Measuring the Speed of Waves (OCR GCSE Physics A (Gateway))
Revision Note
Measuring the Speed of Waves
Sound waves in air are a type of longitudinal wave
The speed of sound can be measured
Experiments to Determine the Speed of Sound
There are several experiments that can be carried out to determine the speed of sound
Three methods are described below
The apparatus for each experiment is given in bold
Method 1: Measuring Sound Between Two Points
Measuring the speed of sound directly between two points
Two people stand a distance of around 100 m apart
The distance between them is measured using a trundle wheel
One person has two wooden blocks, which they bang together above their head
The second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear the sound
This is then repeated several times and an average value is taken for the time
The speed of sound can then be calculated using the equation:
Method 2: Using Echoes
Measuring the speed of sound using echoes
A person stands about 50 m away from a wall (or cliff) using a trundle wheel to measure this distance
The person claps two wooden blocks together and listens for the echo
The person then starts to clap the blocks together repeatedly, in rhythm with the echoes
A second person has a stopwatch and starts timing when they hear one of the claps and stops timing 20 claps later
The process is then repeated and an average time calculated
The distance travelled by the sound between each clap and echo will be (2 × 50) m
The total distance travelled by sound during the 20 claps will be (20 × 2 × 50) m
The speed of sound can be calculated from this distance and the time using the equation:
Method 3: Using an Oscilloscope
Measuring the speed of sound using an oscilloscope
Two microphones are connected to an oscilloscope and placed about 5 m apart using a tape measure to measure the distance
The oscilloscope is set up so that it triggers when the first microphone detects a sound, and the time base is adjusted so that the sound arriving at both microphones can be seen on the screen
Two wooden blocks are used to make a large clap next to the first microphone
The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time difference between them
This is repeated several times and an average time difference calculated
The speed can then be calculated using the equation:
Measuring Wave Speed in Water
Ripples on water surfaces are used to model transverse waves
The speed of these water waves can be measured
Creating ripples in water
Choose a calm flat water surface such as a lake or a swimming pool
Two people stand a few metres apart using a tape measure to measure this distance
One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple
The second person then starts a stopwatch to time how long it takes for the first ripple to get to them
The experiment is then repeated 10 times and an average value for the time is calculated
The average time and distance can then be used to calculate the wave speed using the equation:
Worked Example
Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up and down hitting the surface of the water. The diagram below shows a cross-section of the ripple tank and water.
Which letter shows:
a) The amplitude of a water wave?
b) The wavelength of the water wave?
Answer:
Part (a)
Step 1: Recall the definition of amplitude
Amplitude = The distance from the undisturbed position to the peak or trough of a wave
Step 2: Mark the undisturbed position on the wave
This is the centre of the wave
Step 3: Identify the arrow between the undisturbed position and a peak
The amplitude is arrow D
Part (b)
Step 1: Recall the definition of wavelength
Wavelength = The distance from one point on the wave to the same point on the next wave
Step 2: Draw lines on each horizontal arrow
This helps to identify the points on the wave the arrows are referring to
Step 3: Identify the arrow between two of the same points on the wave
The wavelength is arrow C
Examiner Tips and Tricks
When you are answering questions about methods to measure waves, the question could ask you to comment on the accuracy of the measurements
In the case of measuring the speed of sound:
Method 3 is the most accurate because the timing is done automatically
Method 1 is the least accurate because the time interval is very short
Whilst this may not be too important when giving a method, you should be able to explain why each method is accurate or inaccurate and suggest ways of making them better (use bigger distances)
For example, if a manual stopwatch is being used there could be variation in the time measured which can be up to 0.2 seconds due to a person's reaction time
The time interval could be as little as 0.3 seconds for sound travelling in air
This means that the variation due to the stopwatch readings has a big influence on the results and they may not be reliable
Evidence for Energy Transfer
Waves transfer energy and information
Waves are described as oscillations or vibrations about a fixed point
For example, ripples cause particles of water to oscillate up and down
Sound waves cause particles of air to vibrate back and forth
In all cases, waves transfer energy without transferring matter
For water waves, this means it is the wave and not the water (the matter) itself that travels
For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels
Objects floating on water provide evidence that waves only transfer energy and not matter
Worked Example
The diagram below shows a toy duck bobbing up and down on top of the surface of some water, as waves pass it underneath.
Explain how the toy duck demonstrates that waves do not transfer matter.
Answer:
Step 1: Identify the type of wave
The type of wave on the surface of a body of water is a transverse wave
This is because the duck is moving perpendicular to the direction of the wave
Step 2: Describe the motion of the toy duck
The plastic duck moves up and down but does not travel with the wave
Step 3: Explain how this motion demonstrates that waves do not transfer matter
Both transverse and longitudinal waves transfer energy, but not the particles of the medium
This means when a wave travels between two points, no matter actually travels with it, the points on the wave just vibrate back and forth about fixed positions
Objects floating on the water simply bob up and down when waves pass under them, demonstrating that there is no movement of matter in the direction of the wave, only energy
Examiner Tips and Tricks
There is a key distinction between the wave itself and the particles in the wave. The wave causes the particles to move, but the particles themselves are not the wave, since they are not transferring energy, which a wave does.
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