Title
Activity: Standing Waves
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts
NGSS Disciplinary Core Ideas
Table of Contents
Materials
- Fig. 4.5
- Fig. 4.6
- Table 4.3
- Long wave tank
- One paddle that fits snugly in the width of the wave tank
- Water
- Three rulers
- Metronome or sound recording of 100 beats per minute and 120 beats per minute
- Construction paper
- Masking tape
- Pencil
- Towels
Image
Image caption
Fig. 4.5. Making a watermarked wave profile picture in a long wave tank (This image is not to scale; the paddle, paddlestop, and ruler have been enlarged relative to the size of the tank.)
Image copyright and source
Image by Byron Inouye
Image
Image caption
Fig. 4.6. Analysis of a watermarked wave profile picture
Image copyright and source
Image by Byron Inouye
Procedure
Safety Note: If your wave tank is sliding on the table, place towels underneath it. Immediately mop up all water spills to prevent slipping.
A. Generate standing waves.
- Set up the long wave tank as shown in Fig. 4.5.
- Fill the wave tank about halfway with water.
- Tape one ruler to the end of the wave tank as a backstop. The backstop should be positioned to prevent the paddle from going past vertical.
- Tape a second ruler along the top edge of the wave tank (the yellow ruler in Fig. 4.5)
- Tape a third ruler 5 cm in front of the backstop as the paddle-stop. The paddle-stop will help you control the amount of water pushed by limiting the distance the paddle can move.
- Set your paddle in the paddle groove and practice generating standing waves. Standing waves do not advance; they appear to move up and down in place.
- Predict what will happen when you move the paddle to create wave pulses. Draw or describe your predictions.
- Take turns creating waves. Observe as others create waves and let the wave maker know when a standing wave has been created.
- Determine the best method for creating a standing wave.
- Record your best method for creating a standing wave.
B. Compare effects of wave frequency and length of wave pulse on wavelength and wave height.
- Make watermarked profiles of a standing wave at a frequency of 120 beats (waves) per minute and a paddle-stop distance of 5 cm.
- Check that the distance from the backstop to the paddle-stop is 5 cm.
- Use the metronome or sound recording to generate waves at a consistent and accurate frequency of 120 waves per minute. The paddle should hit the front paddle-stop once per beat.
- Use your best method for forming a standing wave from Part A Step 2.
- Print a watermark profile of the standing waves.
- Tape construction paper together to make a long sheet two-thirds the length of the wave tank.
- Hold the paper lengthwise near the top corners just above the water level (see Fig. 4.5).
- To make a wave profile, quickly but carefully dip the paper in and out of the water. The watermarks on the paper will show the profile of the waves at the instant the paper was dipped into the water.
- Trace the profile of the waves with pencil before the paper dries.
- Label your wave watermark profile (Fig. 4.6).
- Record the paddle-stop setting (5 cm) and wave frequency (120 waves per minute) on your wave profile.
- Identify and label the wave crests and troughs.
- Devise a method for measuring each of the following on your wave profile (Fig. 4.6):
- Wavelength
- Wave height
- Record your measurements in Table 4.3.
- (Optional) Print another watermark profile. Calculate the average wavelength and wave height among your trials.
- Determine the wave period and wave speed. Record in Table 4.3.
- Predict and give reasoning for what will happen when you change the frequency or paddle-stop settings as follows:
- Frequency of 100 waves (beats) per minute; paddle-stop distance of 5 cm
- Frequency of 120 waves (beats) per minute; paddle-stop distance of 10 cm
- Frequency of 100 waves (beats) per minute; paddle-stop distance of 10 cm
- Test your predictions from Step 4. Repeat steps 1–5 for the following conditions:
- Frequency of 100 waves (beats) per minute; paddle-stop distance of 5 cm
- Frequency of 120 waves (beats) per minute; paddle-stop distance of 10 cm
- Frequency of 100 waves (beats) per minute; paddle-stop distance of 10 cm
Activity Questions
- Describe how you created standing waves in the wave tank. In your description, include information about the paddle and your timing.
- How did you identify wave crests and troughs on your wave watermark profile?
- Did your results from Step 5 (Table 4.3) match your predictions from Step 4? Explain your answer.
- If the paddle stop is constant, what happens to the wavelength as the frequency decreases?
- Describe the relationship between frequency and wavelength that you observed. How could you express this relationship mathematically?
- How did frequency and paddle-stop setting affect the wave height? Explain your answer.
- One equation for wave speed is speed = frequency × wavelength (S = FL).
- Using the information in the data table (Table 4. 3), calculate wave speed (cm/s) for each of the four standing waves you measured.
- Record your results in the data table (Table 4. 3).
- Use your results to make a graph of frequency (X-axis) versus speed (Y-axis) for each paddle-stop setting.
- Use your graph to make a general statement about how increased frequency affects wave speed.
- The period of a wave is the inverse of its frequency (T = 1/F).
- Using the information in the data table (Table 4. 3), calculate wave period in seconds per wave for each of the four standing waves you measured.
- Record your results in the data table (Table 4. 3).
- Do your results support the claim that waves with a longer period generally move faster than waves with a shorter period? Explain.
- What is the relationship between wave period and wave speed?
- How are the waves you created in the tank similar to and different from waves in the ocean?
- Standing waves form in the ocean under natural conditions when two wave sets of the same height travel towards each other. Explain how it is possible to simulate standing waves in a wave tank.
