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An army marches toward an enemy castle. Thousands of soldiers are ready to fight, but they wish, more than anything, that they could just go home. The general calls out the order. Hundreds of archers aim toward the castle and draw back their bows. It takes energy to pull the bows back, but that energy is now potential energy, stored in the strings themselves. The general calls out. The archers let go and a cloud of arrows flies toward the castle. But just then the army looks up the hill to their right. An enemy soldier is standing next to what looks like a wooden wall. He chops at some ropes and suddenly the wall collapses and hundreds of big, heavy logs come tumbling down the hill. The logs have kinetic energy as they gain speed, rolling toward the army. We witness these same types of energy anytime something moves, whether in a battle, at school, or at a sports game. Energy is what allows things to move.

Energy is the ability to do work, and to do work means to move something. Every time you or any of your cells move (which is all the time), you are using energy. The movement of Earth around the sun, the crashing of ocean waves, the sound pumping out of speakers when Beyoncé performs at Madison Square Garden, the beating of our hearts: It all takes energy.

Humans get all of our energy from eating things. We get calories (a unit of measurement of energy) from dead animals and plants. Plants get their energy from the sun. In fact, nearly all the energy on our planet comes from the sun. That big, bright ball will keep shooting energy toward us until it explodes.

Because the sun is a giant fusion reactor, it constantly bombards Earth with energy, which is transferred, stored, and used in many ways. You can feel it by simply standing outdoors on a sunny day: Light energy from the sun is converted into thermal energy in your skin. Soon your skin feels hot and you may even get sunburn. This is an example of energy conversion or transfer.

Energy conversion is the transfer of energy from one form to another. For example, chemical energy is stored in the cells of green plants through a process called photosynthesis. This chemical energy is really solar energy, which is converted in the cells of green plants using the sun's light energy, water, and carbon dioxide. The light energy breaks down the chemical bonds in carbon dioxide molecules and converts them to oxygen and carbohydrates (molecules of hydrogen and carbon). When you eat something, the muscles in your body use this stored chemical energy from the sun by splitting the carbohydrate molecules to gain their stored, potential energy.

Over millions of years, the chemical energy stored by green plants can be converted into oil or coal. When these are burned, their chemical energy is released as thermal energy that can be used to drive turbines and form electrical energy. In this way machines convert or transfer energy as well.

Energy can be converted into various forms, but it can never go away. When you die, the chemical energy in your muscles and blood becomes nutrients that feed the Earth. Ashes to ashes, dust to dust.

According to the law of conservation of energy, energy can't be created or destroyed, just used and converted into different forms. The amount of energy in a system does not change. But we are not able to use all of the energy we receive. For example, some energy is wasted as friction between the moving parts of machines; some energy may be converted into thermal energy and radiated into the air, which we can't use. Today, we are faced with the problem of finding the best sources of energy for us to use with the least amount of waste.

We divide our energy resources into two categories: renewable and nonrenewable. Non-renewable energy resources are those that we can't easily replace. For example, that chemical energy from plants stored as petroleum, natural gas, or coal is called fossil fuels. As their name implies, fossil fuels took millions of years to produce; they're non-renewable because we can't wait millions of years to produce more.

Renewable energy resources are those that can be replaced in a short time. Solar energy and wind energy, for instance, are renewed constantly by the sun and wind.

Solar energy is collected in large panels called solar cells, which collect sunlight and convert it into electrical or thermal energy. One benefit of solar power is that it is constantly renewed, but drawbacks are that we can't produce solar power at night and it's difficult to collect and convert a great deal of thermal energy at one time.

Wind energy is caused by the uneven heating of the Earth's surface. The kinetic power of wind can be used to turn windmills, which can then be used to generate electricity. The process works well in places where there's almost always wind, but unfortunately, wind does not blow so steadily in most places.

Water energy is really a form of gravitational potential energy collected by storing water in a reservoir behind a dam. As the water falls through pipe, its potential energy is converted

into kinetic energy and used to turn a turbine. The turbine produces a stream of electrons or electrical energy called hydroelectric power. One benefit of this power is that we can control the amount of water and when we use it. The main drawback is that only a limited number of rivers in the world can have hydroelectric dams built in them.

