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Chapter 15: Energy
15.1 Energy and Its Forms
Energy &Work are closely related:
 Energy is the ability to do work. Energy is transferred by a force moving an object
through a distance.
 Any object that has energy has the ability to created a force.
 Work: is a transfer of energy. When work is done on an object, energy is transferred
to that object.
 Work is done by forces. Work is done on objects.
 Work in a physics problem means force times distance.
Work = Force x Distance
W = Fd
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The joule is the SI unit of work & energy.
Energy is measured in joules, the same units as work. That is because energy is really
stored work.
If you push a box with a force of one Newton (N) for a distance of one meter (m), you
have done exactly one joule of work.
= 1 joule
Joules = Newtons x Meters
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Energy can take many different forms and can always be measured in joules.
Kinetic & Potential Energy: 2 general types of energy.
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Kinetic energy: the energy of motion
The kinetic energy of any moving object depends upon its mass & speed.
Kinetic energy = ½ x mass x velocity squared
KE = ½ mv2
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Kinetic energy increases as the square of the speed. This means:
o If you go twice as fast, your energy increases by 4 times (22 = 4)
o If your speed is three times higher, you energy is 9 times bigger. (32 = 9)
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Kinetic energy increases with mass.
Double the mass = double the KE.
Kinetic energy increases with speed.
Double the speed = quadruple the KE
? Why would tripling the speed at which a car is
moving have a greater effect on its kinetic energy
than tripling its mass?
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Potential Energy: energy that is stored, energy with the potential to do work.
Potential energy comes from the position of an object relative to the Earth.
Potential energy that depends upon an object’s height is called gravitational potential
energy
o An object’s gravitational potential energy depends on its mass, its height, and
the acceleration due to gravity.
Potential Energy = mass x acc. of gravity x height
PE = weight x height
o Elastic Potential Energy: the potential energy of an object that is stretched or
compressed.
Forms of Energy: each of these forms of energy can be converted into other forms of
energy.
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Mechanical Energy: the energy associated with the motion and position
of everyday objects is mechanical energy.
Mechanical energy = KE + PE
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Thermal Energy: Heat, the particles that make up matter are always in
random motion.
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Chemical Energy: the energy stored in the chemical bonds in
compounds. When these bonds are broken, the release energy can do
work. All chemical compounds store energy.
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Electrical: The energy associated with electric charges, these charges can
exert forces that do work.
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Electromagnetic Energy: a form of energy that travels through space in
the form of waves.
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Nuclear Energy: The nucleus of an atom is held together by strong and
weak nuclear forces, which can store an enormous amount of potential
energy.
o Fission: a process that releases energy by splitting nuclei apart.
o Fusion: release energy when less massive nuclei combine to form a
more massive nucleus.
15.2 Energy Conversion & Conservation:
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Energy can be converted from one form to another.
Energy conversion: the process of changing energy from one form to another
When energy changes from one form to another, the total energy remains unchanged
even though many energy conversions may occur.
The work done by friction changes kinetic energy into thermal energy.
The law of conservation of energy states that energy cannot be created or destroyed.
The gravitational potential energy of an object is converted to the kinetic energy of
motion as the object falls.
When friction is small enough to be ignored, an object’s mechanical energy does not
change. If you apply the law of conservation of energy to a process, you obtain an
equation for the conservation of energy.
Conservation of Mechanical Energy
(KE + PE) beginning = (KE + PE) end