A student investigates the speed of sound using the apparatus shown in Fig. 15.1. S is a starter pistol. M1 and M2 are microphones separated by a distance d...

(a) Speed of sound [2 marks]

Convert 7.1 ms to seconds: \( 7.1 \text{ ms} = 0.0071 \text{ s} \)

\( v = \frac{d}{t} = \frac{2.40}{0.0071} \) [1]

\( v = 338 \text{ m/s} \) [1]

(b) Why the timer measures M1-to-M2 time, not S-to-M2 [2 marks]

M1 is closer to the starter pistol than M2, so it receives the sound first and starts the timer. [1] The timer therefore measures only the time for the sound to travel the known distance \( d \) between M1 and M2. This eliminates the unknown and unmeasured distance from S to M1, giving a precise measurement over a known distance. [1]

(c) Advantage of an electronic timer [1 mark]

An electronic timer removes human reaction time error and provides a much more precise measurement (to the nearest millisecond or better) than a hand-operated stopwatch. [1]

(d) Mean time and improved speed of sound [2 marks]

\( \bar{t} = \frac{7.1 + 7.0 + 7.2 + 7.0 + 7.1}{5} = \frac{35.4}{5} = 7.08 \text{ ms} = 0.00708 \text{ s} \) [1]

\( v = \frac{2.40}{0.00708} \)

\( v = 339 \text{ m/s} \) [1]

Taking the mean of multiple readings reduces the impact of random errors and gives a more reliable result.

(e) Source of error other than timing [1 mark]

The distance \( d \) between the microphones may not be measured accurately (e.g. parallax when reading the ruler, or the microphones not being perfectly aligned). Alternatively, air draughts or wind could affect the speed at which sound travels. [1]

(f) How to improve accuracy [1 mark]

Increase the distance \( d \) between the two microphones. A larger distance gives a longer time interval, reducing the percentage error in the time measurement. [1]