2211_S11+Lecture+27+part2+and+supplement

2211_S11+Lecture+27+part2+and+supplement - Physics 2211:...

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Physics 2211: Matter and Interactions Chapter 9: friction PRS: A block of mass M is pulled at constant speed with force F a distance d across a surface. Using the energy principle, what can you say about the change in thermal energy of the block? (1) ΔE thermal > 0 (2) ΔE thermal = 0 (3) ΔE thermal < 0
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Physics 2211: Matter and Interactions Chapter 9: friction PRS: Solution Point Particle System Real System There seems to be a paradox! When objects rub up against each other they tend to get hot. K trans = F f d = 0 K trans  E thermal = F f d E thermal = 0
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Physics 2211: Matter and Interactions Chapter 9: friction
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Physics 2211: Matter and Interactions Chapter 9: friction Since, from observation, we know that the thermal energy increases it must be that the applied force and the friction force do not move through the same displacement At the atomic level surfaces are not perfectly flat As few as three asperities or teeth will make contact between the two surfaces K trans = 0 E thermal = Fd fd eff E thermal = F d d eff
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Physics 2211: Matter and Interactions Chapter 9: microscopic friction The very high loads at these contact regions result in adhesion; asperities deform before the bond is broken This deformation leads to a difference in the energy equation for the point-particle and real system After separating, the asperities move back to their equilibrium positions resulting in an increase in thermal energy
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Matter and Interactions Chapter 9: friction Example: Imagine a penguin of mass M dropped on a conveyor belt moving with speed v . When the penguin first lands on the belt there is slippage, until the speed of the penguin catches up to the belt. If the coefficient of friction between penguin and belt is μ , what is the distance d the penguin travels before reaching speed v ? How hot does this make the penguin? d
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2211_S11+Lecture+27+part2+and+supplement - Physics 2211:...

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