Energy

Energy is the ability to do work or cause change. In science, "work" means moving something or heating it up. Anything that moves, glows, grows, or warms up is using energy. Energy is one of the most important ideas in all of physics.

Energy comes in many forms. Moving things have kinetic energy. A rolling ball, a running kid, and a blowing wind all have kinetic energy. Things that are stored up or lifted up have potential energy. A book on a high shelf has potential energy, because gravity could pull it down. A stretched rubber band has potential energy too.

Heat is a kind of energy. So is light. So is sound. So is electricity. Food has energy stored inside it in the form of chemical energy. Your body breaks the food apart and uses that energy to move your muscles and keep you warm. Batteries store chemical energy too. The sun gives off huge amounts of light and heat energy, and almost every form of energy on Earth can be traced back to it.

The most important rule about energy is this: it cannot be created or destroyed. It can only change from one form to another. Scientists call this the law of conservation of energy. When you turn on a flashlight, chemical energy in the battery changes into electrical energy, then into light and a little bit of heat. The total amount of energy stays the same. It just moves and changes shape.

You can see this rule in action all around you. A roller coaster is pulled to the top of a hill, giving it potential energy. As it drops, that potential energy changes into kinetic energy, and the coaster speeds up. At the bottom, it trades some of its kinetic energy back for potential energy as it climbs the next hill. A little bit is lost as heat and sound along the way, but none of it disappears.

Energy also powers human life. We burn coal, oil, and natural gas to heat buildings and run cars. We use falling water, sunlight, and wind to make electricity. Scientists are working on cleaner ways to get energy, because burning fuels adds gases to the air that warm the planet.

Albert Einstein showed something amazing in 1905. He proved that mass and energy are really two forms of the same thing. A tiny bit of matter holds a huge amount of energy.