Class 9 – Science Extra Questions – Ch. 11 – Sound

Ans: C

Explanation: A supersonic aircraft is defined as an aircraft that can travel at a speed faster than the speed of sound. The speed of sound in air is approximately 343 meters per second (or 767 miles per hour) at sea level and 20 degrees Celsius. Therefore, a supersonic aircraft’s speed is greater than the speed of sound. Option D is incorrect as the speed of light is significantly faster than the speed of sound and is the ultimate speed limit in the universe.
Correct Option: C

Ans: C

Explanation: Frequency is directly related to the pitch of a sound. Higher frequency means higher pitch. Amplitude is related to the loudness of a sound. Since Wave A has a higher frequency than Wave B, it will have a higher pitch. The amplitudes are the same, so there is no difference in loudness based on the information provided.
Correct Option: C

Ans: B

Explanation: The speed of a wave (v) is directly proportional to its frequency (f) and wavelength (λ). This relationship is described by the fundamental wave equation, which states that the speed of a wave is equal to the product of its frequency and its wavelength. Therefore, v = fλ.
Correct Option: B

Ans: B

Explanation: When a slinky is pushed sharply along its length, it creates compressions and rarefactions that travel along the slinky. The particles of the slinky oscillate parallel to the direction of wave propagation. This is the defining characteristic of a longitudinal wave. A transverse wave, on the other hand, has oscillations perpendicular to the direction of wave propagation. A stationary wave is formed by the superposition of two identical waves traveling in opposite directions.
Correct Option: B

Ans: C

Explanation: The loudness of a sound wave is determined by its amplitude. Amplitude refers to the maximum displacement or pressure variation from the equilibrium position. A higher amplitude means a greater displacement of particles in the medium, resulting in a louder sound. Option A describes frequency, which relates to pitch. Option B describes wavelength, which also relates to pitch. Option D describes the speed of sound, which is dependent on the medium, not the loudness of the sound itself.
Correct Option: C

Ans: D

Explanation: The length of a vibrating string is directly related to the frequency of the sound it produces. A shorter string vibrates faster, producing a higher frequency sound (higher pitch), while a longer string vibrates slower, producing a lower frequency sound (lower pitch). Pressing down on a fret shortens the effective vibrating length of the string, thus increasing its frequency and producing a higher note. Loudness (amplitude) is related to how hard the string is plucked, and timbre is related to the overtones present, which are influenced by the material and construction of the guitar.
Correct Option: D

Ans: D

Explanation: The relationship between wave speed (v), frequency (f), and wavelength (λ) is given by the formula: v = fλ.
To find the wavelength, we can rearrange the formula to λ = v/f.
Given:
Wave speed (v) = 400 m/sec
Frequency (f) = 100 Hz
Substituting the given values into the formula:
λ = 400 m/sec / 100 Hz
λ = 4 m
Therefore, the wavelength of the wave is 4 meters.

Looking at the options provided:
A. 6 m
B. 2.8 m
C. 3 m
D. 4 m

The calculated wavelength matches option D.

Correct Option: D

Ans: B

Explanation: The loudness of a sound is determined by its amplitude. Amplitude is the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. A larger amplitude means more energy is carried by the sound wave, resulting in a louder sound. Frequency determines the pitch of a sound (high frequency is high pitch), velocity is how fast the sound travels, and wavelength is related to frequency and velocity.
Correct Option: B

Ans: A

Explanation: Non-mechanical waves, such as electromagnetic waves (light, radio waves, X-rays), do not require a medium to propagate. They can travel through the vacuum of space. Mechanical waves, on the other hand, like sound waves or water waves, do require a medium to travel. Therefore, non-mechanical waves can propagate in a vacuum as well as in a medium.
Correct Option: A

Ans: B

Explanation: Wavelength is defined as the spatial period of a periodic wave, the distance over which the wave’s shape repeats. It is the shortest distance between two points in a vibrating medium that are in the same phase, meaning they are moving in the same direction and with the same displacement. Amplitude refers to the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. Frequency is the number of oscillations or cycles per unit of time. Phase describes the position of a point in time on a sequence of periodic events, and it is used to compare the relative position of two waves or oscillations. Therefore, wavelength is the correct term for the shortest distance between two points in a vibrating medium that are moving in the same manner.
Correct Option: B