levo

Waves

Measuring waves, and calculating depth from wave velocity

  • 3 things to know
  • Paper 1

Overview

Depth and distance from echo times for sonar, ultrasound, thunder and radar, ripple tank and stretched string measurements of wavelength, frequency and speed, and the speed of a wave in a solid from the note of a struck metal rod.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Key term

Explain why the time for an echo must be halved before a distance is worked out from it.

Numbers

Thunder is heard 6.0 s after the lightning flash is seen, with sound travelling at 340 m/s. Calculate the distance to the storm.

Key term

Name the apparatus used to produce and observe water waves in the laboratory when wave speed is investigated.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    A sound pulse sent down from a ship returns from the seabed 0.40 s later, with sound travelling at 1500 m/s in seawater. Calculate the depth of the water.

    Model answer

    The round trip is 1500 × 0.40 = 600 m, so the depth is 300 m.

  2. 2

    Explain why the time for an echo must be halved before a distance is worked out from it.

    Model answer

    The wave travels out to the object and back, so only half the journey is the distance to it.

  3. 3

    An ultrasound pulse travelling at 6000 m/s in steel returns from a crack 0.0010 s after it is sent. Calculate how far below the surface the crack lies.

    Model answer

    The round trip is 6000 × 0.0010 = 6.0 m, so the crack is 3.0 m below the surface.

  4. 4

    Thunder is heard 6.0 s after the lightning flash is seen, with sound travelling at 340 m/s. Calculate the distance to the storm.

    Model answer

    Distance = 340 × 6.0 = 2040 m.

  5. 5

    A radar pulse travelling at 3.0 × 10⁸ m/s returns from an aircraft 2.0 × 10⁻⁴ s after it is sent. Calculate the distance to the aircraft.

    Model answer

    The round trip is (3.0 × 10⁸) × (2.0 × 10⁻⁴) = 6.0 × 10⁴ m, so the aircraft is 3.0 × 10⁴ m away.

  6. 6

    An echo sounder is used where the water is 45 m deep and sound travels at 1500 m/s. Calculate the time for the pulse to return.

    Model answer

    The round trip is 90 m, so the time is 90 ÷ 1500 = 0.060 s.

Other ways to work this

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Key Concepts of Physics
Motion and Forces
Conservation of Energy
Waves

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2.5 × = 9000 s.
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ALength is measured in metres, with the symbol m.
BMass is measured in kilograms, symbol kg.
CTime is measured in seconds, symbol s.

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Watt means the unit of power, symbol W.
TrueFalse

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Spot the fault in a statement. The traps are the mistakes students actually make.

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