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GRADE 9 PRACTICE

Physics Problems: Mechanical waves and sound, Grade 9

Types of waves, length, speed, properties and perception of sound.

18 problems across 3 sub-topics
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Problem statements

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18 problems
Position in this set: 1 from 18

Problem #34

Mechanical waves

What concept does the description correspond to: “In a longitudinal wave, particles oscillate along the propagation, and in a transverse wave, particles oscillate perpendicular to it.”?

Answer options

  1. \text{Longitudinal and transverse waves}
  2. \text{Mechanical wave}
  3. \text{Wavelength}
  4. \text{Point particle}
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Given
\text{Description}
\text{In a longitudinal wave, particles oscillate along the propagation direction; in a transverse wave, they oscillate perpendicular to it.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    Sound in a gas is longitudinal, while a wave on a taut string is transverse in the simple model.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Longitudinal and transverse waves}

Answer\text{Concept} = \text{Longitudinal and transverse waves}

Position in this set: 2 from 18

Problem #35

Mechanical waves

What concept does the description correspond to: “Propagation of vibrations and energy in an elastic medium without transfer of matter over long distances.”?

Answer options

  1. \text{Mechanical wave}
  2. \text{Longitudinal and transverse waves}
  3. \text{Wavelength}
  4. \text{Point particle}
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Given
\text{Description}
\text{Propagation of vibrations and energy in an elastic medium without transfer of matter over long distances.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    Particles of the medium oscillate about equilibrium, passing the disturbance to their neighbours.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Mechanical wave}

Answer\text{Concept} = \text{Mechanical wave}

Position in this set: 3 from 18

Problem #36

Mechanical waves

The wave travels at a speed of 340 m/s and has a frequency of 170 Hz. Find the wavelength.

Answer options

  1. 2\,\text{м}
  2. 0{,}5\,\text{м}
  3. 170\,\text{м}
  4. 57800\,\text{м}
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Given
v
340\,\text{м/с}
\nu
170\,\text{Гц}
Find

\lambda

  1. v=\lambda\nu
  2. \lambda=\frac{v}{\nu}\lambda=\frac{340}{170}\,\text{м}\lambda = 2\,\text{м}

Answer2\,\text{м}

Position in this set: 4 from 18

Problem #37

Sound, echo and resonance

What concept does the description correspond to: “Pitch is mainly related to frequency, loudness is related to intensity, and timbre is related to the spectral composition of sound.”?

Answer options

  1. \text{Pitch, loudness and timbre}
  2. \text{Infrasound and ultrasound}
  3. \text{Sound reflection and echo}
  4. \text{Point particle}
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Given
\text{Description}
\text{Pitch is mainly related to frequency, loudness to intensity, and timbre to the sound's spectral composition.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    The same note played by different instruments differs in timbre because their overtones differ.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Pitch, loudness and timbre}

Answer\text{Concept} = \text{Pitch, loudness and timbre}

Position in this set: 5 from 18

Problem #38

Sound, echo and resonance

What concept does the description correspond to: “Infrasound has a frequency below the human audible range, and ultrasound has a frequency above it.”?

Answer options

  1. \text{Infrasound and ultrasound}
  2. \text{Pitch, loudness and timbre}
  3. \text{Sound reflection and echo}
  4. \text{Point particle}
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Given
\text{Description}
\text{Infrasound has a frequency below the human audible range, and ultrasound has a frequency above it.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    Ultrasound is used in diagnosis and echolocation, with attention to intensity and exposure safety.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Infrasound and ultrasound}

Answer\text{Concept} = \text{Infrasound and ultrasound}

Position in this set: 6 from 18

Problem #39

sound

The echo returned after 0.8 seconds. Speed ​​of sound 340 m/s. Find the distance to the obstacle.

Answer options

  1. 136\,\text{м}
  2. 68\,\text{м}
  3. 272\,\text{м}
  4. 425\,\text{м}
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Given
t
0{,}8\,\text{с}
v
340\,\text{м/с}
Find

s

  1. s=\frac{vt}{2}s=\frac{340\cdot0{,}8}{2}\,\text{м}s = 136\,\text{м}

Answer136\,\text{м}

Position in this set: 7 from 18

Problem #130

Mechanical waves

Wave speed 12 m/s, frequency 3 Hz Find the wavelength.

Answer options

  1. 4\,\text{м}
  2. 8\,\text{м}
  3. 2\,\text{м}
  4. 12\,\text{м}
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Given
v
12\,\text{м/с}
f
3\,\text{Гц}
Find

\lambda

  1. \lambda=\frac{v}{f}\lambda=\frac{12}{3}\lambda=4\,\text{м}

    In one period, a wave travels one wavelength.

