Law of Conservation of Energy

7.2 Conservation of Energy

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Systems change as energy flows and
changes from one part of the system
to another.

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Each change transfers energy or
transforms energy from one form to
another.

Multiple Choice

The faster an object moves, the ________ kinetic energy it has. 

• Less

• More

• None of the above

7.2 Energy flow
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How can we predict
how energy will
flow?

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One thing we can always be sure of is that systems tend to move from higher to lower energy.

7.2 Sources of energy

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The chemical potential energy stored in
the food you eat is converted into
simple sugars that are burned as your
muscles work against gravity as you
climb the hill.

Multiple Choice

Which way does energy flow?

• It doesn't flow, stays the same

• Low to High

• High to low

7.3 Conservation of Energy

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The idea that energy tranforms from one form into another
without a change in the total amount is called the law of conservation of energy.

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The law of energy conservation
says the total energy before the
change equals the total energy
after it.

Multiple Choice

If a system has 18 joules of total ME and 3 joules of KE how many PE joules are in the system?

• 18 J

• 15 N

• 15 Joules

• 18 N

7.3 Conservation of Energy

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When you throw a ball
in the air, the energy
transforms from
kinetic to potential
and then back to
kinetic.

Solving Problems

�A 2 kg car moving with a speed of

2 m/sec starts up a hill.

�How high does the car roll before

it stops?

1.

Looking for:

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…height of hill

2.

Given

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… mass = 2 kg, v = 2 m/s

3.

Relationships:

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Energy transformed from EK to EP

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EK = ½ mv2

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EP =mgh

Solving Problems

1.

Solution

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Find beginning EK

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EK = ½ (2 kg) (2 m/s)2 = 4 Joules

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Assume energy before = energy after
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EK = EP

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EP =mgh 4 J = mgh

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h = (4 N∙m)/(2 kg)(9.8 N/kg) = .2 m

Solving Problems

7.3 Conservation of Energy

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Many people are concerned about
“running out” of energy.

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What they worry about is running out
of certain forms of energy that are
easy to use, such as fossil fuels like
oil and gas.

Multiple Choice

Energy sources that we can use are only petroleum, wind, solar, uranium (nuclear), coal, natural gas, and hydropower. True or False?

• True

• False

7.3 Conservation of Energy

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It took millions of years to accumulate these fuels
because they are derived from decaying, ancient
plants that obtained their energy from the Sun when
they were alive.

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Because it took a long time for these plants to
grow, decay, and become oil and gas, fossil fuels
are a limited resource.

7.3 Conservation of Energy

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Other forms of
energy, such as
thermal energy,
flowing water,
wind, and solar
energy are not as
limited.