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Announcements 10/5/12

Announcements 10/5/12. Prayer Handout – Adding together two cosine waves Colloquium: Did you notice “Fourier transforms”? I just got the exams from the Testing Center, TA & I will work on grading them today & this weekend. Non Sequitur. From warmup. Extra time on?

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Announcements 10/5/12

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  1. Announcements 10/5/12 • Prayer • Handout – Adding together two cosine waves • Colloquium: Did you notice “Fourier transforms”? • I just got the exams from the Testing Center, TA & I will work on grading them today & this weekend. Non Sequitur

  2. From warmup Extra time on? how exactly can an amplitude absorb a complex number when it itself is not complex? Is it related to the way you lump a constant into +C after taking an integral? Other comments? (none in particular)

  3. Adding together two cosine waves In short: “The amplitude and phase of the answer were completely determined in the step where we added the amplitudes & phases of the original two cosine waves, as vectors.” Don’t worry about writing each step completely. Don’t write “Real( )” Don’t write “ei(3x)”

  4. HW 16.5: Solving Newton’s 2nd Law Simple Harmonic Oscillator (ex.: Newton 2nd Law for mass on spring) Guess a solution like what it means, really:  there’s an understood “Real{ … }”

  5. Complex numbers & traveling waves Traveling wave: A cos(kx – wt + f) Write as: Often: …or where = “A-tilde” = a complex number the amplitude of which represents the amplitude of the wave the phase of which represents the phase of the wave often the tilde is even left off

  6. Clicker question: Which of these are the same? (1) A cos(kx – wt) (2) A cos(kx + wt) (3) A cos(–kx – wt) (1) and (2) (1) and (3) (2) and (3) (1), (2), and (3) Which should we use for a left-moving wave: (2) or (3)? Convention: Use #3, Aei(-kx-wt) Reasons: (1) All terms will then have same e-iwt factor. (2) Whether you have kx then indicates the direction the wave is traveling. “Wavevector”

  7. From warmup • What was wrong with the first solution that was tried in the reading today (PpP section 3.2)? What assumption did it start with and how could Dr. Durfee tell that that assumption was wrong? • it started by assuming that the wave passed straight from one rope to the next and was wrong because that would lead to the wave having the same velocity on both ropes. • How did the next guess (section 3.3) build on the first? • He then guessed that a wave was partially reflected, instead of solely transmitted

  8. Reflection/transmission at boundaries: The setup x = 0 • Why are k and w the same for I and R? (both labeled k1 and w1) • “The Rules” (aka “boundary conditions”) • At boundary: f1 = f2 • At boundary: df1/dx = df2/dx Region 1: light string Region 2: heavier string transmitted wave in-going wave Goal: How much of wave is transmitted and reflected? (assume k’s and w’s are known) reflected wave

  9. Boundaries: The math x = 0 Goal: How much of wave is transmitted and reflected? and

  10. Boundaries: The math x = 0 Goal: How much of wave is transmitted and reflected?

  11. Boundaries: The math x = 0 • Like: and • How do you solve? Goal: How much of wave is transmitted and reflected? 2 equations, 3 unknowns?? Can’t get x, y, or z, but can get ratios! y = -0.25 x z = 0.75 x

  12. Boundaries: The results x = 0 • Recall v = w/k, and w is the same for region 1 and region 2. So k ~ 1/v • Can write results like this: Goal: How much of wave is transmitted and reflected? The results…. “transmission coefficient” “reflection coefficient”

  13. Special Cases x = 0 • Do we ever have a phase shift in reflected or transmitted waves? • If so, when? And what is it? • What if v2 = 0? • When would that occur? • What if v2 = v1? • When would that occur? The results….

  14. Reflected & Transmitted Power x = 0 • Recall: (A = amplitude) • Region 1: m and v are same … so P ~ A2 • Region 2: m and v are different… more complicated …but energy is conserved, so easy way is: r,t = ratio of amplitudes R,T = ratio of power/energy

  15. Clicker question: • A wave at frequency ω traveling from a string to a rope. At the junction, 80% of the power is reflected. How much power would be reflected if the wave was going from the rope to the string instead? • Much less than 80% • A little less than 80% • About 80% • More than 80% • It depends on the color of the rope.

  16. Demo • Reflection at a boundary. Measure v1 and v2.

  17. Now, on to sound!

  18. Clicker question: • Sound waves are typically fastest in: • solids • liquids • gases

  19. Sound Waves • What type of wave? What is waving? • Demo: Sound in a vacuum • Demo: tuning fork • Demo: Singing rod • Sinusoidal? • Demo: musical disk

  20. Speed of sound • Speed of sound… • in gases: ~300-1200 m/s • in liquids: ~1000-1900 m/s • in solids: ~2000-6000 m/s • v = sqrt(B/r) compare to v = sqrt(T/m) • Speed of sound in air • 343 m/s for air at 20C • Dependence on temperature (eqn in book and also given on exam)

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