Lesson 24: Resonance

Download here: Ontario Curriculum Expectations

The world will never starve for want of wonders, but for want of wonder.

Gilbert K. Chesterton

An object will emit a certain sound, or vibrate at a certain frequency when hit. The frequency at which the object vibrates is called its natural frequency.

If an opera singer sings a note at the correct frequency, she can shatter glass. To do this she must sing at the natural frequency of the glass. The glass does this because of resonance.

Resonance is the increase in the amplitude of an object’s vibration caused by repeated pushes that have the same frequency as the natural frequency of the object.

A swing is a form of pendulum. The frequency at which a pendulum swings back and forth is determined by its length. The longer the pendulum, the longer it takes to swing back and forth. The shorter the pendulum, the faster it swings back and forth. The same is true of a swing. If you want to push someone on a swing, you intrinsically know that there is specific time when you should apply the push. It is just after the person on the swing has been moving backwards, has reached their peak height, and is just starting to move forwards.

Let’s assume that the swing has a natural frequency of 2.0 s. Every 2.0 s the swing starts a new cycle and starts to swing forwards. If you are going to push your friend on the swing, you will need to apply a push every 2.0 s, which is with the same frequency as the natural frequency of the swing. Through these repeated pushes, you are able to transfer energy of vibration from yourself to the swing. This results in the swing going higher. You have increased the amplitude of the swing’s vibration.

Examples of Resonance: Tuning Forks

Two tuning forks that vibrate at the same frequency can quite easily resonate with each other. That means that the vibrations of one tuning fork can cause the second tuning fork to start to vibrate, and therefore emit a sound.

You can see an example of this in the tuning forks video.

Examples of Resonance: Similar Pendulums

In the image to the right, you can see two pendulums suspended from a string. Both of the pendulums are the same length and therefore have the same natural frequency. If we start the pink one swinging back and forth, it will pass its energy of vibration to the blue one and cause it to start swinging too. As the two pendulums resonate with each other, they will pass this energy of vibration back and forth. You can see this ooccur in the video below.

Watch the video below to see a similar example of resonance.

Example of Resonance?  The Tacoma Narrows Bridge

Enter the phrase “Tacoma Narrows Bridge collapse” into an internet search engine or video sharing website to see video footage of the bridge before, during, and after its collapse.

Textbooks usually chalk this up to resonance. Watch the video below to get a different interpretation: