Concept: Potential and Kinetic Energy 1 Discovery Communications, LLC

Getting to Know: Potential and Kinetic Energy What have you learned about energy so far? You might know that energy can exist in many different forms. There is chemical energy in the foods we eat and in the fossil fuels that power our cars and planes. There is electrical energy that travels through the power grid and helps light our homes (and that is another form of energy: light). There is sound energy, heat energy, and so on. Did you know, though, that no matter the form of energy, it can be divided into one of two types of energy? Those two types of energy are potential and kinetic energy.

Before we dive into that, can we quickly review one question: What is energy? That is a great place to start. Energy is defined as the ability to do work. Energy is what allows things to move, whether they are cars, people, or atoms. Energy can move between objects and change form. Every object has energy…even if it is not moving, and most objects have many types of energy.

Even small movements have kinetic energy.

Thanks for the review. What is kinetic energy? Kinetic energy is the energy of motion. The word kinetic comes from the ancient Greek word kinetikos, which means, simply, “to move.” Kinetic energy can be large scale, like the movements of the planets within our solar system, or on a small scale, like the tiny motions of electrons around an atom. Most of the time, we see it in visible objects, like a soccer ball sailing through the air or a motorcycle driving down the highway. When you move any part of your body, even to wink, that is kinetic energy. Any time an object moves, it has kinetic energy.

Are there different types of kinetic energy? Yes, kinetic energy exists in many different forms. One form is electromagnetic energy, like the energy in the light waves from the sun. Other forms include heat energy (heat is motion because molecules move faster and faster as they increase their thermal energy) and electrical energy (the movement of electrons). Sound is another form of kinetic energy because sounds cause air molecules to move.

Are there ways to calculate how much kinetic energy something has? We can calculate an object’s kinetic energy based on its mass and the velocity at which it travels. For example, it takes much more energy to throw a bowling ball than a basketball because the bowling ball has more mass. Similarly, a rapidly moving bowling ball has more energy than a slowly moving one—it can knock down more pins. The relationship is KE=½mv2, or “kinetic energy equals one-half of the mass times the velocity, squared.” In this

Concept: Potential and Kinetic Energy 2 Discovery Communications, LLC

lesson, you will likely solve a few equations to get familiar with the concept.

Misconception 1: Is it true that only objects that are moving have energy?

No, all objects, whether they are moving or not moving, possess energy. Even a rock lying on the ground has energy! It can have heat energy, which is the kinetic energy of the molecules that make up the rock wiggling around. It could also have potential energy depending on its position, which you will learn about next.

Okay, that makes sense. Let us move on to potential energy. What is that? The other main category of energy is potential energy, which is the energy stored in an object because of its position, its structure, or its chemical composition. It is called potential energy because it describes the potential of an object to be able to move. Think about a ball lying on the ground. If you lift it up, you are using kinetic energy to change its position. That energy is now stored in the ball with its position. If you let go of it, the potential energy will be converted back into kinetic energy and it will fall. Thus, higher objects have more gravitational potential energy than lower objects, because they have the potential to release more kinetic energy when they fall.

Are there different types of potential energy? Yes, there are different types of potential energy just as there are different types of kinetic energy. Gravitational potential energy is the one we just talked about. This is the energy something has because of its position and the force gravity can exert on its position.

Elastic energy is the energy an object has when it is stretched (think about stretching a rubber band; the further you stretch it, the greater its potential to fly across the room!). Electrical energy, the movement of electrons, is a type of kinetic energy. However, when it is stored in a charged object, like your shoes that have rubbed across the carpet, this is a form of potential energy that can be discharged in a spark. Chemical bonds in batteries and foods store chemical potential energy, which can be released to turn on your laptop or help you move. Nuclear energy is the energy stored in the nucleus of an atom, which can be released in radioactive decay.

This girl is holding a ball, which has

potential energy.

Misconception 2: When an object stops moving, its energy is gone. Is this correct?

No, energy is never really gone. Instead, it changes form. So, when people say that energy is lost, what they really mean is that some energy was converted into an unusable form, like heat energy. When a ball stops moving across the floor, it is because friction (the interaction of the floor and the ball) transforms the kinetic energy into heat.