When we talk with friends about energy, sometimes we’re talking about how tired or invigorated we feel. Other times we’re referring to how much charge is left in the battery on our phones. But in science, the word energy has a very specific meaning. It refers to the ability to perform some type of work on an object. That could be lifting the object off the ground or making it speed up (or slow down). Or it could be kick-starting a chemical reaction. There are lots of examples.
Every object in motion has kinetic energy. This could be a car zooming along the highway, a soccer ball flying through the air or a ladybug slowly walking along a leaf. Kinetic energy depends on just two quantities: mass and speed.
But each has a different impact on kinetic energy.
For mass, it is a simple relationship. Double something’s mass and you’ll double its kinetic energy. A single sock tossed toward the laundry basket will have a certain amount of kinetic energy. Ball up two socks and toss them together at the same speed; now you’ve doubled the kinetic energy.
For speed, it’s a squared relationship. When you square a number in math, you multiply it by itself. Two squared (or 2 x 2) equals 4. Three squared (3 x 3) is 9. So if you take that single sock and throw it twice as fast, you’ve quadrupled the kinetic energy of its flight.
In fact, this is why speed limits are so important. If a car crashes into a light post at 30 miles per hour (about 50 kilometers per hour), which might be a typical neighborhood speed, the crash will release a certain amount of energy. But if that same car is traveling 60 miles per hour (nearly 100 kilometers per hour), like on a highway, the crash energy hasn’t doubled. It’s now four times as high.
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An object has potential energy when something about its position gives it the ability to do work. Usually, potential energy refers to the energy something has because it’s elevated above Earth’s surface. This could be a car at the top of a hill or a skateboarder at the top of a ramp. It could even be an apple that’s about to fall off a countertop (or tree). The fact that it’s higher than it could be is what gives it this potential to release energy when gravity lets it fall or roll down.
An object’s potential energy is directly related to its height above Earth’s surface. Doubling its height will double its potential energy.
The word potential hints that this energy has been stored up somehow. It’s ready for release — but nothing has happened yet. You can also talk about potential energy in springs or in chemical reactions. A resistance band you might use to exercise stores up the energy of your pull as you stretch it past its natural length. That pull stores energy — potential energy — in the band. Let go of the band and it will snap it back to its original length. Similarly, a stick of dynamite has a chemical type of potential energy. Its energy won’t be released until a fuse burns and ignites the explosive.
Conservation of energy
Sometimes kinetic energy becomes potential energy. Later, it may again turn back into kinetic energy. Consider a swing set. If you sit on a motionless swing, your kinetic energy is zero (you’re not moving) and your potential is at its lowest. But once you get going, you can probably sense the difference between the high and low points of your swing’s arc.
At each high point, you stop just for a moment. Then you start swinging back down again. For that instant when you are stopped, your kinetic energy drops to zero. At that same point, your body’s potential energy is at its highest. As you swing back to the bottom of the arc (when you’re closest to the ground), it reverses: Now you’re moving your fastest, so your kinetic energy is also at its max. And since you’re at the bottom of the swing’s arc, your body’s potential energy is at its lowest.
When two forms of energy switch places like that, scientists say that energy is being conserved.
This isn’t the same thing as conserving energy by turning off the lights when you leave a room. In physics, energy is conserved because it can never be created nor destroyed; it just changes form. The thief that captures some of your energy on the swing is air resistance. That’s why you eventually stop moving if you don’t keep pumping your legs.
If you hold a watermelon from the top of a tall ladder, it has quite a bit of potential energy. At that moment it also has zero kinetic energy. But that changes when you let go. Halfway to the ground, half of that melon’s potential energy has become kinetic energy. The other half is still potential energy. On its way to the ground, all of the watermelon’s potential energy will convert to kinetic energy.
But if you could count up all of the energy from all of the tiny pieces of watermelon that explosively hit the ground (plus the sound energy from that SPLAT!), it would add up to the watermelon’s original potential energy. That is what physicists mean by conservation of energy. Add up all of the different types of energy from before something happens, and it will always equal the sum of all of its different types of energy afterward.