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Chapter 6 Wave Motion

Chapter 6 Wave Motion. Types of Waves. mechanical waves require a medium ( water, air, springs) electromagnetic waves require no medium (light, radio, micro). Types of Mechanical Waves. L ongitudinal - particles of the medium vibrate parallel to the direction of the wave. eg . sound waves

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Chapter 6 Wave Motion

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  1. Chapter 6Wave Motion

  2. Types of Waves • mechanical waves require a medium (water, air, springs) • electromagnetic waves require no medium (light, radio, micro)

  3. Types of Mechanical Waves Longitudinal- particles of the medium vibrate parallel to the direction of the wave. eg. sound waves Transverse - particles of the medium vibrate perpendicular to the direction of the wave.eg. guitar strings

  4. Types of Mechanical Waves Wave Pulse - a single disturbance that travels through a medium. Periodic Wave - a series of pulses at regular intervals (travelling wave).

  5. Describing Waves

  6. Properties of a Wave Period (T) - the time needed for one complete cycle or wave. Frequency (Hz) - the number of complete cycles per second. ƒ = 1 T = 1 Tƒ

  7. Ex#1. A swing rocks back and forth 15 times in one minute. • What is the frequency of the swing? • What is the period of vibration of the swing?

  8. Ex#2. A pendulum takes 5 s to complete one swing. • What is the frequency of the pendulum? • How many times will the pendulum swing in one hour?

  9. Wave Velocity • the product of the frequency and the wavelength. We know: v = d or v = λ t T But if T = 1/ƒ Then by substitution: v= ƒλ v= velocity of the wave (m/s) ƒ= frequency (Hz) λ= wavelength (m)

  10. Ex#1. The wavelength of a water wave is 0.55 m. If the frequency of the wave is 4.0 Hz, what is the speed of the wave?

  11. Ex#2. A hiker on Mount Baker shouts across a valley to the other hillside, 750 m away. The echo is heard 4.4 slater.a) What is the speed of sound in air.b) If the sound wave has a frequency of 436 Hz, what is its wavelength?

  12. Ex#3. Mac and Josh stand 8 m apart and demonstrate the motion of a transverse wave on a snakey. The wave has a vertical distance of 32 cm from a trough to a crest, a frequency of 2.4 Hz, and a horizontal distance of 48 cm from a crest to the nearest trough. Determine the amplitude, wavelength and speed of such a wave.

  13. Wave Velocity • Wave speed is dependent upon the medium.eg. spring tension, water depth  • If speed decreases, wavelength decreases (frequency remains constant).  • Frequency can only be changed at the source. • If frequency increases, wavelength will decrease to maintain the same speed. 

  14. Amplitude of a Wave • the amplitude of a wave is its maximum displacement from the rest position. • in order to produce a wave with larger amplitude, more work has to be done.

  15. Wave Interference • the speed of a mechanical wave depends upon the medium. eg. depth of the water, temperature of the air and spring tension.  • When a wave hits a boundary or moves from one medium to another, it will either be reflected, refracted, or absorbed.

  16. Reflected wave: bounces back in the direction it came from • Refracted wave: continues forward but changes speed and direction as it enters the new medium • Absorbed wave: wave energy is converted to other forms such as heat and sound

  17. Reflection of Waves • Reflected wave: bounces back in the direction it came from

  18. Law of Reflection - angle of incidence is equal to the angle of reflection. • reflected wave decreases in amplitude (waves lose energy due to friction and absorption by barrier).

  19. Refraction • waves change direction, wavelength and speed at the boundary between 2 different media (ie. different depths of water)

  20. Refraction • when waves enter shallow water: • frequency is the same (only changed at the source) • wavelength decreases • speed decreases

  21. Boundary Behaviour Less Dense to More Dense: Reflected wave is inverted More dense to less dense: Reflected wave is erect * TRANSMITTED WAVE IS ALWAYS ERECT

  22. Boundary Behaviour Free End (More dense to less dense): Reflected wave is erect. Fixed End (Less dense to more dense): Reflected wave is inverted.

  23. Superposition of Waves • the resultant displacement of a particle caused by two or more waves. • equal to the sum of the displacements produced by individual waves.

  24. Constructive Interference • troughs of the 2 waves occur at the same time; combined wave is larger than each of the separate waves. Destructive Interference • crests of one wave arrives at the same time as the troughs of the other wave; net amplitude is zero. Partially Destructive Interference • when 2 waves meet somewhere between the 2 extremes, or if waves have different amplitudes or different wavelengths.

  25. Diffraction • the spreading of waves around the edge of a barrier. (i.e., light and sound can be heard around a corner) • waves will bend as they pass through an opening or around an obstacle. • longer wavelength = greater diffraction.

  26. Doppler Effect • change in frequency (or pitch) resulting from movement of wave source i.e., change in pitch of ambulance siren as it approaches compared to when it moves away

  27. frequency of source doesn’t change, but the waves are compressed together as the source moves towards the receiver; therefore, the frequency of the detected wave is greater.

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