Kinetic & Potential Energy Notes
Energy Transfer Diagrams Notes
• Energy:
– The ability to
produce change
– Kinetic:
• motion
– Potential:
• stored
1
Work & Power
• Work
• Power
1. The product of the
1.
Force exerted on an
object and the distance
that object travels due
to that force
Work done over time
2. The rate at which
work is done
P
•
symbol:
•
•
W
units:
•
•
symbol:
–
•
Units:
–
N*m= Joule
•
P
J/sec = Watt (W)
1 HP (horsepower) = 746
W
2
Kinetic Energy
• Since a force is required
to change the state of an
If an object is moving,
object’s motion (Izzy’s
2nd), it makes sense that
at any speed, it
to change the energy of
exhibits energy
an object, external Work
– Def.: The energy of an
must be done.
object due to its
• Work-Energy Theorem
motion
• Kinetic Energy
1
2
KE  mv
• symbol:
2
• KE
• units:
• Joules
W  KE
• We will use one kind of
Kinetic energy,
“Translational,” since
any object is moving.
Moving molecules we call
heat, and vibrating
molecules we call sound.
Potential Energy
• Potential Energy
• There are a number
If an object ain’t
of different types of
moving, it may not
Potential Energy
exhibit energy… but it
(since
there
are
so
could!
many positions &
– Def.: The energy of an
conditions for an
object due to its
object)
position or condition
• We will focus on the
• symbol:
position of an object
– PE
with respect to the
• units:
ground–J
Gravitational
Potential Energy
PE  mgy
Conservation of Energy
m = 100 kg
• The Total Energy in a
y = 100 m
system cannot change
– …Unless Work is done
on the system from
outside
• Consider Mr. McGuire on
his Alma Mater, Cass
Tech!(mine is only 1
floor so doesn’t work)
• That Total Energy might
be split between
Potential & Kinetic, and
energy can go back &
forth between the two
v = 0 m/sec
KE = 0 J
PE = 100,000 J
y = 66 m
PE = 66000 J
KE = 34000 J
Energy Transfer Diagrams
• Also known as Energy Flow
Diagrams
• They show:
– Sequence of forms of Energy
– Arrows showing direction of
sequence
– Cause of transfer from one form of
Energy to the next
• What is doing Work on the object
(written above the arrow)
Earth/Gravity
Potential
Kinetic
Energy Transfer Diagrams
Elevator lifting Gladys to the top
of a building
Motor
Electric
Cable
Kinetic
Potential
Energy Transfer
Diagrams
• A taco in a cart is
rolled up a hill, and it
coasts to a stop
Earth
Kinetic
Friction
Earth
Kinetic
Friction
+ Potential
+ Heat
Potential
+ Heat
Energy Bar Charts
• As we have seen, Total Mechanical Energy
(TME) for a system can actually increase
or decrease…
– …only if (external) Work is done on it
Initial Energy + Work = Final Energy
TMEi
+ W
= TMEf
KEi + PEi +
W
= KEf + PEf
Energy Bar Charts
• Example 1: A plate of
lasagna falls to the
ground
– Starts out with only PE
– Ends with KE
• Example 2:
Hernandez, obeying
the 5-sec rule, picks up
the lasagna
– Starts out with Zero KE
& PE
– Hand does Work on
Lasagna
– Ends with PE