# Kinetic Energy: Definition and Description

Table of Content

Kinetic Energy: Consider a baseball flying through the air. The ball is said to have “kinetic energy” by virtue of the fact that its in motion relative to the ground. You can see that it is has energy because it can do “work” on an object on the ground if it collides with it (either by pushing on it and/or damaging it during the collision). The formula for Kinetic energy, and for some of the other forms of energy described in this section will, is given in a later section of this primer. Potential Energy:

Consider a book sitting on a table. The book is said to have “potential energy” because if it is nudged off, gravity will accelerate the book, giving the book kinetic energy. Because the Earth’s gravity is necessary to create this kinetic energy, and because this gravity depends on the Earth being present, we say that the “Earth-book system” is what really possesses this potential energy, and that this energy is converted into kinetic energy as the book falls. Thermal, or heat energy: Consider a hot cup of coffee.

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The coffee is said to possess “thermal energy”, or “heat energy” which is really the collective, microscopic, kinetic and potential energy of the molecules in the coffee (the molecules have kinetic energy because they are moving and vibrating, and they have potential energy due their mutual attraction for one another – much the same way that the book and the Earth have potential energy because they attract each other). Temperature is really a measure of how much thermal energy something has. The higher the temperature, the faster the molecules are moving around and/or vibrating, i. e. he more kinetic and potential energy the molecules have. Chemical Energy: Consider the ability of your body to do work. The glucose (blood sugar) in your body is said to have “chemical energy” because the glucose releases energy when chemically reacted (combusted) with oxygen. Your muscles use this energy to generate mechanical force and also heat. Chemical energy is really a form of microscopic potential energy, which exists because of the electric and magnetic forces of attraction exerted between the different parts of each molecule – the same attractive forces involved in thermal vibrations.

These parts get rearranged in chemical reactions, releasing or adding to this potential energy. Electrical Energy All matter is made up of atoms, and atoms are made up of smaller particles, called protons (which have positive charge), neutrons (which have neutral charge), and electrons (which are negatively charged). Electrons orbit around the center, or nucleus, of atoms, just like the moon orbits the earth. The nucleus is made up of neutrons and protons. Some material, particularly metals, have certain electrons that are only loosely attached to their atoms.

They can easily be made to move from one atom to another if an electric field is applied to them. When those electrons move among the atoms of matter, a current of electricity is created. This is what happens in a piece of wire when an electric field, or voltage, is applied. The electrons pass from atom to atom, pushed by the electric field and by each other (they repel each other because like charges repel), thus creating the electrical current. The measure of how well something conducts electricity is called its conductivity, and the reciprocal of conductivity is called the resistance.

Copper is used for many wires because it has a lower resistance than many other metals and is easy to use and obtain. Most of the wires in your house are made of copper. Some older homes still use aluminum wiring. The energy is really transferred by the chain of repulsive interactions between the electrons down the wire – not by the transfer of electrons per se. This is just like the way that water molecules can push on each other and transmit pressure (or force) through a pipe carrying water.

At points where a strong resistance is encountered, its harder for the electrons to flow – this creates a “back pressure” in a sense back to the source. This back pressure is what really transmits the energy from whatever is pushing the electrons through the wire. Of course, this applied “pressure” is the “voltage”. As the electrons move through a “resistor” in the circuit, they interact with the atoms in the resistor very strongly, causing the resistor to heat up – hence delivering energy in the form of heat.

Or, if the electrons are moving instead through the wound coils of a motor, they instead create a magnetic field, which interacts with other magnets in the motor, and hence turns the motor. In this case the “back pressure” on the electrons, which is necessary for there to be a transfer of energy from the applied voltage to the motor’s shaft, is created by the magnetic fields of the other magnets (back) acting on the electrons – a perfect push-pull arrangement! Electrochemical Energy: Consider the energy stored in a battery. Like the example above involving blood sugar, the battery also stores energy in a chemical way.

But electricity is also involved, so we say that the battery stores energy “electro-chemically”. Another electron chemical device is a “fuel-cell”. Electromagnetic Energy (light): Consider the energy transmitted to the Earth from the Sun by light (or by any source of light). Light, which is also called “electro-magnetic radiation”. Why the fancy term? Because light really can be thought of as oscillating, coupled electric and magnetic fields that travel freely through space (without there having to be charged particles of some kind around).

It turns out that light may also be thought of as little packets of energy called photons (that is, as particles, instead of waves). The word “photon” derives from the word “photo”, which means “light”. Photons are created when electrons jump to lower energy levels in atoms, and absorbed when electrons jump to higher levels. Photons are also created when a charged particle, such as an electron or proton, is accelerated, as for example happens in a radio transmitter antenna.

