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Interest Grabber

VibrationsA wave is a vibration that carries energy from one place to another. But not all vibrations are waves.

Hold a pen lightly between your thumb and index finger. Shake your hand back and forth to make the pen seem to bend like it’s made of rubber. Next, hold a length of string (about 1 meter long) at one end. Shake the end of the string back and forth. Observe the vibrations.

1. Describe the motion of the pen and the motion of the string.

2. In which case did the vibration move from one place to another? In which case did the vibration stay in one place?

Section 17.1

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Mechanical Waves

• A Mechanical wave is a disturbance in matter that carries energy from one place to another.

• Mechanical waves require a medium (solid, liquid or gas) to travel through.

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Transverse Waves

• Transverse Waves

• As the wave travels from one point to another, each individual particle only travels perpendicularperpendicular to the direction the wave travels.

• Each particle experiences no net movement as the energy passes through it.

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Transverse Waves Figure 2

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Longitudinal Waves

• Longitudinal Waves

• As the wave travels from one point to another, each individual particle travels

parallelparallel to the direction the wave travels.

• Each particle experiences no net movement as the energy passes through it.

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Longitudinal Waves Figure 3

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Surface Waves

• Surface waves travel at the surface separating two media.

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Surface Waves Figure 4

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Interest Grabber

Waves Carry EnergyWhen a wave travels through a medium, the wave carries energy, not particles, from one place to another. You can measure the speed of a wave by how fast energy moves from one place to another.

To model a wave’s energy transfer, stand in a line with your classmates. The first person in the line should slowly pass a book to the second person. The second person should pass it to the third person, and so on down the line. Repeat the activity, but this time pass the book quickly.

1. What did the book represent?

2. How would you describe the speed of the wave in each case?

Section 17.2

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Frequency and Wavelength of Transverse Waves

Figures 5 and 6

Frequency is how many cycles in a second

Wavelength is the distance between 2 crests

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Wave speed

• Recall Speed = distance

time

Wave Speed = wavelength x frequency

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Speed of Mechanical Waves

Section 17.2

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Speed of Mechanical Waves

Section 17.2

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Speed of Mechanical Waves

Section 17.2

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Speed of Mechanical Waves

Section 17.2

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Amplitude of Transverse Waves

Section 17.2

Amplitude of a wave is the maximum displacement from the rest position

The more energy a wave has, the greater the amplitude

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Interest Grabber

Reflected WavesWhat happens when a wave hits a wall or some other fixed object? To find out, tie a rope to the back of a chair. Gently shake the rope up and down once to send a single pulse along the rope as shown. Observe what happens when the pulse hits the chair.

1. What happened to the direction of the pulse when it hit the chair?

2. How did the orientation of the reflected pulse compare to the original pulse?

Section 17.3

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Reading Strategy

a. If reflection occurs at a fixed boundary, the reflected wave will be upside down compared to the original wave.

b. Refraction occurs when a wave enters a new medium at an angle because one side of a wave front moves more slowly than the other side.

c. The larger the wavelength is compared to the size of an opening or obstacle, the more a wave diffracts.

d. The types of interference are constructive and destructive interference.

e. A standing wave forms only if a multiple of one half wavelength fits exactly into the length of the vibrating object.

Section 17.3

Identifying Main Ideas

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Reflection & Refraction

• Reflection occurs when a wave bounces off a surface that it cannot pass through.– Reflection does not change the speed or frequency

of a wave, but can be flipped upside down.

• Refraction is the bending of a wave as it enters a new medium and changes speed.– The wave will bend as it speeds up or slows down.

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Diffraction

• Diffraction – bending of a wave around an obstacle or through a narrow opening

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Constructive vs. Destructive Interference of Waves

Figure 12

Constructive interference occurs

when two or more waves combine to produce a larger wave with the

same frequency.

Ex. Two speakers will be louder than one.

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Constructive vs. Destructive Interference of Waves

Figure 12

Destructive Interference occurs when two or

more waves combine to produce a smaller wave of the same frequency.

Ex. Two speakers can cancel each other out and produce a quieter

sound.

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Interest Grabber

Can You Hear Through Your Desk?Sound waves travel through a medium as energy is passed from particle to particle. If the particles are close together, as in a solid, the sound generally travels faster and less energy is lost.

Listen carefully to the sound as you knock on your desk. Note how far your ear is from your hand when you make the sound. Without changing this distance, place your ear against your desk and knock again.

1. In each case, how did the sound travel to your ears?

2. What was the difference in the sound when you held your ear against the desk?

Section 17.4

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The Speed of Sound in Different Mediums

Figure 14

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Sound Waves

• Sound waves are longitudinal- 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

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• Intensity is the rate at which a wave’s energy flows through a given area.

• Sound intensity depends on both the wave’s amplitude and the distance from the source.

• Sound intensity levels are measured in decibels (dB)

• Loudness is a physical response to the intensity of sound.

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• Pitch is the frequency of sound as you perceive it.

• Pitch depends on a wave’s frequency.

• High frequency-high pitch

• Low frequency- low pitch

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• Ultrasound-sound frequencies lower than 20 hertz

• Human hearing- 20 - 20,000 hz

• Ultrasound is used in sonar and ultrasound imaging

• Sonar is a technique for determining the distance to an object under water. (time it takes a sound to reach an object under water and the echo to return.)

• Ultrasound uses a pulse or short burst of a wave 1/8000 of a second. Computer software makes a detailed map of the image.

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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.

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The Doppler Effect Figure 18

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The outer ear

• The outer ear gathers and focuses sound into the middle ear.

• The middle ear receives and amplifies the vibrations using the Hammer, Anvil, and Stirrup.

• The inner ear uses nerve endings to sense vibrations and send signals to the brain.

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The Anatomy of the Ear

Figure 19

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• Most musical instruments vary pitch by changing the frequency of standing waves.

• Resonance is the response of a standing wave to another wave of the same frequency.