Activities in High-Speed Photography

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Speed of Sound Activities

*These materials were originally written by Kelly
Cooke for middle-school teachers and their students.
With modification, they're also appropriate for high-school
students. The speed of sound measurement using the high-speed
clock makes a good demonstration at the middle-school
level and above.*

Introduction

This program is designed to use hands-on methods to give students of 6th-8th grade level an understanding of the magnitude of the speed of sound by using both visual and auditory senses for measurement. The program consists of an introductory discussion and outdoor activity on the speed of sound, a session on measuring the speed of sound with high-speed techniques, and follow-up exercises that could be led by the students' science or mathematics teachers. The exercises require multiplication and division of fractions and proportional reasoning.

- Parts I and II are written as teacher's guides.
- Parts III and IV (rtf format) guide the students through the mathematics needed to calculate the speed of sound. Follow-up problems are given. These parts are provided in rich-text format to allow for convenient editing by teachers.

Part I. Introductory activity and discussion

Materials

meter stick

large clear area (at least 100 meters in length)

large cymbals or bass drum

* note: if these items are unavailable, others can be used; the only requirements are that it needs to be large enough for the children to see distinct movement, and it needs to make a very loud, sharp sound (two metal trash can lids, two large blocks of wood, etc.)

Procedure for Outdoor Activity

(1) Begin with discussion of sound travel, pointing out the measurable difference between the speeds of sound and light. The following example might be helpful.

"Let's think about a thunderstorm. When lightning strikes, do you see the lightning or hear the thunder first? That's right, you see the lightning usually a few seconds before you hear the thunder, if the storm is a few miles away. This is because sound is a lot slower than light. One way to tell how far away you are from the storm is to count the number of seconds between when the light from lightning reaches your eyes and when the sound of thunder reaches your ears. Every 5 seconds you count means the storm is about 1 mile away."

(2) Take the students out to a field which has been pre-measured and marked off at a distance of 100 meters. (It need not be extremely accurate: take 1m ~ 1 adult giant step). Have at least one student stand at one end of the field with a pair of cymbals or other noisemaker. The other children should stand at the other end. Let them watch and listen as the cymbals (or pieces of wood, etc.) are clapped, making sure they notice the difference between the moment they see the two objects hit and the moment they first hear the sound. There should be at least a noticeable difference in the two times.

(3) Next the children will need to measure the field in units of their armspans. Simply string the students out (fingertip to fingertip or holding hands) across the field. It is unlikely that there will be enough of them to cover the entire field. If this is the case, an explanation beforehand using the meter stick should make the activity a bit easier for them to follow. Show them that to measure a distance of 1.5 meters with only one meter stick, one must first measure the meter, then swing the stick around its endpoint to measure the other 0.5 m. In this way the students might measure the field without having a large enough number to stretch all the way across. Stretch part of the way, then hold the child on the end still while the others swing around him to measure the rest.

(4) After taking the measurement, you will want to show the students how to break up large units of distance into smaller ones. This might be done with a short talk about, "If it takes sound a certain length of time to go the whole 100 meters (60 children), how long does it take sound to go half that distance, or 50 meters (30 children)? It takes about half the time. What about to go 1/4 of the distance, or 25 meters (15 children)? It takes about 1/4 of the time." Now the students should be ready for the next activity on the speed of sound.

Part II. High-speed measurement of the speed of sound

The method for this measurement is described in the html document: A Demonstration Measurement of the Speed of Sound.

The script below is written in a free-flowing style intended to provide notes for the demonstrator.

- this disk is a sort of clock, but it's not exactly like the ones you normally see

:what are the differences? it has no numbers and it moves very quickly, so it can be used to measure very fast things

- to be able to use the clock to time things, we must first time the clock to see how fast it spins; we can do this by using this stroboscope light

- the stroboscope is a light that you might have seen before, because it's often used at parties to make motion look choppy

;it does this by a very fast flashing light; because it can show things in short jumps instead of continuous motion and show us things that happen really fast; it is also used to study machinery

- for instance, if we shine the light on the disk, we can now see a few lines on it

;this is because although the disk has only one line painted on it, the stroboscope light is flashing when the line is in different positions, so that we see it as many different lines

- to measure how fast the disk spins, we need to make the light flash once every time the line moves in a complete circle

;what we will see is one stationary line, because each time the light flashes, the line, having rotated one complete turn, will be in the same place as the time before

- when we see that stationary line, then the frequency at which the light is flashing will also be the frequency at which the disk is spinning

;we can see that now, when the stroboscope is flashing 50 times per second

- since the disk is rotating at the same time the stroboscope is flashing, we know that the motor moves at about 50 rotations per second,

or does one rotation in about 1/50 s.

- using this, we can measure how long it takes sound to travel a certain distance

- for instance, I wonder how long it takes sound to travel my armspan, from fingertip to fingertip? you can try to measure it with a normal classroom clock, but I bet it won't work. Let's use this high-speed clock instead.

- if we set up these two sound triggers as far apart as the distance from one of my hands to the other (about 1.7 meters), with each one connected to a flash unit, then make a loud sound, we should be able to measure the time it takes for the sound to move the distance of my armspan

- what will happen is that when we make the sound in line with the sound triggers, it will pass the first trigger first, causing a flash to go off and show us the position of the line on the disk

- then it takes a certain amount of time for the sound to travel from the first to the second trigger, and in this time the disk keeps spinning, so that when the second trigger finally "hears" the sound and causes the second flash to go off, the disk has moved a certain distance

- so the time it takes sound to go from one trigger to the other (the distance of my armspan) can be seen by the difference in the position of the white line on the clock

;let's try that now

- since the two images of the line are one-quarter turn (90 degrees) apart, we know that the time it took sound to get from the first trigger to the second (to go across my armspan) is the same time it took for the clock to rotate one-quarter turn

- we can look at this now like a regular clock

- it takes the second hand of a clock 60 seconds to move around one time

- if it goes one-quarter turn, it takes 1/4 times 60 seconds, or 15 seconds

- remember that each complete rotation of the disk takes about 1/50 s; so for one-quarter turn, it takes about (1/4) x (1/50 s) = 1/200 s

- we can see now that it takes sound about 1/200 s to travel about 1.7 meters, or about one adult armspan

- so when I hold my arms apart and snap the fingers of one hand, it takes the sound 1/200 s to reach the other hand

- if the sound traveled 1.7 m in 1/200 s, how far would it travel in 1 s? 200 times as far or 1.7 m x 200 = 340 m, so the the speed of sound is 340 meters per second