Chapter 17

1. Chapter 17 Mechanical Waves and Sound

2. DO NOW: • Why does a wave topple over on itself when it approaches the shore? • Why can you hear the pencil sharpener next door? • What is a wave pool?

3. Learning Goals • I can describe the cause of mechanical waves. • I can identify the three main types of mechanical waves and label their parts.

4. 17.1 Mechanical Waves • A mechanical wave is a disturbance in matter that carries energy from one place to another. • Medium- The material through which a wave travels. • A mechanical wave is created when a source of energy causes a vibration to travel through a medium. In a wave pool, the waves carry energy across the pool

5. 3 Types of Waves • 1. Transverse Waves- shake a rope up and down. • Crest- highest point • Trough- lowest point • A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels.

6. IDENTIFY THE PARTS OF THE DIAGRAM What type of wave? Transverse Wave F. Crest H. Trough G. Amplitude J. Wavelength

7. 3 Types of Waves • 2. Longitudinal Waves- • Compression- area where the particles in a medium are spaced close together • Rarefaction- area where the particles in a medium are spread out • longitudinal waves, the vibration of the medium is parallel to the direction the wave travels.

8. IDENTIFY THE PARTS OF THE DIAGRAM • What type of wave is this? Longitudinal Wave A. Compression B. Rarefaction

9. 3 Types of Waves • 3. Surface Waves- • A wave that travels along a surface separating two media • Bobber moves up and down like a transverse wave and back and forth like a longitudinal wave • Most waves do not transfer matter. • Close to the shore, a surface wave topples over on itself because friction with the shore slows the bottom of the wave. The medium, and anything floating in it, travels to the shore.

10. Learning Goal Check • 5 , 3, or zero fingers… • I can describe the cause of mechanical waves. • I can identify the three main types of mechanical waves and label their parts.

11. Exit Ticket • Give one example for each of transverse, longitudinal and surface waves. • What does a mechanical wave need to travel through a medium? • What three things can act as a medium? • Homework: Complete Reviewing concepts 17.1

12. Do Now: • What are the three types of mechanical waves? • Draw and label the parts of a transverse wave.

13. Learning Goals I can describe what determines the frequency of a wave. I can describe how frequency, wavelength, and speed are related. I can describe how the amplitude of the wave is related to the wave’s energy.

14. 17.2 Properties of Mechanical Waves • Frequency and Period • Periodic Motion- any motion that repeats itself at regular time intervals. • Period- time required for 1 cycle, a complete motion that returns to its starting point. • Frequency- number of complete cycles in a given time. • For a wave – number of wave cycles that pass a point at a given time. • Frequency is measured in cycles per second or hertz. (Hz) • A wave’s frequency equals the frequency of the vibrating source producing the wave. 1 vibration = 1 Hz

15. Frequency is the number of complete cycles in a given time. AA wave vibrating at one cycle per second has a frequency of 1.0 Hz. B A wave vibrating at two cycles per second has a frequency of 2.0 Hz

16. Wavelength • Wavelength- distance between a point on one wave and the same point of the next cycle of the wave. • Transverse = crest to crest or trough to trough • Longitudinal = compression to compression or rarefaction to rarefaction • Increasing the frequency of the wave decreases its wavelength.

17. Wave Speed • speed = wavelength x frequency • (m/s) (m) (Hz) • The speed of a wave can change if it enters a new medium or if variables such as pressure or temperature change.  • If speed is constant, wavelength is inversely proportional to frequency. • Example Problem: One end of a rope is vibrated to produce a wave with a wavelength of 0.25 meters. The frequency of the wave is 3.0 hertz. What is the speed of the wave?

18. Math Practice • 1.A wave on a rope has a wavelength of 2.0 m and a frequency of 2.0 Hz. What is the speed of the wave? • 2.A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a frequency of 4 Hz and a wavelength of 0.1 m. What is the speed of the wave? • 3.What is the speed of a wave in a spring if it has a wavelength of 10 cm and a period of 0.2 s? (v=λ/period) • 4.What is the wavelength of an earthquake wave if it has a speed of 5 km/s and a frequency of 10 Hz?

19. Amplitude • Amplitude- maximum displacement of the medium from its rest position. • The more energy a wave has, the greater its amplitude. • For longitudinal waves, more energy means more compression. . A The amplitude of a transverse wave equals the distance to the highest point above the rest position. B This wave's amplitude is one half the amplitude of the wave in A

20. Independent Practice • Complete Wave Math Worksheet 1-13 • Write answers on separate sheet of paper and hand in

21. Learning Goals I can describe what determines the frequency of a wave. I can describe how frequency, wavelength, and speed are related. I can describe how the amplitude of the wave is related to the wave’s energy.

