Have you heard of a Ripple Tank? No? I didn't think so. Don't worry, it sounds scary and intimidating, but it's really not. It's just a shallow tank filled with water that has a wave generator attached. A light shines down on top of the system so the shadows of the waves are recreated on the whiteboard underneath. Not so threatening any longer, right?
By using this contraption, we can measure the speed of a wave not one way... not two ways... but three ways! Can you believe it? Pretty cool, huh? Guess what else! All three ways should give you just about the same speed! Woah! Physics is getting crazy up in here! Why? Because the one factor that changes wave speed is the depth of the water in the tank and we're keeping that constant.
Let's begin! After you get the waves going so that you can actually watch them on the whiteboard without getting a migraine, draw a 30cm line perpendicular to the waves on the whiteboard. There's a picture below for your reference. Choose a wave and follow it across the line with your finger. It's pretty tricky at first, but after you do it a few times, you'll be a pro! Once you have become one with the waves, that is, once you have a steady pace going, we'll time how long it takes you to get from one end of the line to the other. We'll do this a few times just to make sure we're accurate. After you calculate an average time, which should be about .57 seconds, you can determine the speed by dividing the distance (.3 meters) by the time (.57 seconds). The speed equals about .53 m/s. Did you get the same? Fantastic! Easy, right? I told you so.
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| Method 1 |
The next way is a bit more complicated and takes some practice. But that's okay. We've got time. First, we need to find the wavelength- the distance a wave travels during one cycle. The easiest way to find this is to measure from crest to crest (the highest point above the equilibrium). You really just need to eyeball this one. It looks to be about 2cm or .02m. Okay, next! See the spinny thing in the picture below? It's a strobe disk. What do you do with it, you ask? You turnandturnandturnandturnandturnandturnandturn. Good! Just like that! Now look through the slits at the waves. You know you're spinning the strobe disk right when the waves look stationary and are the same size through the slits as they are on the whiteboard. No, no! Don't stop. It'll take a while to get right, but when you do, it's so cool! I'm going to go make some more tea. You keep working.
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| Method 2 |
Okay, I'm back. Look at you go! Now that you've got this down, we'll find the frequency. The textbook definition of frequency is the number of cycles or vibrations per unit of time. For waves, this means the number of crests that hit a certain point in a specific unit of time. It's measured in 1/second because it's the number of cylces/time. This unit is also known as Hertz (Hz). To find frequency using the strobe disk, time how long it takes to rotate it 10 times. I calculated about 4.2 seconds. Then, divide this by 10 to determine how long one rotation would take (.42 seconds). Divide the number of slits in the strobe disk (12) by the time (.42s). The frequency should equal 28.6Hz.
Since we have both frequency and wavelength, we can now calculate the speed by multiplying the two together: .02 m * 28.6 1/s = .57 m/s. That's pretty close to the speed the calculated the first way! Nice! We are on fire.
The third way to calculate speed isn't as much fun- there's no spinny thing, excuse me, there's no strobe disk to play with and you don't need to chase waves with your finger. However, you do need a computer and webcam. Technology is always interesting within itself... For this video analysis method, you place a ruler down the same way you did in the first method. The computer then videos the waves moving and doing their thing. To determine the speed, the video is analyzed by Vernier which allows us to see each individual point a wave passes! This is then made into a Distance (m) vs Time (s) graph and VOILA! The slope of the graph is the speed! I made the graph below as big as possible, but just in case you still can't see, it says the slope is about .59m/s.
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| Method 3 |
The last thing we have to do is compare the three values we calculated for slope. The video analysis is the most accurate method, so we'll compare the first two to the last one. The percent error formula is ((calculated value - accepted value)/accepted value)*100%. By plugging and chugging a few numbers, we determined the percent error of the first method to be about 10%. The percent error of the second one is only about 3%. You're getting good at this physics stuff!
