Lect21-Phys172s11-(11.4-11.9-Angular_momentum)

# Lect21-Phys172s11-(11.4-11.9-Angular_momentum) - PHYS 172...

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Spring 2011 PHYS 172: Modern Mechanics Lecture 21 – Angular momentum Read 11.4 – 11.9 EXAM 3 is next week Time: 8:00-10:00 pm Wed Apr 13th Place: Elliott Hall Material: lectures 15-22 and respective HWs Problems: multiple choice, 14 questions (70 points) write-up part, hand graded (30 points) Equation sheet : provided Practice exam: will be posted Note: no lecture on April 14th Conservation of angular momentum , , 0 A system A surroundings L L Δ + Δ = Example: Important: both L ’s must be about the same point (axis)

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A comet A A L r p = × 1 1 2 2 rmv r mv = 1 1 2 2 rv r v = A A net A dL r F dt τ = × = A r grav F CLICKER: What is the direction of the torque on the comet in point B about the star due to gravitational pull? A) Into the page B) Out of the page C) It is zero B (nonrelativistic) Example: Kepler and elliptical orbits Kepler, 1609: “a radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time” Can be easily proven using conservation of angular momentum See book p. 430 (11.4)
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Lect21-Phys172s11-(11.4-11.9-Angular_momentum) - PHYS 172...

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