Resonance: Swinger
 

	Fun Experiment to Try at Home!
		Purpose To determine how a small force can cause a large motion.   Materials - metal washer - 12-inch (30-cm) piece of string - transparent tape   Procedure 1.  Tie the washer to one end of the string. 2. Tape the free end of the string to the edge of a table. 3. Pull the string slightly to the side and then release it. 4. As the string swings, gently tap the top of the string near the edge of the table. Each tap should be made as the string swings forward (away from you). 5. Repeat step 4 at least 10 times. Observe the amplitude (extent) of the motion. 6. Repeat steps 4 and 5, this time tapping the string when it swings backward (toward you).   Sum It Up! Resonance can be thought of as the reinforcement of vibration. Every object has a frequency at which it will vibrate if disturbed. When an outside force with the same frequency as the natural frequency of the object causes the object to vibrate, this action is called resonance, or sympathetic vibration. Resonance can be produced by a small force applied at the right frequency, such as walking in step across a bridge or pumping a swing. When a force close to the natural frequency of an object is applied, the amplitude of the object's vibration can increase substantially. A pendulum is a weight hung from a rod or string from a fixed point in a way that allows the weight to swing back and forth. Tapping the string pendulum as it moves forward gives the pendulum more energy so that it moves to a greater amplitude, but the frequency (swings per second) does not change. Tapping the pendulum as it moves backward cancels the motion of the pendulum, and it eventually stops swinging. Soldiers marching out of step cancel the vibration of a bridge.

Soldiers purposely march out of step when crossing a bridge.
 
As a troop of soldiers walk across a bridge, their movement causes the bridge to vibrate. If the soldiers are in step and if the frequency of their step is the same as the natural vibration of the bridge, the bridge could collapse. This is because each step that the troop makes in unison will cause the amplitude (size) of the bridge's vibrations to become higher and higher. If the amplitude of the vibrations becomes high enough, the bridge can collapse. To prevent this, soldiers purposely march out of step when crossing a bridge.

If you have ever pumped your legs on a swing to make yourself swing higher, you have observed the use of a small force to cause a large movement. Pumping a swing and walking in step across a bridge are examples of sympathetic vibration, or resonance. Resonance is a condition in which the natural frequency (number of vibrations per second) of an object is matched in frequency by an outside source of vibration energy, such as pumping or marching. The natural or resonant frequency of an object is the frequency at which the object will easily vibrate. Resonance results in an increase in the amplitudeof the vibrations.

For more experiments about motion and vibrations, read "Janice VanCleave's Physics for Every Kid." (New York: Wiley, 1991)