Lect_31 - Energy. Intensity. Interference

# Lect_31 - Energy. Intensity. Interference - Lecture 31...

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Lecture 31 Energy. Intensity. Interference. Beats.

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ACT: A sixth sense? A large ammunition factory and a town are separated by a rocky hill, at a horizontal distance of about 5 km. An accident produces a huge explosion in the middle of the night. What do the town folks experience? A. First the room shakes, and then they hear an explosion. B. First they hear an explosion, and then the room shakes. C. They hear an explosion and the room shakes at yp the same time.
5 km Time for sound wave to reach the town: Through hill (granite): Through air: 5000 m 0.8 s 6000 m/s x t v == = 14.6 s 343 m/s = granite granite Y ρ ⎛⎞ ⎜⎟ ⎝⎠ 14 seconds later! This happened in California during WWII. Most people woke up (distressed…) to the light quake and then heard the explosion. Many ttributed this to a “sixth sense” that had warned them of the imminent attributed this to a sixth sense that had warned them of the imminent disaster. The “sixth sense” was just the laws of wave propagation…

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Wave energy ork is clearly being done: > as hand Work is clearly being done Fdr 0 as hand moves up and down. • This energy must be moving away from your hand (to the right) since the kinetic energy (motion) of the end of the string grabbed by the hand stays the same. P
Transfer of energy The string to the left of x does work on the string to the right of x, just as your hand did: x Energy is transferred or propagated.

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Power Energy of a mass in SHM (attached to a spring k) Energy of a mass n SHM (attached to a spr ng k) 22 2 1 2 Ek A ω =∝ 2 k m ⎛⎞ = ⎜⎟ ⎝⎠ This energy propagates at speed v.
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## This note was uploaded on 04/05/2010 for the course PHYS 221 taught by Professor Herrera-siklody during the Spring '08 term at Iowa State.

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Lect_31 - Energy. Intensity. Interference - Lecture 31...

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