1. Announcements 10/4/10 Prayer Exam goes until Saturday a. a.Correction to syllabus: on Saturdays, the Testing Center gives out last exam at 3 pm, closes at 4 pm. Complex numbers next time. The Colton “Complex numbers summary” should be helpful. (We’ll go over today, if enough time.) Homework survey

2. k and  What’s the difference between these: General form of cosine wave: …sometimes written as: k = “wavevector”;  = “angular frequency”  = …

3. Quick Writing If I have a longitudinal wave passing down my slinky, described by the function s(x,t), what does s represent, what does x represent, and what does t represent? Other good questions: For a fixed time, what would s(x) represent? For a fixed x, what would s(t) represent?

4. Demos Shive wave machine Web demo: http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf

5. Reminder: the Wave Equation Why is it called the wave equation? a. a.Because traveling waves are solutions of the equation! Any function that has “x-vt” will work! …or “x+vt”

6. Analysis: A section of rope T2T2 T1T1 x x+  x 11 11  = small; cos   1 T2T2 22 T1T1 11  = small; sin   tan   = mass/length “linear mass density”

7. A section of rope, cont. What is tan  2 in picture?  tan  2 = opp/adj = rise/run = slope! (at x +  x) T2T2 T1T1 x x+  x T2T2 22 T1T1 11

8. Demo: wave speed vs tension I think mass of spring is 0.276 kg. (Check with balance.) Can we predict how fast wave will travel?

9. Reading Quiz A wave pulse traveling on a string hits the end of the string, which is tied to a post. What happens? a. a.The pulse reflects, flipped over b. b.The pulse reflects, not flipped over

10. Reading Quiz A wave pulse traveling on a string meets an interface, where the medium abruptly switches to a thicker string. What happens? a. a.The pulse continues on, but flipped over b. b.The pulse continues on, not flipped over c. c.The pulse reflects, flipped over d. d.The pulse reflects, not flipped over e. e.The pulse partially reflects and partially transmits Advertisement: We’ll figure out the equations for reflection and transmission on Friday

11. Power: energy transfer (Proved in book; most important is P ~ A 2 )

12. The Wave Equation: Linear Why is it called the linear wave equation? a. a.Because we don’t have nonlinear terms like f 2, x 2, x  f, e x, etc., in the equation itself. Properties of linear differential equations: a. a.If f 1 is a solution, then so is C  f 1 b. b.f 1 and f 2 are solutions, then so is (f 1 +f 2 ). Consider: Any function that has “x-vt” will work!

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

14. Complex Numbers – A Summary What is “i”? What is “-i”? The complex plane Complex conjugate a. a.Graphically, complex conjugate = ? Polar vs. rectangular coordinates a. a.Angle notation, “A  ” Euler’s equation…proof that e i  = cos  + isin  a. a.  must be in radians