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Announcements 10/1/12

Updates on class activities, demonstrations, and exam schedules. Get ready for complex numbers and wave equations. Review key physics concepts online and join the interactive demos to enhance your understanding.

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Announcements 10/1/12

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  1. Announcements 10/1/12 • Prayer • Exam going on, ends Thurs night • No office hours for me today, sorry. (Clement still has his.) • The four Taylors series that all physics majors need to know • Complex numbers next time. The Colton “Complex numbers summary” handout on class website should be helpful. The Far Side

  2. From warmup Extra time on? partial derivatives: What do the funny little backward sixes mean? I don't understand partial derivations at all so that all just confused me. what do we do for the partial derivatives in section 16.6?!?! What is that symbol that looks like an upside down e in section 16.6? Can we have a demo on the wave transmittance between a heavier string to a lighter one? Other comments? rate my professor you put a generous curve on the tests. Will you?

  3. Review: k and w • Reminder: what’s the difference between these: • General form of cosine wave: …sometimes written as: • k = “wavenumber”; w = “angular frequency” v = w/k v = lf k = 2p/l w = 2p/T

  4. Clicker question: • A wave pulse traveling on a string hits the end of the string, which is tied to a post. What happens? • The pulse reflects, flipped over • The pulse reflects, not flipped over

  5. Demos • Rubber tubing From warmup: String attached to a wall. Which of the following will decrease the time required for the the pulse to reach the wall? Mark all that apply. Moving your hand up and down more quickly but by the same amount Moving your hand up and down more slowly but by the same amount Moving your hand the same speed but farther up and down Moving your hand the same speed but a shorter distance up and down Using a heavier string of the same length under the same tension Using a lighter string of the same length under the same tension Using the same string of the same length but under more tension Using the same string of the same length but under less tension • Shive wave machine • Web demo: http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf

  6. The (Linear 1D) Wave Equation Why is it called the wave equation? Because traveling waves are solutions of the equation! What’s that funny symbol? C = v2 Any function that has “x-vt” will work! …or “x+vt”

  7. 1 1 T2 q2 q1 T1 Analysis: A section of rope T2 q = small; cosq 1 T1 x x+Dx m = mass/length “linear mass density” q = small; sinq tanq

  8. T2 q2 q1 T1 A section of rope, cont. What is tanq2 in picture?  tanq2 = opp/adj = rise/run = slope! (at x + Dx) T2 T1 x x+Dx

  9. Demo: wave speed vs tension • Can we predict how fast wave will travel on spring (or slinky)? • We need to measure some things: • mass of spring • length of spring • tension of spring • Do the experiment! Time the wave!

  10. Clicker question: • A wave pulse traveling on a string meets an interface, where the medium abruptly switches to a thicker string. What happens? • The pulse continues on, but flipped over • The pulse continues on, not flipped over • The pulse reflects, flipped over • The pulse reflects, not flipped over • The pulse partially reflects and partially transmits Advertisement: We’ll figure out the equations for reflection and transmission in the class after next

  11. Power: energy transfer • What does everything stand for? • Proved in book; most important thing for us now is P ~ A2 • From warmup: Explain how each of the following (keeping everything else the same) would impact the energy transfer rate of a wave on a string: • reducing the mass density of the string • reduces P linearly, assuming that the velocity is unchanged • doubling the wavelength • irrelevant (if no dispersion) • doubling the tension • increases v and therefore P (by sqrt(2)) • doubling the amplitude • increases by 4

  12. The Wave Equation: Linear • Why is it called the linear wave equation? • Because we don’t have nonlinear terms like f2, x2, xf, ex, etc., in the equation itself. • Properties of linear differential equations: • If f1 is a solution, then so is C  f1 • If f1 and f2 are solutions, then so is (f1 + f2) Consider a medium with v = 3 m/s: Any function that has “xvt” will work! It’s a perfectly acceptable wave!

  13. Clicker question: • What happens when two wave pulses on a linear medium run into each other head on? • They reflect off of each other and go back the way they came. • Part of each wave is reflected and part transmitted. • They pass right through each other. • Demo: Shive wave machine interference • Web demo again

  14. Complex Numbers – A Summary What is the square root of 1… 1 or -1? • What is “i”? • What is “-i”? • The complex plane • Complex conjugate • Graphically, complex conjugate = ? • Polar vs. rectangular coordinates • Angle notation, “Aq” • Euler’s equation…proof that eiq = cosq + isinq •  must be in radians • Where is 10ei(p/6) located on complex plane?

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