Physics lab assignments
Transcript: PHY 21041 Lab 4
In this lab today, we’re going to measure the speed of sound in two ways; one direct, one indirect. I’m here at the base of the library tower. If you don’t live in Kent, find a place where there’s a bell tower in your town and start your cell phone stopwatch exactly on some signal you decide. I’m going to use the first gong after the four‐part Winchester chime. Let’s wait for that. Okay, here it is. I’m going to start now. Instead, you’ll go somewhere else in town where you can still hear the library chimes. We’re here at the practice field by Music and Speech. When I hear the gong now, and stop my watch it won’t show one hour. It will show one hour plus the extra few seconds for sound to get to me from the library tower back there. Those extra few seconds, that’s the time for the sound to travel. Sound travels as fast as a speeding bullet. But even so, it will be a significant amount of time to get from back there to over here. I would then go to Google Maps, I’d find this location on the map, I’d find the library tower on the map, and use the scale on the computer to figure out the exact distance from there to here. I’d divide that by those extra few seconds, and just like that, with the power of physics, there is the speed of sound. Let’s go back inside for the second part of our lab. In the rest of Lab 4 we’re going to measure the speed of sound in a really clever, indirect way. We’re going to catch a wave, or at least part of a wave and measure its wavelength. And from that calculate its speed. Here are some of things you’re going to need. First you’ll need a sun source that will make a reliable frequency. I downloaded a little signal generator app on my phone. You can also use your laptop with the Audacity program that you’ve downloaded. And either a computer speaker, or maybe better yet an ear bud or headphone. You need a container of water. This is a giant, cool vase I happen to have at home. Anything that’s about a foot deep will do. You could use a bucket, a deep sink, or even an aquarium, the fish probably wouldn’t mind too much. You need some kind of a pipe or tube. You could buy a very inexpensive piece plastic from Lowe’s or Home Depot. You could even use the cardboard tube from a paper towel roll, toilet paper would be too short, but a paper towel roll could work. It will get soggy, but it will last long enough to do the experiment. And we have also, is supplied in the kits you’ll have for this class, is a piece of plastic transparency film. With a piece of tape, you can make it into a pipe. And use a ruler to measure the length of the pipe and its diameter. Let me show you what to do, how you can capture a wave and measure the speed of sound. You may have a tube or type of some kind already. If you want to make one from this sheet we gave you, just roll this up long ways. It doesn’t matter exactly what diameter you use, all that matters is that it’s not real big or real small. And you want it to be as straight as possible, so it isn’t wider at one end than it is at the other. You can just roll this up and tape it closed. And you have a tube just like that, that you can use.
This will show better on camera, I’m going to use this one instead. Set your sound source for any frequency of about 320 hertz or higher. I have my phone set to generate a tone of 400 hertz. What I’m going to do is hold speaker with my thumb. If you’re using your laptop, you can use a headphone or ear bud at this location. Hold it a little bit above the end of the tube, and the lower it down into the water. You’ll hear something remarkable. Let’s try this. Ah‐ha! At one point the sound will be a lot louder. Why because you’re hearing two sound sources; this one, plus the air inside singing along at the same frequency. This is another example of resonance, the easy transfer of energy to one medium to another. What you want to do is fine tune this. Move this up and down gradually, until you hear the loudest possible sound. You may need a friend to help you, then, to measure how long this is and to get the most precise measurement you can. You’ll also need to measure the inside diameter‐not the outside‐the inside diameter of the pipe or tube or whichever you use. We need to know how big this is across, on the inside. And do this as accurately as you can in centimeters and millimeters. From the lab directions, you’ll see what to do with those measurements to figure out the speed of sound in its clever, indirect way. Now the speed of sound depends on temperature as well. You’ll need to know the temperature of the room where you’re doing the experiment or the temperature of the air outside when you do that part of the experiment inside. Have fun!