Read the following information on energy. Once you have read the information, follow the link at the bottom to answer the energy questions.
Energy
When work is done by an archer in drawing a bow, the bent bow has the ability to do that amount of work on the arrow. When work is done to raise the heavy ram of a pile driver, the ram acquires the ability to do that work on the object it hits when it falls. When work is done to wind a spring mechanism, the spring acquires the ability to do work on various gears to run a clock, ring a bell, or sound an alarm.
In each case, something has been acquired. This “something” that is given to the object enables the object to do work. It may be in the form of a compression of atoms in the material of an object; it may be a physical separation of attracting bodies; it may be a rearrangement of electric charges in the molecules of a substance. This “something” that enables an object to do work is energy. Energy is measure in a unit called Joules. It appears in many forms. For now we will focus on mechanical energy – the energy due to the position or the movement of something. Mechanical energy may be in the form of either potential energy or kinetic energy.
Potential Energy
An object may store energy by virtue of its position. The energy that is stored and held in readiness is called potential energy (PE), because in the stored state it has the potential for doing work. A stretched or compressed spring, for example, has the potential for doing work. When a bow is drawn, energy is stored in the bow. A stretched rubber band has potential energy because of its position, for if it is part of a slingshot, it is capable of doing work.
The chemical energy in fuels is potential energy, for it is actually energy of position on a microscopic scale. This energy is available when the positions of the electric charges within and between molecules are altered, that is, when a chemical change takes place. Any substance that can do work through chemical action possesses potential energy. Potential energy is found in fossil fuels, electric batteries, and the food we eat.
Work is required to elevate objects against earth’s gravity. The potential energy due to elevated position is called gravitational potential energy. Water in an elevated reservoir and the ram of a pile driver have gravitational potential energy.
The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in lifting it. The work done equals the force required to move it upward times the vertical distance it is moved (W=Fd). The upward force required (while moving at constant velocity) is equal to the weight mg of the object, so the work done in lifting it through a height h is given by the product mgh:
Gravitational potential energy = weight x height
PE = mgh
Note that the height h is the distance above some reference level, such as the ground or the floor of a building. The potential energy mgh is relative to the level and depends only on mg and the height h. The potential energy of an object on a ledge does not depend on the path taken to get it there.
Kinetic Energy
Push on an object and you can set it in motion. If an object moves, then by virtue of that motion it is capable of doing work. It has energy of motion, or kinetic energy (KE). The kinetic energy of an object depends on the mass of the object as well as its speed. It is equal to half the mass multiplied by the square of the speed.
Kinetic energy = ½ mass x speed2
KE = ½ mv2
When you throw a ball, you do work on it to give it speed when it leave your hand. The moving ball can then hit something and push it, doing work on what it hits. The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest, or the work the object can do in being brought to rest:
net force x distance = kinetic energy
or in shorthand notation,
Fd = ½ mv2
Note that the speed is squared, so that if the speed of an object is doubled, its kinetic energy is quadrupled (22 = 4). This means it takes four times the work to double the speed; also an object moving twice as fast take four times as much work to stop. Accident investigators are well aware that a car going 60 mph has four times the kinetic energy it has at 30 mph. So a car going 60 mph will skid four times as far when its brakes are locked than it would at 30 mph. This is because speed is squared for kinetic energy.
Kinetic energy underlies other seemingly different forms of energy such as heat (random molecular motion), sound (molecules vibrating in rhythmic patterns), and light (originating from the motions of electrons within atoms). There is much in common among the various forms of energy.
Conservation of Energy
More important than being able to state what energy is, is understanding how it behaves – how it transforms. You can understand nearly every process or change that occurs in nature better if you analyze it in terms of a transformation of energy from one form to another.
As you draw back the stone in a slingshot, you do work in stretching the rubber band; the rubber band then has potential energy. When released, the stone has kinetic energy equal to this potential energy. It delivers this energy to its target, perhaps a wooden fence post. The slight distance the post is moved multiplied by the average force of impact doesn’t quite match the kinetic energy of the stone. The energy score doesn’t balance. But if you investigate further, you’ll find that both the stone and the fence post are a bit warmer. By how much? By the energy difference. Energy changes from on form to another. It transforms without net loss or net gain.
The study of the various forms of energy and their transformations from one form into another lead to one of the greatest generalizations in physics, known as the law of conservation of energy:
Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.
When you consider any system in its entirety, whether it be as simple as a swinging pendulum or as complex as an exploding galaxy, there is one quantity that does not change: energy. It may change form, or it may simply be transferred from one place to another, but the total energy score stays the same.
This energy score takes into account the fact that the atoms that make up matter are themselves concentrated bundles of energy. When the nuclei (cores) of atoms rearrange themselves, enormous amounts of energy can be released. The sun shines because some of this energy is transformed into radiant energy. In nuclear reactors much of this energy is transformed into heat.
Powerful gravitational forces in the deep hot interior of the sun crush the cores of hydrogen atoms together to form helium atoms. This welding together of atomic cores is called thermonuclear fusion. This process releases radiant energy, some of which reaches the earth. Part of this energy falls on plants, and the plant energy is later stored as coal. Another part supports marine life in the ocean’s food chain that begins with plants, and part of this energy later becomes oil. Part of the energy from the sun goes into the evaporation of water from the ocean, and part of this returns to the earth as rain that may be trapped behind a dam. By virtue of its position, the water in a dam has energy that may be used to power a generating plant below, where it will be transformed to electric energy. The energy travels through wires to homes, where it is used for lighting, heating, cooking, and to operate electric toothbrushes. How nice that energy is transformed from one form to another!
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Energy Questions
