Blowing Up a Phone Book

Don't let the title fool you. This experiment does not involve any explosions. Instead, we are going to explore the science of resonance. Resonance involves putting in small amounts of energy, at just the right time, to get stronger results. A good example is pushing a swing. Each push causes the person in the swing to go higher. We will lift a phone book high into the air by blowing on it.

To do that, you will need:

  • a phone book or large book
  • some strong string
  • something to tie the string to that is higher than your head and that will allow the string to swing freely. A tree limb, the bar of a swing set, a nail at the top of a door frame or a shower curtain rod are all good things to try.

Tie one end of the string around the book. Tie the other end to the nail, rod, limb, etc., so that the book hangs at about chest level. Now blow very hard against the book. How far did it move? Not very far. Now, using resonance, we will lift it much higher. Blow against the book. Watch it swing forward and back. Just as it is moving forward, blow again. The book will swing a little higher. On each swing, just after it begins to swing forward, give it another strong breath. Be careful not to let it hit you in the face as it swings back. Soon, you will be lifting the book quite a way into the air, with just the force of your breath.

Knowledge about resonance is used in many things, from musical instruments to building bridges. The hole under the strings of an acoustic guitar allows the sound waves of the vibrating strings to resonate and makes the sounds louder. If you cover up the hole, the sound is much softer. Bridge builders have to be careful of resonance, as small, regular bumps can build up, causing the bridge to swing violently. Nikola Tesla used electromagnetic resonance in his Tesla coils, allowing him to generate millions of volts of electricity.

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