Conservation of Mechanical Energy
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Transcript Conservation of Mechanical Energy
Conserved Quantities
When
a quantity is conserved, it
remains constant
The amount of a conserved
quantity is the same at the
beginning of a situation as it is at
the end of the situation
Conservation of Energy
The
TOTAL amount of energy in a
closed system will remain constant
CLOSED SYSTEM: Nothing in or
out – no interaction with the
surroundings
TOTAL ENERGY: all kinds of
energy (kinetic, PEE, PEG,
chemical, heat, etc.)
Mechanical Energy
Mechanical
Energy is the sum of
kinetic and potential energy
ME = KE + PE
ME = KE + PEG + PEE
“Useful” energy
“Can the energy break the window?”
Conservative Forces
Gravity
and elastic forces
Conservative forces DO NOT change
the amount of mechanical energy of
a system
Energy is converted from one form
to another, but the total amount of
mechanical energy remains the
same
Conservation of Mechanical Energy
When
there are only conservative
forces acting on an object (gravity,
springs), MECHANICAL ENERGY IS
CONSERVED
MEi = MEf
KEi + PEGi + PEEi = KEf + PEGf + PEEf
Conservative Forces
Conservation of Mechanical Energy
PEG decreases & KE increases, but total ME
remains constant (in the absence of air friction)!
Conservation of ME & Falling
Objects
10000 J
5000 J
Time (s)
Conservation of ME & Pendulums
PEG = mgh
KE = 0
KE = ½ mv2
PEG = 0
Is Mechanical Energy Conserved
Here?
Nonconservative Forces
When nonconservative forces act on a
system, mechanical energy IS NOT
conserved
WNC = ΔME
This is the Work-Energy Theorem
Friction
Friction does negative work on the
system, so the mechanical energy is
decreased
While ME is not conserved when
nonconservative forces act on an object,
TOTAL ENERGY is always conserved
Total energy would include heat energy
Non-Conservative forces turn ME into
non-ME (usually heat), but the total
amount of energy stays the same
Friction & Pendulums