Echoes and SONAR: Class 9 practice
An echo travels to the reflector and back, so distance = speed × time ÷ 2. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Speed = distance/time
The idea behind echoes and sonar
An echo travels to the reflector and back, so distance = speed × time ÷ 2. A clear echo needs at least 0.1 s delay, so the reflector must be at least about 17 m away in air (v ≈ 344 m/s).
Step-by-step method
- Total path = v × t.
- Distance to the reflector = half of that.
- Use 1500 m/s in sea water for SONAR.
Worked example 1 (Basic)
A shout returns after 3 s (speed 340 m/s). How far is the cliff?
- d = 340 × 3 ÷ 2
- Answer: 510 m
Worked example 2 (Standard)
A ship's SONAR signal returns after 2 s (speed 1500 m/s). How far is the sea bed?
- d = 1500 × 2 ÷ 2
- Answer: 1500 m
Worked example 3 (Challenge)
What is the minimum distance from a wall for a clear echo if sound travels at 340 m/s?
- Echo needs 0.1 s
- d = 340 × 0.1/2 = 17 m
- Answer: 17 m
Common mistakes and misconceptions
- Forgetting to halve
- Using the speed of light
Practice questions
- (Basic) A shout returns after 3 s (speed 344 m/s). How far is the cliff?
- (Basic) A shout returns after 5.5 s (speed 344 m/s). How far is the cliff?
- (Standard) A ship's SONAR signal returns after 6 s (speed 1500 m/s). How far is the sea bed?
- (Standard) A ship's SONAR signal returns after 4 s (speed 1500 m/s). How far is the sea bed?
- (Challenge) What is the minimum distance from a wall for a clear echo if sound travels at 330 m/s?
- (Challenge) What is the minimum distance from a wall for a clear echo if sound travels at 344 m/s?
Answer key with full working
- Q1: d = 344 × 3 ÷ 2 → Answer: 516 m
- Q2: d = 344 × 5.5 ÷ 2 → Answer: 946 m
- Q3: d = 1500 × 6 ÷ 2 → Answer: 4500 m
- Q4: d = 1500 × 4 ÷ 2 → Answer: 3000 m
- Q5: Echo needs 0.1 s → d = 330 × 0.1/2 = 16.5 m → Answer: 16.5 m
- Q6: Echo needs 0.1 s → d = 344 × 0.1/2 = 17.2 m → Answer: 17.2 m
Syllabus note
This skill is listed under "Sound" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is echoes and sonar in?
"Sound", Class 9 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.