[Music] hi and welcome back to three sighs lessons code okay by the end of this video you should be able to describe how to use a ripple tank to measure the wavelength frequency and speed of water waves and this is a required practical so you need to learn the details I'll be covering the second part of this practical in the next video okay now a ripple tank is used to observe the features of water waves and I'm showing you a picture of the ripple tongue here a ripple tank is a shallow tray of water and in the water is a vibrating bar the bar is connected to a power pack when the bar vibrates it creates waves across the surface of the water above the ripple tongue we've got a lump on below the tank we've got a sheet of white paper when light shines through the water it produces an image of the ways on the paper so we're going to use this setup to measure the wavelength frequency and speed of the water waves now the easiest way to do this is to record the waves using a mobile phone that allows us to play back the recording at different speeds or to freeze the image completely so we're going to start by measuring the wavelength of the waves this is quite straightforward to measure the wavelength we place a ruler on the paper we then freeze the image of the waves like this we now measure the distance between one wave and 10 waves further in other words a total of 10 wavelengths so in this case 10 wavelengths equals 26 centimeters in other words not 0.26 meters to find one wavelength we now divide this by 10 this gives us a value of naught point naught to 6 meters for the wavelength of these waves ok finding the frequency is a bit harder remember that the frequency is the number of waves passing a point each second to find the frequency we need to place a timer next to the paper and then count the number of waves passing a point in one second that's hard to do so it's more accurate to count the number of waves in 10 seconds and then divide by 10 again this is easier if we record this and then watch it in slow motion but remember we need to record the timer as well as the waves so I'm going to count the number of waves that pass the arrow in 10 seconds starting now okay so the total above ways was 24 in 10 seconds so the frequency is 24 divided by 10 this gives us a frequency of 2.4 waves per second in other words 2.4 Hertz okay we're going to finish now by looking at how to determine the speed of the waste and this is fairly straightforward we already know the wavelength and the frequency of the waves we can now use the wave equation to determine the speed remember that the wave speed is the frequency in Hertz multiplied by the wavelength in meters in this case the frequency is 2.4 Hertz and the wavelength is naught point naught 2 6 meters multiplying these together gives us a wave speed of naught point naught 6 to 4 meters per second another way to determine the wave speed is to select a wave and then measure the time it takes to move the length of the tank we calculate the speed by dividing the distance traveled by the time taken I should point out that we might get slightly different results using these two methods that's due to measurement errors for example in time remember you'll find plenty more questions on this required practical in my vision workbook and you can get that by click on the link above [Music]