Geothermal energy is energy that comes from the heat found inside Earth's crust. Like the sun, nuclear reactions deep inside the Earth generate thermal energy. Where it can be found close to the surface of the crust, geothermal energy is almost unlimited; however, it is only found at thin points in the crust, in areas prone to volcanoes and earthquakes.

Other forms of renewable energy are biomass and tidal energy. Biomass is the chemical energy stored in plants by photosynthesis. We can burn these plants to produce thermal energy and use that to turn turbines, producing electrical energy. A benefit is that we can renew biomass by growing more plants, but when we do that, we cause pollution by putting carbon dioxide and smoke into the air.

Tidal and wave energy are generated by rising and falling tides and/or ocean waves. This is an almost unlimited form of energy, but it's difficult to collect and convert this energy, and it can only be gathered where there's sufficient tidal or wave action.

We call energy that is moving or can do work for us mechanical energy. This energy is either doing work or has the potential to do work, which brings us to the two types of mechanical energy: kinetic and potential. The energy of an object in motion is kinetic energy; the energy of an object that has potential to move is potential energy. The amount of kinetic energy an object has depends on its mass and its speed.

Imagine a truck, a baseball, and a spitball flying toward you. The baseball and the spitball may be traveling at about the same speed, but since the baseball has more mass, it has more kinetic energy. When it hits you it will hurt, while the spitball will simply go splat. Of course, if you don't get out of the way, the truck will not only hurt you but will knock you down and do serious damage. It has far more mass and kinetic energy than the baseball and spitball combined.

Potential energy is stored energy. An object has potential energy if it's able to move and thus capable of doing work. A stretched rubber band has potential energy, as does a gymnast standing on a balance beam, a water balloon held out a window, and a gallon of gasoline about to be burned.

Kinetic energy is the energy of motion. The amount of kinetic energy in an object is measured by an equation: Kinetic energy equals mass times velocity squared over two.kinetic energy = MV/2 2

The mechanical energy of an object is the sum of its potential energy and its kinetic energy. It too is measured by an equation: Mechanical energy equals potential energy plus kinetic energy.mechanical energy = potential energy + kinetic energy

There are many kinds of potential energy, but one we experience all the time is called gravitational potential energy. This is the amount of work that can be done when an object falls to Earth. You feel it every time you drop something on your toe. Gravitational potential energy is based on the mass of the object and the distance or height from which it falls. When you lift an object, say a 50- kilogram barbell, you are using energy to lift it off the ground. The higher you lift it, the more energy you are using and the more potential energy you are putting into the barbell. Gravitational potential energy is measured by an equation: Gravitational potential energy equals mass times height above the ground.gravitational potential energy = mass x height

In science we have special terms for expressing mass, height, and energy. As we've seen, the mass of an object is expressed in newtons. The potential or kinetic energy of an object is measured in units called joules. For height we use meters. Using the equation above, this means that joules equal newtons times meters, or one joule equals one newtonmeter.joules = newtons x meters

Mechanical energy explains why a big dude, who has more mass (and weighs more), can make a bigger splash into a swimming pool than you, a smaller person, can. Mechanical energy also explains why a person jumping from higher up will likewise make a bigger splash. If two people with the same mass both prepare to jump into the pool, but one jumps from the diving board three meters high, that person will have more potential energy to make a bigger splash.

Kinetic energy and potential energy can both come in many different forms, and energy is often transferred or converted from one form to another. This is how we use energy to do work. Gravitational potential energy is only one form of energy that we can use; others include thermal, chemical, electrical, sound, light, and nuclear. And by converting different forms of kinetic energy into potential energy, we can store it for future use.

Chemical energy is the energy stored in the chemical bonds that hold molecules together. It's a form of potential energy, and can be used when these chemical bonds are altered. Consider your digestive system. After a meal, chemicals are released in your body to break down the molecular bonds in the food you ate. This releases the chemical energy that was stored in the food, thus powering your muscles, warming your body, and allowing you to move. Chemical energy stored in wood can be converted to thermal energy by burning the wood.

