University of Virginia
Physics Department

## Wavelength and Frequency of Vibration

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.8

Objectives

Students will

• understand that sound is produced by a vibrating object;
• see how the vibrations form a wave, and will be able to analyze the properties of the wave.

Motivation for Learning

Demonstration of Sound Vibrations

Materials
• Tuning fork
• Wooden block
• Plastic cup
• Water

Procedure

1. Ask students what they think will happen when a tuning fork is placed in water. Will there be a difference if the fork is sounding or not?
2. Fill a plastic cup or tub with water. Put the tuning fork in the water and observe the ripples, if any.
3. Now strike the tuning fork on the wooden block or on the bottom of a rubber-soled shoe and slowly place it in the water.

• Why did the water move (splash)? The tuning fork is vibrating.
• What property of sound does this demonstrate? That sound transmits due to vibration.

Background Information

The pitch of a sound is determined by the frequency of vibration of the source, in other words, how many times it vibrates per second.

### Student Activity

Materials

• Plastic or wooden ruler
• Bicycle wheel
• Tape
• Graph Paper
• Rubber band
• Marker

Procedure

Follow directions in Data Sheet and record observations.

Data Sheet

 1. Press a plastic or wooden ruler down on the edge of a desk so that half of it hangs over the edge. Give the end a flick and observe the vibration and note the pitch. Vary the position of the ruler and where you hold it down. Note how this affects the frequency and pitch. Was the sound higher when the ruler moved up and down more quickly, or more slowly?   Was the sound higher when the ruler end was longer or shorter?   Using the above two answers, how are pitch, frequency, and wavelength related?     2. Hold a bike wheel by the axle and give it a spin. Hold the ruler so the spokes strike it, making a flapping noise. The faster the wheel spins, the greater the frequency and the higher the pitch. What happened to the pitch when the wheel was spun faster? Does this agree with your results from the ruler?     3. Use the rubber band to attach the marker to the ruler so that the marker tip extends beyond the end of the ruler by about one inch. Lay the ruler on a book so that it extends about halfway over the edge. Tape the ruler securely in place (rubber bands work also). Hold the paper in front of the marker so that the marker's tip touches the paper on the left side. Have a partner pull down on the ruler and let go. Move the paper from left to right, and try to get a waveform on your paper. It may take a couple of tries. Try to pass the paper by at the same speed each try. Now move the ruler so that it extends way beyond the table, and repeat. Try to get at least two different, good waveforms. Look at the graph paper. How do the wave forms compare for when the ruler stuck way out versus when there was a shorter portion hanging over the edge of the table?   Measure the number of squares between the tops of two humps on each of the curves. How do they compare?   Count the number of waves between two vertical lines five squares apart. How do the graphs compare? Does this agree with your earlier conclusion?

1.
• The sound should be higher when the ruler moves more quickly because it is a higher frequency.
• The sound should be higher when the ruler end is shorter because it has a shorter wavelength.
• Pitch increases as frequency increases, pitch increases as wavelength decreases; therefore frequency increases as wavelength decreases.
2.
• The pitch increases when the wheel is spun faster, because it is at a higher frequency. This agrees with the results from the ruler.
3.
• The shorter ruler length should have more waves that are closer together. Therefore it has a higher frequency and a shorter wavelength.
• The shorter length of ruler should have a shorter distance, or wavelength.
• There should be more waves for the shorter length. This means that the shorter length has a higher frequency because it had more waves in the same time interval. This should agree with the earlier conclusion.

Students with Special Needs

All students should be able to participate in this activity. Students who are unable to manipulate the ruler or wheel may work with partners.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.