SMDEP Physics

Waves: Standing Waves, Sound Waves

Webpage updated: http://www.astro.yale.edu/krines/smdep

Harmonic Motion and Waves in the Real World:

• Harmonic Motion: springs (car shocks, slinkys, DNA molecules)– Pendulums (clocks, swings, skyscrapers)– Periodic motion: orbiting satellites, planets, stars, galaxies

• Waves: Sound– Sonar (bats, submarines)– Ultrasound (reflection/transmission of waves depends on

density)• Waves: Light (Electromagnetic waves)

– X-rays (see Ultrasound)– Lasers (pointers, grocery stores, cat toys, space weapons)– Radio (AM/FM, satellite), TV (broadcast)– Microwave (microwave ovens, cordless phones, cell

phones)• Waves: Compressional (density waves)

– Traffic slowdowns– Spiral galaxies– Sound

Ch 16, #3: distance

0%

94%

0%2%4%0%

1. 5 m2. 500 m3. 50 m4. 5000 m5. Other 6. Didn’t finish

Ch 16, #21

0% 2% 0%0%

98%

0%

1. 93 dB2. 97 dB3. 80 dB4. 87 dB5. Other6. Didn’t finish

Ch 16, #61: horn frequency?

0% 0% 0%0%0%

100%1. 132 Hz 2. 126 Hz3. 120 Hz4. 66 Hz5. Other6. Didn’t finish

Ch 16, #63: beat frequency?

0%

96%

2%0%0%2%

1. 45 Hz2. 91 Hz3. 182 Hz4. 450 Hz5. Other6. Didn’t finish

What is required to produce a standing wave?

13%

0%

88%

0%0%

1. Two oppositely traveling waves

2. Same frequency3. Same wavelength4. Same amplitude5. All of the above

Consider the sum of two similar waves (same amplitude, wavelength,

frequency) traveling in opposite directions. What is the amplitude of

the standing wave they produce?

9%

64%

2%

26%

0%

1. Same as either wave2. Twice the amplitude

of either wave3. Half the amplitude

of either wave4. Zero5. None of the above

Consider the sum of two similar waves (same amplitude, wavelength,

frequency) traveling in opposite directions. What is the wavelength of

the standing wave they produce?98%

0%0%2%

1. Same as original wavelength

2. Twice the original wavelength

3. Half the original wavelength

4. Cannot tell without knowing the phase difference

Consider the sum of two similar waves (same amplitude, wavelength,

frequency) traveling in opposite directions. What is the frequency of

the standing wave they produce?1. Same as original

frequency2. Different from the

original frequency

Two sound waves combine to form beats, which occur 3 times a second. Two different sound waves combine to

form a different set of beats, which occur 2 times a second. For which pair

of waves is the frequency difference greater?

1. The first pair (3 beats a second)

2. The second pair (2 beats a second)

3. Same for both4. Need more info

Doppler shift

source moves toward detectorshorter (higher f)

source moves away from detectorlonger (higher f)

detector moves away from sourcelonger (higher f)

detector moves toward sourceshorter (higher f)

Doppler shift

Doppler shift formula

• f’ = f (v vD)• _____• (v vS)

Two sound waves travel in the same region and thus interfere with each other. What is required to produce

100% destructive interference?

10

0of70

20%20% 20%20%20%1. Waves have same frequency

2. Waves have same amplitude

3. Waves differ in phase by radians

4. All of the above5. None of the above

Joe hears a fire engine siren coming towards him at speed v. For Jane to hear the identical pitch (frequency) from the siren of a stationary fire

engine, she would have to:

10

0of70

20% 20% 20%20%20%1. Be standing still2. Be moving towards the

fire engine at speed v3. Be moving towards the

fire engine at some other speed

4. Be moving away from the fire engine at speed v

5. Need more info

A submerged submarine sends a sonar wave into the surrounding ocean, and

it (the wave) reflects off a distant object. If the sub is moving away from the object that reflects the sonar wave, the frequency of the reflected wave is

10

0of70

20% 20% 20%20%20%

1. Smaller than2. Equal to3. Larger than4. (depends on motion of

distant object)5. (need more info)

… the frequency of the emitted wave