[Music] hi and welcome back to three sighs lessons cold UK by the end of this video you should be able to describe how to measure the wavelength frequency and speed of ways in a solid and this is part two of the required practical on waves again you need to make certain that you learn the details as this may come up in your exam in the last video we saw how to measure the features of water waves in a ripple tank in this video we're measuring the features of waves in a solid to do that we use this apparatus we've got a string with one end attached to a vibration generator at the other end of the string we've got a hanging mass and the mass keeps the string taut the vibration generator is attached to a signal generator and the signal generator allows us to change the frequency of vibration of the string so when we turn on the power the string vibrates now it's extremely difficult to see those vibrations however at a certain frequency we get a wave like this this is called a standing wave and that's due to an effect called resonance I should point out that in the exam you will not be required to explain how standing waves all resonance happen now we find standing waves in stringed musical instruments such as a guitar we're going to measure the wavelength of this standing wave and to do that we use a ruler like this we need to measure the total length of the standing wave from the wooden bridge to the vibration generator in this case the length is 21 point five centimeters in other words naught point two one five meters we can now use this to calculate the speed of the wave and to do that we need to use the wave equation which we saw in a previous video the wave speed equals the frequency multiplied by the wavelength we read the frequency from the signal generator let's say that in this case the frequency is 10 Hertz and we know that the wave length is naught point two on five meters multiplying 10 by Noor point two on five gives us a wave speed of two point one five meters per second now if we go back to a standing wave what happens if we increase the frequency well at a certain frequency the standing wave changes to this in this case we now have three half wave lengths and I'm showing those here now it may seem strange having half a wavelength but that is possible on a standing wave like this so how do we calculate the wavelength of this wave well as we saw before the distance from the wooden bridge to the vibration generator is not 0.2 1 5 meters to calculate the wavelength we need to divide the total length by the number of half wave lengths and then multiply by 2 so the distance is naught point to on 5 meters dividing no.21 5 by 3 and then multiplying by 2 gives us a wavelength of naught point 1 4 3 meters the frequency used to create this wave was 15 Hertz so I'd like you to calculate the wave speed pause the video now and try this yourself ok to calculate the wave speed we multiply the frequency by the wavelength multiplying 15 by naught point 1 4 3 gives us a wave speed of 2 point 1 5 meters per second to two decimal places so as you can see the wave speed of the string does not depend on the frequency or the wavelength in fact the wave speed depends on the tautness of the string on the mass per centimeter now in the exam you could be shown any standing wave aren't us to calculate the wavelength I'm showing some examples here so again to calculate the wavelength you need to divide the total length by the number of half wavelengths and then multiply by 2 remember your fan plenty of questions on this required practical in my vision workbook and you can get that by clicking on the link above [Music]