Physics Unit 8 : Waves

1. Physics Unit 8:Waves

2. L x F mg Part 1 Simple Harmonic motion

3. AmplitudeA Periodic Motion Simple periodic motion is that motion in which a body moves back and forth over a fixed path, returning to each position and velocity after a definite interval of time. Period, T, is the time for one complete oscillation. (seconds,s) Frequency, f, is the number of complete oscillations per second. Hertz (s-1)

4. x F Simple Harmonic Motion, SHM Simple harmonic motionis periodic motion in the absence of friction and produced by a restoring force that is directly proportional to the displacement and oppositely directed. A restoring force, F, acts in the direction opposite the displacement of the oscillating body.

5. Period and Frequency for a mass and spring For a vibrating body with an elastic restoring force: Recall that F = ma = -kx: The frequencyfand the periodTcan be found if the spring constant k and mass m of the vibrating body are known. Use consistent SI units.

6. L mg The Simple Pendulum The period of a simple pendulum is given by: For small angles q.

7. Part 2 – Wave Characteristics

8. What is a Wave? • A disturbance that carries energy through matter or space.

9. What is a Wave?

10. 2 Categories of Waves • Mechanical Waves • Require a medium (material) to travel through • Examples: water waves, sound waves • Electromagnetic Waves • Do not require a medium • Example: light travels through empty space

11. Wave Description • Crest • Trough • Amplitude • Intensity of the wave. • How much energy is transferred. • Wavelength • The distance from one point on a wave to the same point on the next wave.

12. Wave Description • Frequency • How frequently a wave pulse occurs. • Unit • Hertz (Hz) • 1 Hz = 1/seconds or s-1 • Equation

13. Wave Description • Period • The time (in seconds) that it takes for one wave to pass. • Equation • T = Period (in seconds)

14. Wave Description • Frequency and Period are Inversely Related

15. Types of Waves • Transverse Waves • Matter moves perpendicular to the direction of wave movement.

16. Types of Waves • Longitudinal Waves • Matter moves parallel to the direction of wave movement.

17. Other Types of Waves! • Surface (water) Waves

18. Other Types of Waves! • Raleigh Waves • Seismic (earthquake) waves.

19. Wave Interactions • When two (or more) waves are travelling through the same medium they will interact with each other. • The net displacement is the sum of the two waves.

20. Wave Interactions • Constructive Interference • The two waves are in-phase. • The Amplitudes add together.

21. Wave Interactions • Destructive Interference • Waves are out-of-phase. • Amplitudes “cancel out”

22. Wave Interactions • Example: • Shown to the right are four different pairs of wave pulses that move toward and interact with each other. Rank, from most to least, the height of the peak that results when the two pulses coincide. • Answer:A,B,D,C

23. Standing Waves • A repeated wave produces a pattern caused by the rotation of constructive and destructive interference.

24. Standing Waves

25. Wave Motion • Waves transfer energy! • Matter has no net movement.

26. Wave Motion • Waves transfer energy! • Matter has no net movement.

27. Wave Speed • Equation • Examples • A wave with a wavelength of 2 meters has a frequency of 50 Hertz. What is the velocity of the wave? • Answer: 100 m/s v = wave velocity (in m/s)f = frequency (in Hertz)λ= wavelength (in meters)

28. Wave Speed • Examples • Ocean waves are washing up on shore with a velocity of 5 m/s. If they each have a length of 1.5 meters, what is the frequency of the ocean waves? • Answer: 3.33 Hz

29. Part 2 - SOUND

30. What is Sound? • A sound wave is a mechanical, longitudinal wave produced by a vibration of matter.

31. What is Sound? • A sound wave is a mechanical, longitudinal wave produced by a vibration of matter.

32. What is Sound? • Pitch • Determined by the frequency of the sound wave. • Humans can hear frequencies from 20Hz – 20,000Hz • (dogs can hear up to 40,000 Hz)

33. What is Sound • Pitch • Infrasonic • Sound waves with frequencies below 20 Hz. • Ultrasonic • Sound waves with frequencies above 20,000 Hz.

34. What is Sound? • Amplitude • The intensity of a sound wave • How “loud” it is. • “Loudness” is measured in Decibels (dB)

35. Sound in Air • Speakers (and other objects) produce sound by vibrating and compressing air.

36. Sound in Air • Compressions • Higher pressure air. • Rarefactions • Lower pressure air.

37. The Speed of Sound in Air • 340 m/s or 761 mph • The speed of sound varies. • It is affected by the medium and temperature. • More dense medium = faster • Warmer = faster

38. Sound Behaviors • Reflection • Sound “bouncing” off of an object. • The angle of incidence equals the angle of reflection.

39. Sound Behaviors • Refraction • Bending of a wave caused by a change in medium.

40. Sound Behaviors • Diffraction • The bending of a wave around an obstacle.

41. Sound Behaviors • Sound Wave Interference • Sound waves can interfere with one another and cause beats.

42. Natural Frequency • The special sound an object has when it vibrates. • Every object has a specific frequency.

43. Harmonics • A harmonic is a whole number multiple of the natural frequency.

44. Resonance • The frequency of forced vibrations in an object matches its natural frequency, causing it to resonate. • This creates a huge increase in amplitude.

45. The Doppler Effect • A change in the frequency of a wave caused by an objects motion.

46. The Doppler Effect • Shock Wave • Created by an object moving faster than the speed of a wave in a medium.

47. The Doppler Effect • Sonic Boom • A loud sound created by a shockwave when something is travelling faster than the speed of sound.

48. The Doppler Effect • Weather Radar

49. The Doppler Effect • Astronomers use Red Shift and Blue Shift to understand the Universe!

50. The End !!!