Answer4\,\text{м}

Position in this set: 8 from 18

Problem #131

Mechanical waves

Wave speed 20 m/s, frequency 5 Hz Find the wavelength.

Answer options

  1. 8\,\text{м}
  2. 4\,\text{м}
  3. 2\,\text{м}
  4. 12\,\text{м}
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Given
v
20\,\text{м/с}
f
5\,\text{Гц}
Find

\lambda

  1. \lambda=\frac{v}{f}\lambda=\frac{20}{5}\lambda=4\,\text{м}

    In one period, a wave travels one wavelength.

Answer4\,\text{м}

Position in this set: 9 from 18

Problem #132

Mechanical waves

Wavelength 0{,}5 m, for 10 with source committed 40 hesitation. Find the speed of the wave.

Answer options

  1. 4\,\text{м/с}
  2. 2\,\text{м/с}
  3. 1\,\text{м/с}
  4. 6\,\text{м/с}
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Given
l
0{,}5\,\text{м}
t
10\,\text{с}
N
40\,\text{колебаний}
Find

v

  1. f=\frac{N}{t}f=\frac{40}{10}f=4\,\text{Гц}

    Find the frequency of the source.

  2. v=\lambda fv=0{,}5 \cdot 4v=2\,\text{м/с}

    The frequency of the wave matches the frequency of the source.

Answer2\,\text{м/с}

Position in this set: 10 from 18

Problem #133

Mechanical waves

Wavelength 0{,}8 m, for 20 with source committed 50 hesitation. Find the speed of the wave.

Answer options

  1. 4\,\text{м/с}
  2. 1\,\text{м/с}
  3. 2\,\text{м/с}
  4. 6\,\text{м/с}
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Given
l
0{,}8\,\text{м}
t
20\,\text{с}
N
50\,\text{колебаний}
Find

v

  1. f=\frac{N}{t}f=\frac{50}{20}f=2{,}5\,\text{Гц}

    Find the frequency of the source.

  2. v=\lambda fv=0{,}8 \cdot 2{,}5v=2\,\text{м/с}

    The frequency of the wave matches the frequency of the source.

Answer2\,\text{м/с}

Position in this set: 11 from 18

Problem #134

Mechanical waves

In the first medium the wave speed 12 m/s, length 3 m. In the second environment the speed 8 m/s. How many wavelengths of the second medium fit at a distance 20 m? The frequency at the border does not change.

Answer options

  1. 20\,\text{длин волн}
  2. 5\,\text{длин волн}
  3. 10\,\text{длин волн}
  4. 30\,\text{длин волн}
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Given
v_1
12\,\text{м/с}
l_1
3\,\text{м}
v_2
8\,\text{м/с}
S
20\,\text{м}
Find

N

  1. f=\frac{v_1}{\lambda_1}f=\frac{12}{3}f=4\,\text{Гц}

    First we find a constant frequency.

  2. \lambda_2=\frac{v_2}{f}\lambda_2=\frac{8}{4}\lambda_2=2\,\text{м}

    In the second medium the wavelength changes.

  3. N=\frac{S}{\lambda_2}N=\frac{20}{2}N=10\,\text{длин волн}

    Divide the distance by the new wavelength.

Answer10\,\text{длин волн}

Position in this set: 12 from 18

Problem #135

Mechanical waves

In the first medium the wave speed 20 m/s, length 4 m. In the second environment the speed 10 m/s. How many wavelengths of the second medium fit at a distance 30 m? The frequency at the border does not change.

Answer options

  1. 30\,\text{длин волн}
  2. 7{,}5\,\text{длин волн}
  3. 45\,\text{длин волн}
  4. 15\,\text{длин волн}
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Given
v_1
20\,\text{м/с}
l_1
4\,\text{м}
v_2
10\,\text{м/с}
S
30\,\text{м}
Find

N

  1. f=\frac{v_1}{\lambda_1}f=\frac{20}{4}f=5\,\text{Гц}

    First we find a constant frequency.

  2. \lambda_2=\frac{v_2}{f}\lambda_2=\frac{10}{5}\lambda_2=2\,\text{м}

    In the second medium the wavelength changes.

  3. N=\frac{S}{\lambda_2}N=\frac{30}{2}N=15\,\text{длин волн}

    Divide the distance by the new wavelength.

Answer15\,\text{длин волн}

Position in this set: 13 from 18

Problem #136

Sound, echo and resonance

The echo came back through 0{,}4 s after a short sound. Speed of sound 340 m/s. Find the distance to the reflective wall.

Answer options

  1. 136\,\text{м}
  2. 68\,\text{м}
  3. 34\,\text{м}
  4. 204\,\text{м}
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Given
t
0{,}4\,\text{с}
v
340\,\text{м/с}
Find

S

  1. S=\frac{vt}{2}S=\frac{340 \cdot 0{,}4}{2}S=68\,\text{м}

    The sound travels the distance to the wall twice.

