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Potential energy in a crossbow When we think of the concept of energy, we often think of objects in motion: a car rolling down a hill or a jogger running up a hill. This kind of energy-in-motion, called kinetic¹ energy, isn’t the only kind of energy. Another type, which motionless objects can have, is called potential energy. Potential energy is stored energy; that is, energy that can be tapped later. To better understand the concept of potential energy, consider how a crossbow works. If you took a crossbow, pulled its string back, and cocked it so that the arrow remained in place until you decided² to shoot it, you would have created energy, energy that can be released by pulling the trigger. You have increased the potential energy of the crossbow. The amount of potential energy in the cocked crossbow is equal to the amount of work you put in it by pulling it back and cocking it. This potential energy becomes kinetic energy when you pull the trigger. With the release of the trigger, the potential energy of the cocked crossbow converts into kinetic energy as the arrow flies through the air. Except for a bit of energy lost to friction³, the kinetic energy of the flying arrow is equal to the potential energy that was stored in the cocked crossbow. Of course, potential energy doesn’t only exist in crossbows. Other examples include a boulder⁴ on the edge of a cliff, and gasoline in the tank of a car. In effect, potential energy is simply kinetic energy waiting to happen.

 

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