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2026Q7(iii)(a)(1)Sound
NumericalModerate1.5 marks
7(iii)(a)

Two persons A and B are standing in front of a cliff in the same line 170 m apart as shown in the diagram. Person B fires the gun and hears the echo in 3 s. Then the person A standing in front of the person B fires the gun. (Speed of sound in air is 340 m s-1) Calculate:

Figure for this question

Calculate the distance of the person B from the cliff.

Topic: Echo and distance

Question wording verified against source paper

Answer

Exam answer

Given: B fires the gun and hears the echo after t = 3 s

speed of sound v = 340 m s⁻¹

The sound travels from B to the cliff and back, so 2d = v × t 2d = 340 × 3 2d = 1 020 d = 1 020 ÷ 2 d = 510 m

Distance of person B from the cliff = 510 m

Explanation

The standard echo relation, 2d = vt. The sound makes a round trip, so halve the total distance. Forgetting the 2 gives 1 020 m.

The 170 m between A and B is not needed for this part — it belongs to part (2). Read each part for what it actually asks rather than trying to use every number given.

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