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Physics 101: Lecture 21 Waves

Preflight 5. Which concepts did you find the most difficult when preparing for this lecture? . Standing waves was difficult to understand. What does it mean and can you give another explanation as to how to calculate it? Also, reflection was not explained enough. What is the difference between a fi

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Physics 101: Lecture 21 Waves

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    1. Physics 101: Lecture 21 Waves Today’s lecture will cover Textbook Chapter 11

    2. Preflight 5 Which concepts did you find the most difficult when preparing for this lecture?

    3. Waves Overview Types Speed Harmonic Superposition Standing

    4. Stadium wave here, for fun.Stadium wave here, for fun.

    5. Slinky Preflight 3 Suppose that a longitudinal wave moves along a Slinky at a speed of 5 m/s. Does one coil of the slinky move through a distance of five meters in one second? 1. Yes 2. No

    6. Velocity of Waves Act Long Spring and slinky equal length (8 feet each) speeds up a LOT in slinky because mu is small and T is same.Long Spring and slinky equal length (8 feet each) speeds up a LOT in slinky because mu is small and T is same.

    7. Add stuff hereAdd stuff here

    9. Harmonic Waves Exercise y(x,t) = A cos(wt –kx) Label axis and tic marks if the graph shows a snapshot of the wave y(x,t) = 2 cos(4t –2x) at x=0. Recall: T = 2 p /w Students make plot at x=0, give them line, they mark ticsStudents make plot at x=0, give them line, they mark tics

    10. Preflight 1+2 Suppose a periodic wave moves through some medium. If the period of the wave is increased, what happens to the wavelength of the wave assuming the speed of the wave remains the same? 1. The wavelength increases 2. The wavelength remains the same 3. The wavelength decreases

    11. ACT

    12. ACT Solution

    14. Interference and Superposition When two waves overlap, the amplitudes add. Constructive: increases amplitude Destructive: decreases amplitude Do this with springsDo this with springs

    15. Reflection Act A slinky is connected to a wall at one end. A pulse travels to the right, hits the wall and is reflected back to the left. The reflected wave is A) Inverted B) Upright Fixed boundary reflected wave inverted Free boundary reflected wave upright Use large spring as demo hereUse large spring as demo here

    16. Standing Waves Fixed Endpoints Fundamental n=1 (2 nodes) ln = 2L/n fn = n v / (2L) Two people with large spring, can produce all of these as demo.Two people with large spring, can produce all of these as demo.

    17. Standing Waves: Calculate tension in guitar string given mass density and frequency. Ch 11 # 45Calculate tension in guitar string given mass density and frequency. Ch 11 # 45

    18. Summary Wave Types Transverse (eg pulse on string, water) Longitudinal (sound, slinky) Harmonic y(x,t) = A cos(wt –kx) or A sin(wt – kx) Superposition Just add amplitudes Reflection (fixed point inverts wave) Standing Waves (fixed ends) ln = 2L/n fn = n v / 2L

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