NEET Waves MCQs & PYQs

Particle velocity in a wave is oscillatory and time-dependent, while wave velocity in a homogeneous medium is constant. So, Assertion (A) is true. Mechanical waves require elasticity for restoring force and inertia for propagation. So, Reason (R) is true. However, (R) explains wave propagation conditions, not the difference in velocities.

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.

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).

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.

Both arms of a tuning fork vibrate as a single system, thus at the same frequency. Assertion (A) is true.

For efficient sound production, the arms vibrate 180 degrees out of phase (in opposite directions), not in phase. Therefore, Reason (R) is false.

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).

Assertion (A) is true. For a closed pipe of length \(L\), fundamental frequency is \(v/(4L)\). For an open pipe of same length, it's \(v/(2L)\). These are different, so the same tuning fork cannot resonate in fundamental mode with both. Reason (R) is false; the primary reason is the fundamental frequency difference (\(f_o = 2f_c\)), not solely end correction.

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.

Assertion (A) is false; a stationary wave stores energy, which oscillates between kinetic and potential forms.

Reason (R) is false; energy is not converted to heat but redistributed to form the stationary wave pattern.

Assertion (A) is true. For a stretched string fixed at both ends, the \(n^{\text{th}}\) normal mode indeed has \(n\) antinodes and \(n+1\) nodes.

Reason (R) is true and correctly explains this: the fixed ends are always nodes, leading to one more node than antinode.