Download Conservation of Energy 2015

Conservation of Energy
(a.k.a Slacker Physics)
Now, Really…Conservation of Energy
In a system, energy can not be created or
destroyed. Energy can change form. The
total amount of energy remains constant.
Total Mechanical Energy = KE + PE
Conservation of Energy
At the top the total energy is all
potential (velocity = 0 m/s).
In between the total energy is
potential + kinetic.
At the bottom the total energy is
all kinetic (height = 0 m).
Gravitational Potential Energy
The elephant is 4 meters
above the ground and has a
mass of 500 kg.
What is his potential energy?
What is his kinetic energy?
Conservation of Energy
The elephant falls off the rock
formation and is 2 meter above
the ground
What is his potential energy?
What is his kinetic energy?
Conservation of Energy
Right before he falls into the
water, what is his potential
energy?
What is his kinetic energy?
What is the magnitude of his
velocity?
Energy of a Pendulum
Energy Transfers from Potential to Kinetic to Potential as the pendulum swings.
PE max
(max height)
KE = 0
(stop and turn)
PE max
(max height)
KE = 0
(stop and turn)
PE 0
(height = 0)
KE = max
Work-energy Theorem
W=∆KE
= KEf - KEi
= ½ mvf2 - ½ mvi2
Work = the change in kinetic energy
Work = final kinetic energy – initial kinetic energy
Work – Energy Theorem
How much work is done
to stop the elephant
when he hits the
ground?
W = KEf - KEi
= 0 J – 19600 J
= -19600 J
Negative and Positive Work
Positive work
The force is in the same direction as the motion.
Negative work
The force is in the opposite direction of motion.
Often, the object being acted upon will be slowed or stopped.
Kinetic Energy Example
A 75 kg skater is moving at 20 m/s across the concrete. How much kinetic
energy does he have?
KE = ½ mv2
Work - Energy Example
A 75 kg skater is moving at 20 m/s across the concrete. The skater slows to 5
m/s to avoid wiping out. How much work is done by the skater?
W=Kef - Kei = ½ mvf2 - ½ mvi2