MCQs / Physics / Wave Intensity and the Doppler Effect

Wave Intensity and the Doppler Effect MCQs — Physics (page 2)

60 multiple-choice questions on Wave Intensity and the Doppler Effect from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

26. A source of frequency 440 Hz moves at 15 m/s away from a stationary observer. The speed of sound is 343 m/s. The observed frequency is:

  1. 422 Hz
  2. 440 Hz
  3. 460 Hz
  4. 379 Hz

Answer: A) 422 Hz

Explanation: fo = fs vv + vs = 440 343343 + 15 = 422 Hz. SLO P-11-D-03 · Application

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27. A wave delivers a power of 12 W uniformly over an area of 0.03 m^2. Its intensity is:

  1. 400 W/m^2
  2. 40 W/m^2
  3. 800 W/m^2
  4. 4000 W/m^2

Answer: A) 400 W/m^2

Explanation: I = PA = 120.03 = 400 W/m^2. SLO P-11-D-01 · Application

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28. The intensity of a wave is defined as:

  1. force per unit area
  2. power per unit area perpendicular to the direction of travel
  3. amplitude squared
  4. energy per unit time

Answer: B) power per unit area perpendicular to the direction of travel

Explanation: Unit W/m^2. SLO P-11-D-01 · Application

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29. A blue shift in the spectrum of a star means it is:

  1. moving away from us
  2. stationary
  3. moving towards us
  4. very hot

Answer: C) moving towards us

Explanation: Shorter observed wavelength. SLO P-11-D-04 · Understanding

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30. The Doppler shift formula fo = fs vv pm vs assumes that:

  1. the air moves with the source
  2. the source is stationary
  3. the speed of sound changes
  4. the observer is stationary relative to the air

Answer: D) the observer is stationary relative to the air

Explanation: Moving-observer case is not required. SLO P-11-D-03 · Application

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31. Doppler echocardiography helps doctors detect:

  1. tooth decay
  2. leaking heart valves and abnormal blood flow
  3. broken bones
  4. eyesight problems

Answer: B) leaking heart valves and abnormal blood flow

Explanation: Studying cardiac problems. SLO P-11-D-04 · Understanding

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32. Intensity of a progressive wave is proportional to:

  1. 1 amplitude
  2. ( amplitude)^2
  3. amplitude
  4. amplitude

Answer: B) ( amplitude)^2

Explanation: I propto A^2. SLO P-11-D-01 · Application

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33. A car horn of 400 Hz is heard as 425 Hz as the car approaches (v = 340 m/s). The car's speed is:

  1. 20 m/s
  2. 40 m/s
  3. 10 m/s
  4. 25 m/s

Answer: A) 20 m/s

Explanation: 425 = 400 340340 - vs Rightarrow vs = 340 - 320 = 20 m/s. SLO P-11-D-03 · Application

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34. A source moves away at a speed equal to the speed of sound. The observed frequency is:

  1. twice the source frequency
  2. half the source frequency
  3. the same
  4. zero

Answer: B) half the source frequency

Explanation: fo = fs v2v. SLO P-11-D-03 · Application

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35. A progressive wave has intensity 10 W/m^2. If its amplitude is multiplied by 3, its new intensity is:

  1. 180 W/m^2
  2. 45 W/m^2
  3. 900 W/m^2
  4. 90 W/m^2

Answer: D) 90 W/m^2

Explanation: I propto A^2, so I = 10 (3)^2 = 90 W/m^2. SLO P-11-D-01 · Application

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36. Weather radar uses the Doppler effect to:

  1. measure the speed and direction of rain and wind in storms
  2. measure air pressure
  3. take satellite photos
  4. predict earthquakes

Answer: A) measure the speed and direction of rain and wind in storms

Explanation: Doppler radar. SLO P-11-D-04 · Understanding

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37. A train sounding its horn passes you at constant speed. The pitch you hear:

  1. stays the same
  2. drops suddenly as it passes
  3. rises as it passes
  4. goes silent

Answer: B) drops suddenly as it passes

Explanation: High while approaching, low while receding. SLO P-11-D-02 · Understanding

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38. A source of frequency 800 Hz moves at 40 m/s away from a stationary observer. The speed of sound is 340 m/s. The observed frequency is:

