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This week, we will continue
our discussion of energy.

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We will introduce the
concept of potential energy--

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which is related to the work
done by a conservative force,

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and also the concept of
total mechanical energy--

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which is defined as the sum
of the kinetic and potential

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energies.

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We will demonstrate that
the total mechanical energy

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of a system remains constant
if only conservative forces are

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acting.

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This is known as the
Principle of Conservation

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of Mechanical Energy.

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We will also see
how the work done

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by any non-conservative
forces causes

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the total mechanical
energy of the system

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to change, usually causing
mechanical energy to be

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lost from the system.

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Finally, we will see how graphs
of a system's potential energy,

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as a function of
position, provide

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an elegant way of
understanding the behavior

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of the system for a given value
of the total mechanical energy.