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Essential Question: What are two main types of energy and what are some examples of these types of energy? Kinetic Energy Kinetic energy is the energy of motion. All moving objects have kinetic energy. Tiny molecules, as well as the planets in outer space, have kinetic energy. The kinetic energyof an object is proportional to its mass and the square of its velocity. Kinetic energy, denoted as KE, is described by the following equation: 1 𝐾𝐸 = 𝑚𝑣 ! 2 The most common examples of kinetic energy are the motions of bodies through water, on land, or through the sky. This includes fish swimming and boats sailing. It also includes people running, cars traveling, fan blades rotating, and carousels spinning. In addition, it includes birds flying and balls moving in arced paths through the air after they are thrown. However, there is kinetic energy in many objects that is not as easily noticeable. When a guitar string is plucked, it vibrates very quickly. Not only does the guitar string have kinetic energy, but it transfers this energy to the molecules of air around it. This causes the air molecules to vibrate as well. In fact, all sound is the result of vibrating molecules in the air. Vibrating molecules in the air are a less obvious example of kinetic energy. This form of kinetic energy is sound energy. Consider also the motion of atoms in a person, such as the atoms in someone’s hands. These atoms are in constant motion, meaning they have kinetic energy. In fact, temperature is a measure of how quickly the atoms in a substance are moving. In other words, it is a measure of the kinetic energy of atoms in a substance. When people rub their hands together, they will notice that their skin feels warmer. This is because the rubbing motion causes atoms at the surface of their skin to move more rapidly. As the atoms move more rapidly, their kinetic energy increases. Thus, the temperature of the skin increases. This form of kinetic energy is thermal energy. Elastic Potential Energy Potential energy is the energy of an object due to its location or position. There are many forms of potential energy. Perhaps the easiest to understand intuitively is elastic potential energy. When a rubber band is stretched to a particular location, it has elastic potential energy. Think of the rubber band as having the “potential” to snap into motion if it were released from this location. Furthermore, the degree to which the rubber band is stretched will determine the amount that the band will snap into motion when released. The more stretched the rubber band, the more potential energy it has. Elastic potential energy is “stored in the rubber band” at the location to which the rubber band has been stretched. In much the same way, springs can store potential energy. When a string is stretched or compressed, it tries to restore itself to its equilibrium position. Gravitational Potential Energy Take a pencil and lift it as far up in the air as possible. What will happen when the pencil is released? It will fall toward the ground. Think of lifting the pencil up into the air in the same way as stretching a rubber band. The lifted pencil has the potential to begin moving once released. This is the same as when a rubber band has been stretched. Recall that gravity acts to pull objects downward toward the surface of Earth. When an object is lifted, it is moved against this force of gravity. Similarly, when a rubber band is stretched, it is moved against the elastic force that acts to keep the rubber band at its equilibrium, or rest, position. The more an object is lifted a gainst the force of gravity, the more energy it will have once it is released. Gravitational potential energy is said to be “stored in the object” when the object is lifted. The gravitational potential energy of an object is typically denoted by PE. Sometimes, to avoid confusion, it is written as PEgrav. The gravitational potential energy is proportional to the height h an object is raised above the ground. Suppose the object has a mass m, then the value of PE is given by the following equation: 𝑃𝐸 = 𝑚𝑔ℎ In this equation, g is the acceleration due to gravity. This is approximately 9.8 m/s2 downward.