Describe how to measure the velocity of sound in air and ripples on water…

The idea

Sound in air is timed over a long distance: two people stand far apart, one claps or strikes a starting sound, and the other times the delay before hearing it, often using a flash of light as the start signal so the observer sees the event instantly. Repeating and averaging improves reliability, and velocity = distance ÷ time. For water ripples, a ripple tank with a vibrating bar produces regular waves; a stroboscope or a camera freezes the pattern so wavelength can be measured across several ripples and divided by the number counted, while frequency comes from the vibration rate. Then velocity = frequency × wavelength. In both methods, measure several cycles or a long distance to reduce timing error.

What earns the marks

• Describes timing a sound over a long measured distance and dividing distance by time to find velocity.
• Uses a light flash or visual signal so the observer starts timing at the instant the sound is made.
• Repeats measurements and averages them to reduce random timing error.
• Describes using a ripple tank with a vibrating bar to generate regular water waves.
• Measures wavelength across several ripples and divides by the number of ripples counted.
• Obtains ripple frequency from the vibration rate or stroboscope and calculates velocity as frequency × wavelength.

Common mistakes and how to put them right

• Times a short distance so reaction time dominates the reading.

  Put it right: Use a long distance and average repeats.

• Counts one ripple gap as the whole wavelength over several ripples.

  Put it right: Measure across many ripples and divide by the number counted.

• Forgets to convert centimetres measured on the tank to metres before calculating; convert first.

• Uses the distance–time equation for ripples without finding frequency and wavelength.

  Put it right: Use frequency × wavelength for the ripple tank.

Exam tips

• Name the measuring instruments and what each one measures in the method.
• Explain why a long distance or many ripples improve accuracy.
• State the equation used at the end of each method and show the substitution.