Whiteboard Work

1. Whiteboard Work • A child on a playground swing makes a complete to-and-fro swing every 2 seconds. • What is the frequency of the swinging? • What is the period of the swinging? Don’t forget the units!

2. Waves Part 1: Basics

3. Objectives • Identify the parts of a wave. • Relate wave speed, frequency, wavelength, and period. • Trace the movement of the medium in a string wave, in a sound wave, and in a compression wave in a spring.

4. What’s the Point? • How are waves described? • Why do waves occur?

5. Week Outline • Waves in One Dimension • Features of Waves • Types of Waves • Combinations of Waves • Interference • Standing Waves

6. Waves and Vibrations • Vibrations: Repeat in time • no net displacement of disturbance • no net displacement of medium • Waves: Repeat in time and space • wave travels • no net displacement of medium

7. crest l l trough Features of a Wave • Crest: high point • Trough: low point • Wavelength: crest-crest distance • Period: crest-crest-timing

8. u A Features of a Wave • AmplitudeA: (crest height – trough height) / 2 • Frequencyf: repeats in a given time (= 1/T) • Velocityu: speed of crest motion

9. Relations between Features • Period T = 1 / f; Frequency f = 1 / T • Velocity u = l / T = lf • Wavelengthl=uT=u / f • Frequencyf = u / l; Period T = l / u

10. Group Question Doubling the frequency of a wave while keeping its speed constant will cause its wavelength to • increase. • decrease. • stay the same.

11. Group Question Doubling the speed of a wave while keeping its wavelength constant will cause its frequency to • increase. • decrease. • stay the same.

12. Group Question Doubling the wavelength of a wave while keeping its frequency constant will cause its speed to • increase. • decrease. • stay the same.

13. Group Whiteboard Work • A wave generator produces 10 pulses each second. The pulses travel at 300 cm/s. • What is the period of the waves? • What is the wavelength of the waves?

14. Wave Pulse in a Rope • Why does the pulse move? • What determines its speed? • What is happening inside the rope?

15. Prediction Increasing the tension on a rope will make waves travel along the rope • faster. • slower. • At the same speed (no effect).

16. Prediction Increasing the length density (mass per meter) of a rope will make waves travel along the rope • faster. • slower. • At the same speed (no effect).

17. a c b d What are the velocity and acceleration of the string particles at the following positions? Why? • middle (leading edge) • crest • middle (trailing edge) • trough Points to Ponder The particles of the string change their motion as the wave travels. What force accelerates them?

18. Types of Waves • Motion of the medium is perpendicular to the direction the wave travels: transverse wave (example: string wave) • Motion of the medium is parallel to the direction the wave travels: longitudinal wave (examples: sound wave, slinky wave) • Animation

19. Question “The Wave” performed by sports fans is an example of • a longitudinal wave. • a transverse wave. • an unnatural wave.

20. Combining Waves

21. Group Work • Sketch the wave resulting from the addition of the two waves shown at one instant. 3 0 –3

22. result Group Work • Sketch the wave resulting from the addition of the two waves shown at one instant. 3 0 –3

23. Interference • Constructive: Sum of waves has increased amplitude • Destructive: Sum of waves has decreased amplitude • Two-wave simulation

24. Beats • Waves of similar frequency combine to give alternating times of constructive and destructive interference • Distinctive “waa-waa” sound with beat frequency equal to the difference in frequency of the component waves (Why?) • (sound files)