Momentum and Energy
What is energy?
An object with energy can change itself or its environment. That’s a pretty abstract definition. Let’s explore some of the many ways an object can have energy and what changes it can cause.
A speeding car has energy—think what damage it can do if it hits a wall or another car. The energy of motion is called kinetic energy. A rotating wheel also has energy—if you try to stop a spinning bicycle wheel with your hand, it may hurt you. This kind of energy is called rotational kinetic energy.
A compressed spring or a stretched rubber band can cause a stone to move. The energy in the squeezed spring or stretched band is called elastic energy. There are a variety of other forms of energy that are stored in a material. The random motion of the atoms that make up the material means that the atoms have kinetic energy. A measure of the amount of the kinetic energy in the random motion of the atoms is called temperature; the more energy, the higher the temperature. Kinetic energy in the random motion of atoms in a material is called thermal energy. If you charge or discharge a battery, like the one in your cell phone, you change the chemical composition of the battery materials. When you charge it you increase its chemical energy. You can also increase the chemical energy of your body by eating. Even mass has stored energy—splitting the nucleus of a uranium atom results in elements that have smaller mass but a large amount of kinetic energy.