Objectives:
At the end of the activity, you will be able to infer that:
1. sound consists of vibrations that travel through the air; and 2. sound is transmitted in air through vibrations of air particles
Materials:
1 rubber band
1 piece of plastic sheet
1 empty large can of powdered milk - 800 g 1 wooden ruler
1 empty small can of evaporated milk - 400 mL rock salt
1 dowel or 1 wooden rod 1 blue bead 4 colored beads 3 inches of tape
2 large books scissors
5 pieces of string paper
slinky spring transistor radio
Procedure:
Part A: Vibrations produce sound
1. Prepare all the materials needed for the activity. Make sure that you find a work area far enough from other groups.
2. Put the plastic tightly over the open end of the large can and hold it while your partner puts the rubber band over it.
72 3. Sprinkle some rock salt on top of the plastic.
4. Hold the small can close to the salt and tap the side of the small can with the ruler as shown in Figure 4.
Q1. What happens to the salt?
5. Try tapping the small can in different spots or holding it in different directions.
Find out how you should hold and tap the can to get the salt to move and dance the most.
Q2. How were you able to make the salt move and dance the most?
Q3. What was produced when you tapped the small can? Did you observe the salt bounce or dance on top of the plastic while you tapped the small can?
Q4. What made the salt bounce up and down?
Q5. From your observations, how would you define sound?
6. Switch on the transistor radio and position the speaker near the large can.
Observe the rock salt.
7. Increase the volume of the radio while it is still positioned near the large can.
Observe the rock salt again.
Q6. What happened to the rock salt as the loudness is increased?
Q7. Which wave characteristic is affected by the loudness or the intensity of sound?
Part B: Transmitting sound
8. Let 2 books stand up as shown in Figure 5. Place the dowel on top of the 2 books.
Figure 4
73 Figure 5. Set up for Activity 1B
9. Cut out an image of a human ear from a magazine and tape it to one of the books.
10. Start with the blue bead. Tape the string to the mark on the dowel that is farthest away from the ear.
11. Then tape the 4 colored beads to the other 4 marks. Make sure that all the beads hang in a straight line.
12. The colored beads represent air particles. Create vibrations (sound) in the air by tapping the blue bead toward the colored beads.
Q8. What happens to the other colored beads when the blue bead is tapped?
13. Create more vibrations by continuously tapping the blue bead and observe the other beads.
Q9. Are there occasion when the beads converge then expand?
14. If the beads represent air particles, what do the converging and expanding of the beads represent?
15. Connect one end of the slinky to a fixed point. Hold the other end then push and pull the slinky continuously. Record your observations.
Q10. Are there converging and expanding parts of the slinky?
Q11. How then is sound classified as a wave?
16. This time shake the other end of the slinky while the other end is still connected to the fixed point. Record your observations.
Tape ear photo here
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Were you able to get good sets of data from the activity? Did you enjoy watching the salt dance and the beads move? The salt and the beads represent particles of air when disturbed. The disturbance encountered by the salt and the beads causes the salt to bounce up and down and the beads to move together and spread alternately. In grade 7, you discussed that energy is transferred or transmitted from one object to another. Bouncing salt is also a manifestation of energy transmission. When sound is created by tapping the small can, the wave (sound) is transmitted by air to the larger can causing the plastic cover of the larger can to vibrate transferring energy to the rock salt. And voila!—dancing rock salt!
What about the beads? Did you observe the alternating converging and spreading of the beads? Compare this to your observations in the slinky spring. The converging portions of the beads match the compressions in the slinky while the spreading portions are the rarefactions of the slinky. With the compressions and rarefactions, what you were able to produce is called a longitudinal wave.
Longitudinal waves are waves that are usually created by pulling and pushing the material or medium just like in the slinky (Figure 6). Alternating compressions and rarefactions are observed. These compressions and rarefactions move along with the direction of the pushing and pulling activity of the material or medium. Thus, the wave moves parallel to the motion of material or the particles of the medium. This is known as a longitudinal wave.
Figure 6. Longitudinal wave
Figure 7. Transverse wave
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Let us compare the longitudinal wave with the other kind of wave known as a transverse wave in Figure 7. The compressions resemble the trough while the rarefactions are the crests. Do you still remember these characteristics of waves?
The trough is the lowest part of a transverse wave while the crest is the highest portion. The distance from one compression to the next or between two successive compressions in a longitudinal wave equals the wavelength. If you count the number of compressions passing by a certain point in 1 second, you are able to determine the frequency of the longitudinal wave. If you multiply the measured wavelength and the computed frequency you will be able to determine the speed of the wave. In equation,
There are other variations in the equation for the speed of the wave. The period of the longitudinal wave is the reciprocal of its frequency . This means that the speed of the wave can be expressed as the ratio of the wavelength and the period,
Let us try to compare the characteristics of longitudinal wave with that of the transverse wave in Activity 2.