22. Exit Ticket • If a wave’s period doubles, how does the wave’s frequency change? • A wave has a wavelength of 25 mm and a frequency of 3.0 Hz. What is its speed? • What is the wavelength of a wave that has a wave speed of 10 m/s and a frequency of .3 Hz?

23. 17.3 Behavior of Waves The ripples visible on the bottom of the pool are caused by light shining through surface waves.

24. Reflection • Reflection-a wave bounces off a surface that it cannot pass through. (like a ball thrown at a wall) • It does not change the frequency of the wave, but the wave can be flipped upside down.

25. Refraction A lawnmower turns when it is pushed at an angle from the grass onto the gravel • Refraction-the bending of a wave as it enters a new medium at an angle. • When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side. • It only occurs when the two sides of a wave travel at different speeds.

26. As an ocean wave approaches the shore at an angle, the wave bends, or refracts, because one side of each wave front slows down before the other side does

27. Diffraction • Diffraction- the bending of a wave as it moves around an obstacle or passes through a narrow opening. • Water waves spread out as they pass through a narrow opening. • A wave diffracts more if its wavelength is large compared to the size of the opening or obstacle.

28. Interference • Interference-waves can occupy the same region of space and then continue on. • There are two types of interference • Constructive Interference • Amplitudes of two waves add together. • occurs when two or more waves combine to produce a wave with a larger displacement • Destructive Interference • occurs when two or more waves combine to produce a wave with a smaller displacement. (reduced amplitude)

29. Interference

30. Standing Waves • Standing Waves - A wave that appears to stay in one place. It does not seem to move through the medium. • Node- point on a standing wave that has no displacement from the rest position. Complete destructive interference  • Antinode- a point where the crest and trough occurs. Midway between two nodes • A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.

31. Standing Waves

32. 17.4 Sound & Hearing • Properties of Sound Waves • Sound waves are longitudinal waves—compressions and rarefactions that travel through a medium • Many behaviors of sound can be explained using a few properties—speed, intensity and loudness, and frequency and pitch.

33. Speed of Sound Waves • speed of sound varies in different media

34. Intensity & Loudness • Intensity is the rate at which a wave's energy flows through a given area. • intensity depends on both the wave's amplitude and the distance from the sound source • Sound intensity levels are measured in units called decibels. • The decibel (dB) is a unit that compares the intensity of different sounds. The decibel scale is based on powers of ten. Lengthy exposure to sounds more intense than 90 decibels can cause hearing damage

35. Loudness is a physical response to the intensity of sound, modified by physical factors. • As intensity increases, loudness increases • loudness also depends on factors such as the health of your ears and how your brain interprets the information in sound waves

36. Frequency & Pitch • The longer the tubing, the longer is the wavelength of the standing wave, and the lower is the frequency of the note produced. • Pitch is the frequency of a sound as you perceive it • Pitch, like loudness, also depends on other factors such as your age and the health of your ears

37. Ultrasound • Most people hear sounds between 20 hertz and 20,000 hertz • Infrasound is sound at frequencies lower than most people can hear • ultrasound is sound at frequencies higher than most people hear. • Ultrasound is used in a variety of applications, including sonar and ultrasound imaging.

38. Ultrasound • Sonar is a technique for determining the distance to an object under water. • sound navigation and ranging • The distance to the object is calculated using the speed of sound in water and the time that the sound wave takes to reach an object and the echo takes to return • Ultrasound imaging • an image of the heart made by sending ultrasound pulses into a patient. A pulse is a very short burst of a wave

39. The Doppler Effect • Doppler effect—a change in sound frequency caused by motion of the sound source, motion of the listener, or both. • As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency. Observer A hears a lower-pitch sound than observer B because the wave fronts are farther apart for observer A

40. Hearing and the Ear • The outer ear gathers and focuses sound into the middle ear, • Middle Ear receives and amplifies the vibrations. • The inner ear uses nerve endings to sense vibrations and send signals to the brain.

41. Resonance • Resonance is the response of a standing wave to another wave of the same frequency. • Think of a child being pushed on a swing. If the pushes are timed at the right frequency, the child can swing higher and higher. • In the same way, one wave can “push” another wave to a higher amplitude. • Resonance can produce a dramatic increase in amplitude.