Discover the fascinating world of sympathetic vibration and resonance in this experiment. Learn how soldiers’ footsteps can affect a bridge’s stability, and how identical objects vibrate in harmony. Explore the impact of distance, size, and material strength on resonance. Uncover the science behind these phenomena and their real-world applications.
SCIENCE – When soldiers march across a bridge, their footsteps cause the bridge to vibrate. If these soldiers walk in rhythm with the natural frequency of the bridge, each step will cause the bridge to vibrate with increasing amplitude. If the amplitude of vibration becomes large enough, the bridge could potentially collapse. This phenomenon, where a repeated force with a small strength causes a vibrating object to have a significantly larger amplitude, is called resonance.
In this experiment, you will demonstrate sympathetic vibration, or resonance, and determine the influence of distance on sympathetic vibration. You will investigate how the size and strength of a structure affect resonance caused by an earthquake. You will also learn how applying a force with the same natural frequency as an object affects the amplitude of the object’s movement.
Starting the Experiment
Learning Objective
To demonstrate sympathetic vibration.
Materials
Two identical 1-liter plastic soda bottles with the same shape and size.
Procedure
Blow air through the mouth of one plastic bottle to create a constant sound. Observe the pattern and intensity of the sound produced.
While blowing air into the first bottle, place the mouth of the second bottle near your ear, as shown in Figure. Observe any changes in the pattern and intensity of the sound produced.
Results
When you blow into the mouth of the first bottle, a sound is produced. When you do the same with the second bottle near your ear, you hear a sound with the same pattern as the first bottle’s sound but louder.
Why?
Because both bottles have the same shape and size, they also have the same natural frequency. Blowing into the first bottle causes the air inside it to vibrate, which in turn causes the air around the mouth of the bottle to vibrate. This vibrating air moves out and causes the air inside the second bottle to start vibrating. Resonance occurs when an external force with the same frequency as the object’s natural frequency causes the object to vibrate at a large amplitude; this is also called sympathetic vibration. The second bottle vibrates without having air blown into its mouth because the vibrating air entering the second bottle has the same frequency as the natural frequency of the bottle. When the vibrations of both bottles are in sync, constructive interference occurs, resulting in a sound wave with a greater amplitude and thus a louder sound. The sound becomes audible when the sound wave reaches the ear. Thus, the sound waves from both bottles together produce a louder sound with the same pitch.
Try a New Approach
How does the distance from the vibration source affect resonance? Repeat the experiment, asking a friend to hold one bottle near their ear while you blow into another bottle’s mouth. Initially, repeat the experiment at a distance of about 1 meter from your friend, then at a distance of about 2 meters. Swap positions and have your friend blow air through the bottle’s mouth while you hold another bottle near your ear.
