[Music] hi and welcome back to three sighs lessons cold dot u K by the end of this video you should be able to use the wave equation to calculate the speed of a wave you should then be able to discover a method to measure the speed of sound waves in air in the last video we looked at three important properties of waves these are the amplitude the wavelength and the frequency remember that the amplitude is the farthest point the wave vibrates from the undisturbed position the wavelength is a distance from a point on one wave to the equivalent point on the adjacent wave and the frequency is a number of waves passing a point each second and the unit of frequency is the Hertz remember that one Hertz as one wave per second now there is one of a key property of waves and that's the speed the wave speed is the speed at which the wave moves through the medium in other words the speed at which energy is transferred and we can calculate wave speed using this equation the wave speed and meters per second equals the frequency in Hertz multiplied by the wavelength in meters and you're not given this equation in the exam so you need to learn it I'm showing you the triangle of this equation here and again it's worth learning this okay so let's take a look at some sample questions here's the first one a wave has a frequency of 165 Hertz and a wavelength of 2 meters calculate the speed of the wave so pause the video now and try this yourself okay so this is a fairly straightforward question by given the frequency in the wavelength and we've got to calculate the speed so the wave speed equals the frequency multiplied by the wavelength the frequency is 165 Hertz under wavelength is 2 meters multiplying 165 by 2 gives us a wave speed of 330 meters per second here's another question for you a wave has a frequency of 10,000 Hertz and a wavelength of 2 centimeters calculate the speed of the wave so again pause the video and try this yourself okay so the question tells us that the frequency is 10,000 Hertz and the wave length is 2 centimeters and again we've got to calculate the wave speed however in this case there's a problem remember that the wave length must be in meters so first we've got to convert to centimeters into meters by dividing by 100 2 centimeters is naught point naught 2 meters now we can use the wave equation to calculate the wave speed the frequency is 10,000 Hertz on the wavelength is naught point naught 2 meters multiplying these together gives us a wave speed of 200 meters per second here's a final question for you a wave has a speed of 500 meters per second under frequency of 200 Hertz calculate the wave length now to answer this question you will need to rearrange the wave equation for wave lengths or if you prefer you can simply use a triangle so pause the video and try this yourself okay so I calculated the wave length to do this we need to divide the wave speed by the frequency the wave speed is 500 meters per second and the frequency as 200 Hertz divided in five hundred by two hundred gives us a wave length of two point five meters now in the exam you could be asked to discover method to measure the speed of sound waves in air I'm showing you here two people separated by distance of 500 meters person a is holding a pair of cymbals on person B is holding a timer person B starts timing when she sees person A Clash the cymbals together person B then stops timing when she hears this sound of the cymbals clashing we can then calculate the speed of the sound waves by dividing the distance travelled by the time taken now there were a couple of problems with this experiment firstly every person has a different reaction time it takes a fraction of a second between seeing the symbols and starting the timer it also takes a fraction of a second between hearing the sound and stopping the timer now we can reduce this error by having a large number of observers with timers we take all of the results underscored any that are anomalous we then calculate a mean value now the second problem is that the time between seeing the cymbals clash and hearing the sound is very short indeed that makes it very difficult to press the timer at the correct times we can reduce that problem by increasing the distance between person a and person B the longer the distance the longer the time that makes it easier to start and stop the timer are the correct times remember you're fine plenty of questions on the wave equation on determining the speed of sound waves in my vision work ball and you can get that by clicking on the link above [Music]