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Lect14-Phys172s11-(7.10-7.12-Energy_macroscopic)

Lect14-Phys172s11-(7.10-7.12-Energy_macroscopic) - PHYS 172...

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Spring 2011 PHYS 172: Modern Mechanics Lecture 14 – Energy in Macroscopic System Read 7.10 – 7.12 EXAM 2 is next week Time: 8:00-10:00 pm Wed March 9 Place: Elliott Hall Material: lectures 9-14, respective HWs and recitations Problems: multiple choice, 14 questions (70 points) write-up part, hand graded (30 points) Note: no lecture on March 10 ! Energy dissipation E universe = const Energy dissipation: “loss” of energy into forms less useful to us Example: air resistance : Free fall of metal ball Free fall of Paper h t 0.03 s 0.06 s 0.09 s 0 s 1 m

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3 Terminal speed mg ° air F ° 0 dp dt = ° ° v const = ±±±±± ° ° terminal speed Mechanism of air resistance Simple model: In average, molecules hit object with larger relative momentum from below than from above, the difference is larger for higher speeds. Too simple! Need to take into account macroscopic “wind”-like phenomenon Fluid dynamics: mathematically complex, beyond the scope of this course
Factors determining air resistance 1. Speed of an object in respect to air?

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