Question 21:
easyAssertion (A): When we start filling an empty bucket with water, the pitch of sound produced goes on decreasing.
Reason (R): The frequency of man voice is usually higher than that of woman.
As water fills a bucket, the air column length decreases, increasing the resonant frequency and thus the pitch. So, Assertion (A) is false. The average frequency of a man's voice is lower than a woman's voice. So, Reason (R) is also false.
Question 22:
easyAssertion (A): Sound is produced due to vibratory motion, but a vibrating pendulum does not produce audible sound.
Reason (R): A vibrating source always produce audible sound.
Sound is produced by vibrations. A vibrating pendulum's frequency is typically below the audible range (infrasound), so it does not produce audible sound. Thus, Assertion (A) is true.
Reason (R) is false because a vibrating source must have a frequency within 20 Hz to 20 kHz to produce audible sound.
Question 23:
easyAssertion (A): With increase in temperature, the speed of sound in a gas increases.
Reason (R): When temperature increases, the gas molecules move faster.
The speed of sound in a gas is \(v = \sqrt{\frac{\gamma RT}{M}}\), so it increases with temperature. Thus, Assertion (A) is true. Higher temperature means gas molecules move faster, leading to quicker sound transmission. So, Reason (R) is true and explains (A).
Question 24:
easyAssertion (A): Sound waves travel faster on a hot summer day than on a cold winter day.
Reason (R): Velocity of sound is directly proportional to the temperature of medium.
Sound speed increases with temperature. A summer day is hotter than a winter day, so sound travels faster. Assertion (A) is true. The velocity of sound in a gas is proportional to the square root of the absolute temperature (\(v \propto \sqrt{T}\)), not directly proportional. So, Reason (R) is false.
Question 25:
easyAssertion (A): Beats are not observed in case of light waves from two independent sources.
Reason (R): The phase difference between two light sources changes randomly.
Stable beats require coherent sources with a constant phase difference. Independent light sources have rapidly and randomly changing phase differences, making stable beats unobservable.
Thus, Assertion (A) is true, and Reason (R) is true and is the correct explanation of (A).
Question 26:
easyAssertion (A): A vibrating tuning fork sounds louder, when its stem is pressed against a desk top.
Reason (R): When a sound wave is incident on the surface of a desk, it is totally reflected.
Assertion (A) is true due to forced vibrations and resonance. The desk provides a larger surface area to vibrate, increasing the loudness.
Reason (R) is false because sound waves are not totally reflected; some energy is transmitted to the desk.
Question 27:
easyIf speed of sound in air at 27°C is v then at what temperature speed of sound becomes 2v?
Since speed \(v \propto \sqrt{T}\), to double the speed, the temperature in Kelvin must quadruple: \(T_2 = 4 \times (27 + 273) = 1200\text{ K}\) or \(927^\circ\text{C}\).
Question 28:
easyIf speed of sound in air at \(27^\circ\text{C}\) is \(v\) then at what temperature speed of sound becomes \(2v\)?
The speed of sound \(v \propto \sqrt{T}\. For the speed to double, \(T_2 = 4 T_1 = 4 \times (27 + 273) = 1200\text{ K}\. Converting to Celsius: \(1200 - 273 = 927^\circ\text{C}\).
Question 29:
moderateSound waves travel at $350text{ m/s}$ through a warm air and at $3500text{ m/s}$ through brass. The wavelength of a $700text{ Hz}$ acoustic wave as it enters brass from warm air:
(2011 Pre)
Frequency $f$ remains constant when a wave changes medium. Velocity is given by $v = f\lambda$, which means $\lambda \propto v$. The ratio of velocities is $v_{brass}/v_{air} = 3500/350 = 10$. Thus, the wavelength increases by a factor of 10.
Question 30:
moderateThe velocity of sound in any gas depends upon:
(1988)
The velocity of sound in a gaseous medium is determined by the formula $v = \sqrt{\frac{E}{\rho}}$, showing dependence on elasticity ($E$) and density ($\rho$).