In previous lessons we’ve learned that energy is the ability to do work, and that work is moving something a distance against a force. The concept of energy is fairly easy to see as far as lifting things or pushing things go. We are exerting energy to lift a box against the force of gravity. We are exerting energy to pedal our bike up a hill. But how does this energy stuff relate to light, electricity, or sound? What’s moving against a force there?

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With energy, what’s happening is that outrageously tiny particles are moving back and forth outrageously tiny amounts, at outrageously high speeds.  With light, you’ve got little photons moving, with electricity little electrons. With sound, you’ve got molecules moving back and forth.

This back and forth motion is called vibration and these vibrations make waves. When one particle moves back and forth it does work on another particle, which does work on another particle and so on. As these particles do work on one another, they cause a wave to move from one place to another.

Energy moves by waves or, in other words, waves are energy-mobiles! Before we get in over our heads talking about waves however, we need to spend some time on this vibration thing.

If you imagine a swing at the park, it’s got a normal resting position (called an equilibrium position). That’s when it’s experiencing a balance of forces: the pull of gravity balances with the tension in the chain holding up the swing, and so forth. Everything adds up to zero, and stays at zero until a kid comes by.

When a kid starts pumping on the swing, the swing is no longer in equilibrium and the swing starts to move back and forth through an arc. This is a slow forced vibration that stops soon after the kid leaves the swing, and the kid needs to keep pumping back and forth to keep swinging at the same rate (or increase the swing rate). Eventually, the kid gets tired and stops pumping, and each repetition of the swing moving back and forth (vibration) is less and less (called damping). Eventually, the swing comes to a stop (equilibrium) until another energetic kid comes along.

Click here to go to next lesson on Damping.

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