Waves RP

In the waves required practicals, you investigate how to measure the speed of a wave using the equation:

Therefore, to find the wavespeed, we need to describe how to measure the frequency and wavelength of the waves, including the equipment used.

A ripple tank is a piece of equipment with a dipper that can be used to make waves in a shallow dish of water. A light is then shone through the dish so the shadow of the waves can be projected onto the surface below.

Measuring Frequency

Choose a fixed point in the ripple tank.

Use a stopwatch to measure the time taken for 10 waves to pass this point.

Divide the total time by 10 to find the time period () for one wave.

Calculate the frequency using:

Measuring Wavelength

• Use a ruler to measure the distance covered by 10 waves.

• Divide this distance by 10 to find the wavelength of one wave ().

Calculating Wave Speed

Use the wave equation:

Improving the Experiment

• Measuring more than ten waves when recording the time taken for the waves to pass, so that it will be a longer time measured, meaning that your reaction time (the time it takes you to respond and hit the stopwatch) will be a smaller fraction of the total time.

• Repeating measurements and calculating a mean improves reliability.

If we vibrate a string attached between two points as shown below, at a certain frequency, a wave will be produced on the string that looks like the one shown below. It oscillates about these points. It is called a standing wave. The string is made to vibrate using a vibration generator. The frequency of vibration is controlled using a signal generator that is connected to the vibration generator. A hanging mass is attached to the string, hung over the edge of a table to keep the string taut (tight).

Method

Set up the equipment as shown.

Turn on the signal generator and adjust the frequency until a clear standing wave pattern is observed.

Measure the length of the vibrating section of the string.

Count the number of loops in the standing wave pattern.

Calculating Wavelength

To calculate the wavelength, we need to know how many waves are on the string. Each loop on the string represents half a wavelength, as shown in the image above. To calculate the number of waves, we multiply half a wavelength by the number of loops.

For example, in the image above, there are loops, so the number of waves would be:

Now that we know the number of waves, we can calculate the wavelength by dividing the length of the string by the number of waves. For example, if we measured the length of the string to be 3 metres,

Measuring Frequency

The frequency can be read directly from the signal generator

Calculating Wave Speed

To calculate the speed of the wave, use the equation:

Problems with the Experiment

• The string is moving, making it very hard to count the loops or measure its length.

• The string may be longer than a metre, making its length hard to measure with a metre ruler.