We feel thermal energy as heat. Because the particles that make up all matter are constantly in motion, they always have kinetic energy. Thermal energy is the amount of kinetic energy or movement in the particles, atoms, and molecules of an object. The faster those particles are moving, the more kinetic energy the object has and the warmer it will feel. For example, the water in a cup of hot coffee has more thermal energy than the same cup of coffee after it's cooled off.

Sound energy is caused when an object, such as a guitar string or a bell, vibrates. This energy is transferred to the air, causing the air to vibrate. Finally, the air transfers the energy to our eardrum, causing it to vibrate. Our brain transfers the vibrations of the eardrum into nerve impulses that tell your brain you hear the sound. It's a lot of vibrating for one sound - imagine the vibrations involved in one 50 Cent song!

Light energy is produced by the vibration of electrically charged particles called photons. Photons travel extremely fast (300 million meters per second!) but have no mass. Unlike sound energy, which needs air to vibrate, light energy can travel through a vacuum - this is why we can see objects like stars or planets that are in outer space. Other kinds of light waves that we can't see are related to visible light. For example, we can't see microwaves, but they can raise the kinetic energy in molecules of food so that it heats up and cooks in a microwave oven.

Electrical energy is the energy of moving electrons, those negatively charged subatomic particles. By causing electrons to move through a wire, we can use them to do work.

Nuclear energy comes from either nuclear fission or nuclear fusion. In nuclear fission, the nucleus of a large atom, such as uranium, is split apart, which we can do by shooting neutrons at the nuclei. When a nucleus splits, it releases a great deal of energy and shoots its own neutrons out. These neutrons in turn may hit more nuclei until a chain reaction is formed, splitting more and more atoms. If left unchecked, the result is a nuclear explosion, which releases tremendous amounts of energy, much of it thermal, and destroys everything nearby. If the process is controlled, then the amount of thermal energy that's released can be harnessed and used to do work. In a nuclear power plant, for example, the thermal energy that's produced is used to boil water, which in turn is used to turn electric turbines and produce electricity.

Nuclear fusion produces even more energy than nuclear fission. Instead of breaking down large atoms, in nuclear fusion, two small hydrogen atoms are fused together to form a larger helium atom. This releases tremendous amounts of energy, which can be measured by Einstein's E = MC 2 equation. Nuclear fusion produces most of the sun's and other stars' energy.

What is your kinetic energy level while running at top speed?

Use the equation:
kinetic energy = MV / 2

Have someone time you running a 100-meter dash. This will give you your velocity or V.

Weigh yourself to calculate your mass to the nearest kilogram.

Insert your mass (M) and Velocity (V) into the equation and see what you get!

Express your answer in kilograms per meter.

When Einstein came up with E = MC 2, he proved that energy and mass, which most people thought of as two totally separate things, were actually the same. He realized that if you could get matter to fall apart, you would create lots and lots of energy. This is the basis of nuclear fission: Radioactive elements fall apart (or "decay") and release tremendous amounts of energy, according to Einstein's formula.

If all the matter in your body were to be annihilated, an insane amount of energy would be released. If you weigh 150 pounds, for example, the energy released would be 6.1 quintillion joules of energy. A quintillion is a billion billion! That's the same amount of energy in 48 billion gallons of gas, and it's all stored within your atoms.

How many different forms of energy can you find in this story?

Shutting off my alarm clock, I stood up, walked into the kitchen, and pulled open the refrigerator door. The lightbulb lit up, and I peered in at the almost empty refrigerator. Grabbing a loaf of bread, I placed a piece in the toaster and turned it on. In a few seconds the toast popped and I spread it with blueberry jam. In a few minutes I finished eating and felt ready to go to school. In the cool morning air, I shivered a few times at the bus stop, until the sun's rays finally warmed my face.

(Sound: alarm clock; potential: when you're standing; kinetic: walking around; light: bulb and sun; electrical: refrigerator and toaster; thermal: toast and sun on face; chemical: food/digestion)

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