But because light can also be described as waves, in addition to being a packet of energy, each photon also has a specific frequency and wavelength associated with it, which depends on how much energy the photon has (because of this weird duality – waves and particles at the same time – people sometimes call particles like photons “wavicles”). The lower the energy, the longer the wavelength and lower the frequency, and vice versa. The reason that sunlight can hurt your skin or your eyes is because it ontains “ultraviolet light”, which consists of high energy photons. These photons have short wavelength and high frequency, and pack enough  energy in each photon to cause physical damage to your skin if they get past the outer layer of skin or the lens in your eye. Radio waves, and the radiant heat you feel at a distance from a campfire, for example, are also forms of electro-magnetic radiation, or light, except that they consist of low energy photons (long wavelength and high frequencies – in the infrared band and lower) that your eyes can’t perceive.

This was a great discovery of the nineteenth century – that radio waves, x-rays, and gamma-rays, are just forms of light, and that light is electro-magnetic waves Sound Energy: Sound waves are compression waves associated with the potential and kinetic energy of air molecules. When an object moves quickly, for example the head of drum, it compresses the air nearby, giving that air potential energy. That air then expands, transforming the potential energy into kinetic energy (moving air). The moving air then pushes on and compresses other air, and so on down the chain. A nice way to think of sound waves is as “shimmering air”.

Nuclear Energy: The Sun, nuclear reactors, and the interior of the Earth, all  have “nuclear reactions” as the source of their energy, that is, reactions that involve changes in the structure of the nuclei of atoms. In the Sun, hydrogen nuclei fuse (combine) together to make helium nuclei, in a process called fusion, which releases energy. In a nuclear reactor, or in the interior of the Earth, Uranium nuclei (and certain other heavy elements in the Earth’s interior) split apart, in a process called fission. If this didn’t happen, the Earth’s interior would have long gone cold!

The energy released by fission and fusion is not just a product of the potential energy released by rearranging the nuclei. In fact, in both cases, fusion or fission, some of the matter making up the nuclei is actually converted into energy. How can this be? The answer is that matter itself is a form of energy! This concept involves one of the most famous formula’s in physics, the formula,       E=mc2. This formula was discovered by Einstein as part of his “Theory of Special Relativity”. In simple words, this formula means: The energy intrinsically stored in a piece of matter at rest equals its mass times the speed of light squared.

Energy comes in two basic forms: potential and kinetic. Potential Energy is any type of stored energy; it isn’t shown through movement. Potential energy can be chemical, nuclear, gravitational, or mechanical. Kinetic Energy is the energy of movements: the motion of objects (from people to planets), the vibrations of atoms by sound waves or in thermal energy (heat), the electromagnetic energy of the movements of light waves, and the motion of electrons in electricity. Each form of energy can be transformed into any of the other forms, but energy isn’t destroyed or created.

Losses of energy can always be accounted for by small transformations to other types of energy, like sound and heat. Power plants convert potential energy or kinetic energy into electricity, a type of kinetic energy, and electricity in turn can be converted back into other forms of energy, like heat in an oven or light from a lamp. Forms of Potential Energy CHEMICAL Chemical energy is stored in the bonds between atoms. (See here for more about atoms. ) This stored energy is released and absorbed when bonds are broken and new bonds are formed –  chemical reactions.

Chemical reactions change the way atoms are arranged. Like letters of the alphabet that can be rearranged to form new words with very different meanings, atoms go through chemical reactions to be reorganized to form new compounds  with vastly different properties. Each compound has its own chemical energy associated with the bonds between the atoms it contains. When we burn sugar (a compound made of hydrogen, oxygen, and carbon) during exercise, it’s components are reorganized into water (H2O) and carbon dioxide (CO2). These reactions both absorb and release energy, but the net reaction releases energy.

Chemical reactions that produce net energy are called exothermic. When gasoline is burned, the reactions taking place are exothermic and thermal energy is released, which can be used to power an engine. Meanwhile, chemical reactions that absorb net energy are called endothermic. NUCLEAR Nuclear energy is the stored potential of the nucleus, or center, of an individual atom. Most atoms are stable on Earth; they retain their identities as particular elements, like hydrogen, helium, iron, and carbon, as identified in the Periodic Table of Elements.

Nuclear reactions change the fundamental identity of elements. Unlike everyday chemical reactions that change how atoms are stuck together (rearranging the letters of a word), nuclear reactions change the name of the atoms themselves. (Sort of as if the letter “m” was split into the letters “r” and “n,” or the letters “l” and “o” combined to make the letter “b”). In nuclear reactions, atoms split apart or join together to form new kinds of atoms, called fission and fusion, respectively. When atoms split apart or fuse together, they release stored nuclear energy, sometimes in huge quantities.

