NCERT Solutions for Class 9th Science Chapter 10 .6.3 Speed of Sound — Pause and Ponder
Book page 19810 Updated on2026-09-08
Q1.
Two friends are standing along a steel fence at a distance of 340 m from each other (Fig. 10.23). Gunjan places her ear over the fence and her friend knocks the fence with a metal object. Using the values of the speed of sound in steel and air given in Table 10.1, calculate the time difference between the sound that reached Gunjan through the air and the steel. Would it have been possible for her to distinguish between the two sounds? (The time interval between two sounds must be at least 0.1 s to be heard separately.)
Answer
The time difference is about 0.93 s, and yes — Gunjan can easily hear the two knocks separately.
One knock sends sound along two paths at once: through the steel fence, and through the air. The distance is the same, 340 m; only the speed differs.
time = distance ÷ speed
Through steel (v = 5000 m s⁻¹):
tsteel = 340 m ÷ 5000 m s⁻¹ = 0.068 s
Through air (v = 340 m s⁻¹):
tair = 340 m ÷ 340 m s⁻¹ = 1.000 s
Time difference Δt = tair − tsteel
= 1.000 s − 0.068 s = 0.932 s ≈ 0.93 s
Can she tell them apart? Two sounds are heard separately only if they are at least 0.1 s apart. Here
0.932 s > 0.1 s → yes, she hears two distinct knocks
The one through the steel arrives first, then almost a second later the same knock arrives through the air.
The same knock, two paths. Steel carries it about 14.7 times faster than air, so the two arrivals are nearly a second apart.
Why the ear needs 0.1 s: the brain merges two sounds arriving closer together than about 0.1 s into a single sensation, which is also why an echo needs a reflecting surface at least 17 m away. Here the gap is nine times larger than that limit, so there is no difficulty at all.