blow leaves around due to drag
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Three Cases of Friction Static Friction In general, if surfaces don’t slide. just big enough to prevent sliding. must use 2 nd law to find out how big Maximum Static Friction Only if surfaces don’t slide, but are on the verge of sliding. f smax = s F N Kinetic Friction In general, if the surfaces do slide. f k = k F N
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In All 3 Cases Normal force may or may not be equal to the weight, mg. Frictional force opposes slipping between surfaces.
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Example #1 Two 5.0-g coins are on a turntable, which is turning at a rate of 2.0 revolutions per second. One coin is 2.3 cm from the center of rotation, the other is 9.9 cm from the center. The coefficient of kinetic friction between the coins and the turntable is 0.29, and the coefficient of static friction is 0.49. Find the frictional force on each coin. Note: k always < s. k and s don’t have units.
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How to determine if it slips Find the force needed to hold it in place. Find the maximum static frictional force. Compare: If F needed < f smax, then does no t slip, and f = f s = F needed. If F needed > f smax, then does slip, and f = f k If F needed = f smax, then on the verge of slipping, and f = F needed = f smax.
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Example 1 – F needed What would it take to hold coin in place? Apply F = ma to horizontal motion: Object: coin Acceleration: centripetal, Forces: friction To hold a coin in place: inner coin: outer coin:
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Example 1 -- f smax Does FN = mg in this case? Yes, no slope, no other vertical force, no vertical acceleration
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Example 1 – Compare Compare: does the force needed to hold coin in place exceed the maximum static frictional force? For the inner coin: 0.0182 N < 0.0240 N F needed < f smax No, inner coin does not slip. Frictional force is static, and equals F needed 0.0182 N directed toward center.
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Example 1 – Compare Compare: does the force needed to hold coin in place exceed the maximum static frictional force? For the outer coin: 0.0782 N > 0.0240 N F needed > f smax Yes, outer coin does slip. Frictional force is kinetic, and equals Coin slides out, so frictional force is toward center, initially.
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Example #2 A 1.0-kg box is sitting on a slope, with a string attached that goes over a pulley, with 2N weight hanging from the string. The slope makes an angle of 10 o with the horizontal. Find the magnitude and direction of frictional force.
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Example 2 –static In this question, the object not slipping so it involves static friction. which is equal to whatever is needed to keep it from slipping must apply 2 nd law Let +x-direction be down slope What is the direction of friction always opposite tendency to slip tension tends to make it slip up, gravity tends to make it slip down, so I’m not sure GUESS – friction is up the slope
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Example 2 –force diagram x y weight N f FTFT
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Example 2 – book slide If the box is not sliding, a=0. x-components: x y weight N f since the answer came out negative, the direction is opposite what we guessed fs=0.30 N DOWN the slope
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Summary Frictional Forces sometimes f = s F N sometimes f = k F N sometimes f = whatever it takes
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