 This will be an experiment to measure the speed of sound. Now we know the speed is the change in distance over the change in time. So our method is going to be to measure how far sound travels in a certain amount of time, and then take the distance it traveled and divide it by the amount of time. Now the way this experiment is usually done is over a much greater distance than we're going to be using in the laboratory here. For example, you might use, you might station people at opposite ends of a football field. And at one end of the field you have somebody smashing two trash can lids together. And at the other end of the field you have someone with a stopwatch who times how long it takes from when that person sees the trash can lids actually contact each other to when they actually hear the sound. And there is a noticeable time delay there. Now since we're just going to be working over the space of this table we don't have those long distances. We're going to have a much shorter period of time which you won't be able to use a stopwatch. We'll have to use a different way to measure the time. But we will be able to do that successfully. Here's some of the equipment that we'll be using. One piece is this flash unit. This is just a standard flash that you might put on top of a camera. Let's give it a flash here. This device is not quite so standard. It's a sound-actuated trigger. It's composed of a microphone right here inside this little audio amplifier. And the way it works is I'll turn it on here and when it receives a sound it automatically sets off that flash. Okay. And we have two of those. Over here is another flash with its own sound trigger. I'll turn that one on. Okay. Now here's basically the plan of the experiment. If I stand right here and produce a sound it will set off this flash first because it reaches this microphone first. And then the sound will take some time to travel along here and it will set off the second one. Let's try that right now. You keep an eye on the two flashes. Once more. Now I'm sure you couldn't tell that there was a time delay between those two flashes. The time interval is just too short. So that means we're going to have to have another piece of equipment in order to measure that short time interval. So I'll show you that piece of equipment next. This is a device that I'll use to measure the short time intervals that we're going to have in this experiment. Now there's nothing special about it. It used to be a fan. One of these clip fans that clipped to the table. I took off the fan blades and replaced it with this cardboard disc painted black with a white line on it. And you can think of this as a high speed clock. And so the white line is the hand of our clock and let's see how fast it goes. It goes so fast that normally you can't actually see it turning. But with the flashes we will be able to see the line at a particular instance of time. Let's see that right now. So I'll begin by turning the flashes around now to face the disc. I'll turn on this trigger. And you can see that when the flash goes off you get a very sharp image of the line on the disc. And that's because this flash is only lasting about a 30,000th of a second. Let me turn the other one on. And now if I make a sound right here you'll see two images of a hand on the clock. And that's because when the sound reaches this microphone the flash goes off and you get one image. And then time passes and it reaches the other microphone and that flash goes off and you get the second image and the disc will have turned during that time. Far enough so that you can see the angle between them. And that angle we will find will be a measure of the amount of time that passed from one sound trigger to the other one. In order to use the clock to measure the amount of time we will need to know the angle between the two images of the clock hand but we need to know something else. We need to know how fast that clock is turning. To find that out we'll use a stroboscope and use a method like was shown to you in a previous clip. I'm going to set the stroboscope for a frequency of 50 flashes per second because I want to make the disc go around 50 times per second. I've got a variable speed control here and I'll adjust that and I get that should be pretty nearly 50 rotations per second. It's a little bit different but it's close enough for what we're doing here. It'll change slightly. So now we know the frequency of the motor. I'll make one more addition to the equipment. I'll put colored filters on the flash units. Green filter for the left flash and a red filter for the right flash. The reason for this is that the hand that shows up on the clock for each of the flashes will be a different color and so we'll easily be able to distinguish between the flashes and see which one went off first. Let's just take a look at that. I'll make a sound here and you can see both the red and the green image on the disc. You can also tell which one went off first because the disc is rotating clockwise and when you look at the hands, the green one appears in that particular case at about the 1 o'clock position and the red at about the 2 o'clock position. The red one went off a little bit later because sound had to travel further to reach it. Now to test the triggers to make sure that they respond in the same way, I put them both together and make the sound out front so that the sound travels the same distance to each microphone. In that case, the two images of the hand should be on top of each other and they should look neither red nor green but maybe probably white and you can see that's the case. Now we're ready to do the experiment. I'll begin by placing the left trigger at the 10 centimeter mark on the meter stick and the right trigger at the 70 centimeter mark that means the distance between them is 60 centimeters. I'll produce a sound with the two blocks out here on the end in line with the two microphones that's so that the time between the flashes will correspond to the distance between the microphones. Here it goes. Now you can see the angle is quite a bit less than the 90 degrees that we're trying to achieve so what I need to do is move the triggers farther apart so I'm going to put this one right in the middle there now at 100 centimeters so the distance between them now is 90 centimeters. Here we go again. Still quite a bit less than 90 degrees. Let's make a bigger change. I'll go out to 140 centimeters there so the distance now is 130 centimeters between sound triggers. Still a little bit less than 90 but we're getting close. Let's try almost the other end of the table, 180. So the difference between them now is 170 centimeters. That looks pretty close to 90 degrees so let's use those particular results. So our data then is the distance between the sound triggers is 1.70 meters. The angle on the disc is 90 degrees and the frequency of the disc is 50 rotations per second so we'll now use that information to calculate the speed of sound.