  1. 644 Hz
  2. 716 Hz
  3. 907 Hz
  4. 800 Hz

Answer: B) 716 Hz

Explanation: fo = fs vv + vs = 800 340340 + 40 = 716 Hz. SLO P-11-D-03 · Application

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39. A source of frequency 500 Hz moves at 20 m/s towards a stationary observer. The speed of sound is 340 m/s. The observed frequency is:

  1. 531 Hz
  2. 584 Hz
  3. 500 Hz
  4. 472 Hz

Answer: A) 531 Hz

Explanation: fo = fs vv - vs = 500 340340 - 20 = 531 Hz. SLO P-11-D-03 · Application

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40. A source of frequency 1200 Hz moves at 60 m/s towards a stationary observer. The speed of sound is 340 m/s. The observed frequency is:

  1. 1410 Hz
  2. 1200 Hz
  3. 1020 Hz
  4. 1460 Hz

Answer: D) 1460 Hz

Explanation: fo = fs vv - vs = 1200 340340 - 60 = 1460 Hz. SLO P-11-D-03 · Application

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41. The Doppler effect applies to:

  1. sound only
  2. all waves, including sound and light
  3. water waves only
  4. light only

Answer: B) all waves, including sound and light

Explanation: Any wave motion. SLO P-11-D-02 · Understanding

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42. A bee buzzes in a circle around your head. You hear:

  1. a constant pitch
  2. the pitch rise and fall periodically
  3. silence
  4. only a rising pitch

Answer: B) the pitch rise and fall periodically

Explanation: It approaches and recedes in turn. SLO P-11-D-02 · Understanding

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43. A progressive wave has intensity 9 W/m^2. If its amplitude is multiplied by 0.5, its new intensity is:

  1. 22.5 W/m^2
  2. 2.25 W/m^2
  3. 4.5 W/m^2
  4. 1.12 W/m^2

Answer: B) 2.25 W/m^2

Explanation: I propto A^2, so I = 9 (0.5)^2 = 2.25 W/m^2. SLO P-11-D-01 · Application

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44. As an ambulance with its siren on approaches a stationary listener, the pitch heard is:

  1. higher than the actual siren frequency
  2. lower than the actual frequency
  3. zero
  4. the same as the actual frequency

Answer: A) higher than the actual siren frequency

Explanation: Wavefronts are bunched up ahead of the source. SLO P-11-D-02 · Understanding

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45. The observed wavelength in front of a source moving at vs is:

  1. lambda' = vfs
  2. lambda' = v + vsfs
  3. lambda' = fsv - vs
  4. lambda' = v - vsfs

Answer: D) lambda' = v - vsfs

Explanation: Each wavefront is emitted vs/fs closer. SLO P-11-D-03 · Application

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46. In front of a moving source, the observed wavelength is:

  1. the same
  2. longer
  3. shorter
  4. zero

Answer: C) shorter

Explanation: Waves are squashed together. SLO P-11-D-02 · Understanding

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47. A point source radiates 100 W of sound equally in all directions. The intensity 5 m away is:

  1. 1 W/m^2
  2. 1.27 W/m^2
  3. 1.59 W/m^2
  4. 0.318 W/m^2

Answer: D) 0.318 W/m^2

Explanation: I = P4pi r^2 = 1004pi(5)^2 = 0.318 W/m^2. SLO P-11-D-01 · Application

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48. A siren's observed frequency is 550 Hz while approaching and 450 Hz while receding (v = 340 m/s). The speed of the siren is:

  1. 100 m/s
  2. 34 m/s
  3. 68 m/s
  4. 17 m/s

Answer: B) 34 m/s

Explanation: Ratio 550450 = 340 + vs340 - vs Rightarrow vs = 34 m/s. SLO P-11-D-03 · Application

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49. The SI unit of intensity is:

  1. dB m
  2. J
  3. W
  4. W m^-2

Answer: D) W m^-2

Explanation: Power per area. SLO P-11-D-01 · Application

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50. A source approaches at a speed equal to half the speed of sound. The observed frequency is:

  1. twice the source frequency
  2. half the source frequency
  3. 1.5 times the source frequency
  4. the same

Answer: A) twice the source frequency

Explanation: fo = fs vv - v/2 = 2fs. SLO P-11-D-03 · Application

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