Belting Out A Physics Lesson

High school science teacher Sam Terfa wanted to demonstrate a fundamental physics principle: resonant frequency. To do so, he found the best singer at Minnehaha Academy and had him serenade a wine glass. It did not turn out well for the glass.

Grade Level: 6th – 8th grade
Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2

Overview

Sound is all around us. Everything we hear in our day-to-day lives has a distinctive sound, from the jingling of keys to the tapping of footsteps in a hallway. Sound is created when objects vibrate. These vibrations cause the air around them to vibrate, sending sound waves in various directions. Some objects tend to vibrate at a specific rate. This is known as their resonant frequency.

In this activity, students will explore the vibrating nature of sound and how it travels from molecule to molecule. Students will experiment with how sound waves or vibrations can be observed, and will compare and contrast the speed of sound through different media. Students also will investigate the resonant frequency of different rubber bands, and how that frequency is affected by various factors.

Activity Materials

Ping-pong ball
String
Tape
Paper clip
Tuning Fork – Online sources for tuning forks (price range: $10 – $20: www.carolina.com, www.fishersci.com, www.sciencekit.com)
Rubber bands in various sizes
Marker
Shoebox or tissue

Vocabulary

Medium – (plural: media) the material (solid, liquid, or gas) through which sound waves travel.
Frequency – the number of vibrations a sound wave produces each second.
Pitch – the highness or lowness of a sound.
Resonant frequency – an object’s natural frequency of vibration, as determined by its physical properties.

What To Do

1. Begin the lesson by having students watch the Science Friday Video, “Belting out a Physics Lesson.” Discuss with students what they know about sound and how sound travels to our ears. Tell students that they will conduct three sound related experiments to learn more about the nature of sound and vibrations.

Activity 1

1. Tell students that sounds are caused when an object vibrates. Vibrations travel in the form of invisible sound waves. Ask students if they can think of ways that we can observe vibrations and their effects on other objects.

2. Have students tape a piece of string to a ping-pong ball and then tape the other side of the string to the edge of a table so that the ping-pong ball hangs freely. Tell students that they are going to gently strike the tuning fork, and then slowly bring the tuning fork near the ping-pong ball. Ask students to explain why the ping-pong ball jumped when gently tapped by the tuning fork.

Activity 2

1. Ask students if they think sound travels more easily through a solid or a gas. Pair the students into teams of two. Designate one student as the “listener” and the other student as the “sound-maker”.

2. Have the sound-maker drop a paper clip on the table while the listener faces in the opposite direction. Is the sound easy to hear?

3. Have the listener press one ear against the table while covering the other ear. Have the sound-maker drop the paper clip on the table. Was the sound easier to hear as it traveled through the table?

4. Repeat the same activity using the tuning fork instead of the paper clip. Instead of dropping the tuning fork on the floor, have students strike the tuning fork and hold it in their hand. Discuss with students the differences heard by using the tuning fork instead of the paper clip.

5. Have the students exchange roles and repeat steps 2 through 4. Have students discuss which was easier to hear and why.

Activity 3

1. Review with students the definition of pitch and resonant frequency. Have students use their own voices to give examples of a high-pitched voice and a low-pitched voice.

2. Have students stretch three rubber bands of varying widths around a shoebox or tissue box with the elastic over the opening. Use the marker to label each rubber band as “A”, “B” or “C”. If there is not enough space on the rubber band, label the side of the box next to the corresponding rubber band.

3. Have students create a chart with three rows (labeled rubber band A, B, and C) and three columns (labeled width measurement, predictions and results). In the width column, have the students measure and record the width of each rubber band. In the prediction column, have students number which rubber band they think will have the highest pitch and which will have the lowest pitch, with 1 being the highest pitch and 2 being the lowest pitch. Have students discuss their predictions.

4. Have students pluck each of the rubber bands and carefully listen to the sounds produced. Have students record their final results on their chart. Ask students to compare and contrast their results. Why did each rubber band make a different sound? Did they observe any differences in the vibrations of the rubber bands? What are some factors that might affect the sounds produced by a particular rubber band? (e.g.,tension, thickness, material, etc.)

What's Happening?

Sound is a type of energy made by vibrations. When an object vibrates, it causes movement in the molecules all around it. These vibrations can be passed on to the surrounding air molecules, causing them to vibrate, too. This is how vibrations travel in the form of invisible sound waves. Even though we cannot see vibrations, we can feel them or see their effect on other objects. Striking a tuning fork will cause the tips of the tuning fork to vibrate hundreds of times every second. These vibrations will cause the ping-pong ball to jump when gently touched by the tuning fork.

Sound also can travel at different speeds through different media. Vibrations are able to travel faster through a solid, since the molecules are closer together, than through a gas, whose molecules are further apart. This is why it is easier to hear the sound of a paper clip being dropped through a solid (the desk), than it is when the vibrations are traveling through a gas (air). Sound waves will travel more than 10 times faster through a wooden table than they do through the air!

The pitch of a sound is measured by its frequency, or the number of vibrations produced in one second. A guitar string, for example, will vibrate at a set number of vibrations per second, producing a specific sound. This specific rate of vibration is known as resonant frequency. All objects have a natural resonant frequency at which they vibrate. When plucked, a stretched rubber band will vibrate at its resonant frequency. Different rubber bands will produce different resonant frequencies, depending on their thickness or width, and how tightly they are stretched. Thicker, heavier, and loosely stretched rubber bands will sound lower pitched, while thinner, lighter, tightly stretched rubber bands will sound higher.

Topics for Science Class Discussion

• What other factors do you think might affect how well sound travels through a medium?

• How is sound created in instruments that don’t have strings?

• How is a shoebox with rubber bands wrapped around it similar to a guitar?

• How are sound waves different than light waves?

Extended Activities and Links

• Experiment further with resonant frequency by collecting several glasses of different shapes or sizes, and gently tapping them with a spoon. The sounds they release are at their individual resonant frequency. Have students organize the glasses from left to right in order of increasing pitch/resonant frequency.

• Sound also can be produced by using the air spaces that certain objects have inside. Gather a few different kinds of plastic or glass bottles or jugs, and have students blow air over the top of the opening of each bottle or jug. Describe the kind of sound produced by each one. Have students try to change the sound produced by filling the bottle/jug half full of water. How does the pitch change? What if the bottle/jug is one third full? A quarter full? Two thirds?

• Assign students to research a musical instrument. Have them explain how and why their instrument produces a distinctive sound.

• Explore sound waves through online interactive animations:

• Explore the science of music through online activities

This lesson plan was created by the New York Hall of Science in collaboration with Science Friday as part of Teachers Talking Science, an online resource for teachers, homeschoolers, and parents to produce free materials based on very popular SciFri Videos to help in the classroom or around the kitchen table.

The New York Hall of Science is a science museum located in the New York City borough of Queens. NYSCI is New York City's only hands-on science and technology center, with more than 400 hands-on exhibits explore biology, chemistry, and physics.