Answer68\,\text{м}

Position in this set: 14 from 18

Problem #137

Sound, echo and resonance

The echo came back through 0{,}6 s after a short sound. Speed of sound 340 m/s. Find the distance to the reflective wall.

Answer options

  1. 204\,\text{м}
  2. 51\,\text{м}
  3. 102\,\text{м}
  4. 306\,\text{м}
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Given
t
0{,}6\,\text{с}
v
340\,\text{м/с}
Find

S

  1. S=\frac{vt}{2}S=\frac{340 \cdot 0{,}6}{2}S=102\,\text{м}

    The sound travels the distance to the wall twice.

Answer102\,\text{м}

Position in this set: 15 from 18

Problem #138

Sound, echo and resonance

The echo sounder signal returned after 80 ms. Speed ​​of sound in water 1500 m/s. Find the depth.

Answer options

  1. 120\,\text{м}
  2. 30\,\text{м}
  3. 60\,\text{м}
  4. 180\,\text{м}
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Given
t
80\,\text{мс}
v
1500\,\text{м/с}
Find

h

  1. T=\frac{t}{1000}T=\frac{80}{1000}T=0{,}08\,\text{с}

    Convert milliseconds to seconds.

  2. h=\frac{vT}{2}h=\frac{1500 \cdot 0{,}08}{2}h=60\,\text{м}

    From the full round trip we get the depth.

Answer60\,\text{м}

Position in this set: 16 from 18

Problem #139

Sound, echo and resonance

The echo sounder signal returned after 120 ms. Speed ​​of sound in water 1500 m/s. Find the depth.

Answer options

  1. 180\,\text{м}
  2. 45\,\text{м}
  3. 270\,\text{м}
  4. 90\,\text{м}
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Given
t
120\,\text{мс}
v
1500\,\text{м/с}
Find

h

  1. T=\frac{t}{1000}T=\frac{120}{1000}T=0{,}12\,\text{с}

    Convert milliseconds to seconds.

  2. h=\frac{vT}{2}h=\frac{1500 \cdot 0{,}12}{2}h=90\,\text{м}

    From the full round trip we get the depth.

Answer90\,\text{м}

Position in this set: 17 from 18

Problem #140

Sound, echo and resonance

A man is between two walls. The first echo returned through 0{,}2 s, second through 0{,}5 s. Find the distance between the walls. Speed of sound 340 m/s; the first reflections from each wall are taken into account.

Answer options

  1. 238\,\text{м}
  2. 59{,}5\,\text{м}
  3. 357\,\text{м}
  4. 119\,\text{м}
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Given
t_1
0{,}2\,\text{с}
t_2
0{,}5\,\text{с}
v
340\,\text{м/с}
Find

S

  1. S_1=\frac{vt_1}{2}S_1=\frac{340 \cdot 0{,}2}{2}S_1=34\,\text{м}

    Find the distance to the nearest wall.

  2. S_2=\frac{vt_2}{2}S_2=\frac{340 \cdot 0{,}5}{2}S_2=85\,\text{м}

    The second echo gives the distance to the other wall.

  3. S=S_1+S_2S=34 + 85S=119\,\text{м}

    A person is between the walls, so the distances add up.

Answer119\,\text{м}

Position in this set: 18 from 18

Problem #141

Sound, echo and resonance

A man is between two walls. The first echo returned through 0{,}3 s, second through 0{,}7 s. Find the distance between the walls. Speed of sound 340 m/s; the first reflections from each wall are taken into account.

Answer options

  1. 170\,\text{м}
  2. 340\,\text{м}
  3. 85\,\text{м}
  4. 510\,\text{м}
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Given
t_1
0{,}3\,\text{с}
t_2
0{,}7\,\text{с}
v
340\,\text{м/с}
Find

S

  1. S_1=\frac{vt_1}{2}S_1=\frac{340 \cdot 0{,}3}{2}S_1=51\,\text{м}

    Find the distance to the nearest wall.

  2. S_2=\frac{vt_2}{2}S_2=\frac{340 \cdot 0{,}7}{2}S_2=119\,\text{м}

    The second echo gives the distance to the other wall.

  3. S=S_1+S_2S=51 + 119S=170\,\text{м}

    A person is between the walls, so the distances add up.

Answer170\,\text{м}

The theory behind these problems

Work through the topic if a problem will not give in.

Mechanical wavesLongitudinal and transverse waves, wavelength, frequency and speed.Sound, echo and resonancePitch, loudness, timbre, frequency ranges and sound reflection.

Formulas by topic

Look up symbols and units of measurement in the reference.

Wavelengthλ · мDistance and pathS, l · мVelocityv · м/сFrequencyν · Гц