p140w07_ct_15

# p140w07_ct_15 - Racquetball Striking a Wall Copyright Loren...

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Physics 140: Winter 2007 Lecture #15 February 22, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy

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Imagine a pair of cylinders which rotate about their long axes. They have the same mass and the same outside radius. Cylinder A is solid, and uniform. Cylinder B has a hollow center, with all it's mass located in a thin shell. Which has the largest rotational inertia? A B 1: Cylinder A 2: Cylinder B 3: Depends on their length
Torque and the cross product • Since this quantity is useful to talk about, a short hand notation exists • This is a “vector product”. Multiply two vectors this way and you get an answer which is a vector • Magnitude = rFsin( θ ) • Direction from right- hand-rule () θτ τ sin rF rF F r F r = = = × = r r r r r F r page into is r θ

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Conditions for equilibrium • For equilibrium we now have two conditions: • Note that the second applies around every possible center of rotation • Any time you see an object at rest (or in constant linear or angular motion) you know that both these conditions are true 0 0 center Every = = Σ = = Σ α τ I ma F r
An equilibrium example: the truck on the bridge

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p140w07_ct_15 - Racquetball Striking a Wall Copyright Loren...

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