Waves on a String Required Practical | GCSE Physics
Required Practical: Waves on a Stretched String
Find stationary waves on a string, measure their wavelength and work out the wave speed using v = fλ
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Drag the two red markers along the ruler onto two nodes to measure the loops. Each loop is half a wavelength. A marker dropped near a node snaps onto it.
Live values
Think first: if you double the frequency, what happens to the number of loops on the string? What happens to the wavelength?
Frequency
Loops on the string
Distance between markers
Wave speed from markers, fλ
Results
Method
Equations
What you see. The vibration generator shakes one end of the string up and down. Waves travel along the string, reflect at the bridge, and overlap with the waves coming the other way. At certain frequencies they combine to make a stationary wave: loops that stay in the same place.
Nodes and antinodes. At a node the string does not move at all. At an antinode it moves up and down the most. Each loop is half a wavelength, so one complete wavelength is two loops.
Measuring the wavelength. Use a metre ruler. For a more accurate value, measure across as many whole loops as you can, then divide: for example, if 4 loops measure 80 cm, one loop is 20 cm and one wavelength is 40 cm.
Wave speed. Read the frequency from the signal generator and calculate v = fλ. If the masses stay the same, the wave speed stays the same: a higher frequency just gives a shorter wavelength and more loops.
Using the graph. Plot frequency against 1/λ for several stationary waves. Because f = v × 1/λ, the points lie on a straight line through the origin, and the gradient is the wave speed.
Changing the tension. Adding masses increases the tension in the string, which increases the wave speed. Moving the bridge changes the length of string that vibrates, which changes which frequencies give a stationary wave, but not the wave speed.
Safety. Keep feet clear of the hanging masses in case the string breaks or the masses fall.
About this simulation
This simulation recreates the GCSE required practical for measuring the speed of waves on a stretched string. A string runs over a pulley, with masses hanging from the end to keep it tight. A vibration generator, connected to a signal generator, shakes the string up and down. At certain frequencies a stationary wave forms: a steady pattern of loops.
Each loop is half a wavelength. To find the wavelength, measure across one or more whole loops with a ruler, divide by the number of loops and multiply by 2. The wave speed is then v = fλ, using the frequency on the signal generator. With 300 g of masses and 80 cm of string, a stationary wave of 2 loops forms at about 55 Hz, giving a wavelength of 0.80 m and a wave speed of about 44 m/s.
Plotting frequency against 1/λ for several stationary waves gives a straight line through the origin, with a gradient equal to the wave speed. Adding masses increases the tension and the wave speed. Moving the wooden bridge changes which frequencies give a stationary wave, but not the wave speed.