Today’s nuclear power plants are fueled by fission, a breaking apart of uranium or plutonium atoms that releases lots of energy. Hydrogen atoms in the sun experience nuclear fusion, combining to form helium and subsequently releasing large amounts of kinetic energy in the form of electromagnetic radiation and heat. ELASTIC Elastic energy can be stored mechanically in a compressed gas or liquid, a coiled spring, or a stretched elastic band. On an atomic scale, the basis for the energy is a reversible strain placed on the bonds between atoms, meaning there’s no permanent change to the material.

These bonds absorb energy as they are stressed, and release that energy as they are relaxed. GRAVITATIONAL Systems can build up gravitational energy as mass moves away from the center of Earth or other objects that are large enough to generate significant gravity (the sun, other planets and stars). For example, the farther you lift an anvil away from the ground, the more potential energy it gains. The energy used to lift the anvil is called work, and the more work performed, the more potential energy the anvil gains. If the anvil is dropped, that potential energy becomes kinetic energy as the anvil moves faster and faster toward Earth.

Forms of Kinetic Energy MOTION A moving object has kinetic energy. A basketball passed between players shows translational energy in the motion that gets the ball from player A to player B. That kinetic energy is proportional to the ball’s mass and the square of its velocity. To throw the same ball twice as fast, a player uses four times the energy. If a player shoots a basketball with backspin or topspin, the basketball will also have rotational energy as it spins through the air. Rotational energy is proportional to how quickly the ball spins, as well as the ball’s mass, and the size and shape of the ball.

A hollow ball needs more energy than a solid ball of equal mass to spin at the same rate. The hollow ball requires more energy because it’s mass is farther from its center. In shooting a basketball, players often try to add rotational energy as backspin, because it results in the greatest slowdown in speed when the basketball hits the rim or the backboard, increasing the chance that the ball stays near the basket. The opposite direction of spin, a topspin, can be used in games like tennis, because it will help speed up a ball after impact and lowers the angle it travels after the bounce. THERMAL ENERGY AND TEMPERATURE

Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating, but we can feel their kinetic energies as temperature, which is a reflection of the energy with which atoms vibrate. When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat. A hot cup of tea in a cool room loses some of its thermal energy as heat flows from the tea to the room. The atoms in the hot tea slow their vibrating as the tea loses heat, and over a few hours the tea cools to the same temperature as the room.

At the same time, the room gains the lost thermal energy from the tea, but because the room is much larger than the tea, the temperature of the room increases by so little a person wouldn’t notice it. Adjacent objects that are different temperatures will spontaneously transfer heat to try to come to the same temperature. However, how much energy it takes to change the temperature of an object is based on what its made of, a principle called heat capacity or thermal capacity. Water has a higher heat capacity than steel, for example.

An empty pot on the stove takes almost no time to get to 212 degrees Fahrenheit (the boiling temperature of water). A pot half-full of water will take much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to get as hot as metal. SOUND Sound waves are made through the transmitted vibration of atoms in bulk — though atoms can also vibrate through heat — and sound can travel by the motion of atoms regardless of whether they are in liquid, solid, or gaseous states.

Sound cannot travel in a vacuum because a vacuum has no atoms to transmit the vibration. Solids, liquids, and gases transmit sounds as waves, but the atoms that pass along the sound don’t travel (unlike the photons in light). The sound wave travels between atoms, like people passing along a “wave” in a sports stadium. Sounds have different frequencies and wavelengths (related to pitch) and different magnitudes (related to how loud). Even though radio waves can transmit information about sound, they are a completely different kind of energy, called electromagnetic.

ELECTROMAGNETIC RADIATION Electromagnetic energy is the same as radiation or light energy. This type of kinetic energy can take the form of visible light waves, like the light from a candle or a light bulb, or invisible waves, like radio waves, microwaves, x-rays and gamma rays. Radiation — whether it’s coming from a candle or nuclear fission of uranium — can travel in a vacuum, and physicists like to think of electromagnetic radiation as divided into tiny energy packets called photons.

Each photon has a characteristic frequency, wavelength, and energy, but all photons travel at the same speed, the speed of light, or nearly 1 billion feet per second. Electromagnetic energy can be converted to stored chemical energy by plants during photosynthesis, the process by which plants, algae, and some other small organisms use the sun’s electromagnetic radiation to turn carbon dioxide gas into sugar and carbohydrates. ELECTRIC Electric energy is to the kinetic energy of moving electrons, the negatively-charged particles in atoms. For more information about electricity, see Basics of Electricity. . .