What is Resonance in Physics
Resonant Phenomena, Musical Instruments, Radio Tuners, and Bridges
Aug 7, 2009
Imagine a father pushing his child on a swing. Small pushes can cause the child to swing fairly high, if Dad pushes at the right time. If Dad pushes as the swing returns to its highest point, gentle pushes are magnified, and the child keeps swinging higher. This case is an example of resonance. If Dad pushes randomly, the pushes cancel out and swing does not go high. Resonance does not occur.
The Physics Terminology of Cycle, Frequency, and Period
If the swing returns to its starting point after 3 seconds, physicists say that the swing has a period of 3 seconds and a frequency of 1/3 cycles per second. The period is the time to complete one cycle, and the frequency is the number of cycles completed per second.
For the child to swing higher, Dad must apply each push at the same point in the cycle. In physics parlance, Dad applies the forces with a frequency of 1/3 cycles per second, which is the same as the swing's natural frequency. The natural frequency, sometimes called the resonant frequency, of an object or oscillation is the frequency at which oscillations will occur if left alone. For example, if Dad just pushes the swing once and lets it oscillate by itself, that is the natural frequency.
Whenever an oscillatory motion receives a periodic force at the same frequency as the natural frequency, resonance occurs. Resonance magnifies the effect of the force or oscillation. If the two frequencies are different resonance does not occur. Forces applied randomly cancel each other out.
Resonance in Musical Instruments
Before electronic sound amplification, musical instruments depended on resonance to amplify the sound. For example, stringed instruments, such as violins and guitars, have resonating sound boxes.
The vibrating string makes a very low volume sound. The string's vibrations resonate in the sound box to amplify the sound volume. Unlike acoustic guitars, electric guitars do not have sound boxes because they amplify the sound electronically.
The sound box must be capable of resonating over the entire range of frequencies the instrument plays. The fact that no one can duplicate the rich sound qualities of a Stradivarius violin demonstrates that the physics of resonating sound boxes is very complex and poorly understood.
Electrical Resonance and Radio Tuners
Electrical circuits containing resistors, capacitors, and inductors have a natural frequency that depends on the properties of the circuit. Radio tuners adjust the value of this frequency, often with a variable capacitor. When the natural frequency of the tuning circuit in a radio matches the frequency of a nearby radio station, the circuit resonates and amplifies the signal from the radio waves. People then enjoy their favorite radio stations.
Resonance in Bridges
Bridges and other structures have a natural frequency that depends on their size and shape. If vibrations match this frequency, the bridge can vibrate wildly and even collapse.
If soldiers march in step across a bridge at the bridge's natural frequency, the magnified oscillations can break the bridge apart. This happened a couple of times in the 19th century, so now, soldiers break step when marching across a bridge.
The Tacoma Narrows bridge collapse in 1940, shortly after it was completed, is the most dramatic example of a resonating bridge collapsing. The bridge vibrated so much it was often called Galloping Gertie. Winds through the narrows vibrated the bridge at its natural frequency. Resonance amplified the vibrations until the bridge collapsed. Engineers now try to eliminate resonances when designing bridges.
Resonant phenomena occur often in nature. The effects can be either good, as in music, or bad, as in collapsing bridges.
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