Transcript The Conservation of Energy

The Conservation of
Energy
Robert Drach
What do we already Know??
• Kinetic Energy: Depends on an objects
motion.
• KE = 1/2 mv2
• Potential Energy: Depends on an
object’s position.
• PE = mgh (gravitational potential energy)
What do we already Know?
• WORK
– Work is done to an object by an external
force.
– Work is measured in Joules, just like
energy!
• Where do we go from here?
What is a closed system?
• Just what it sounds like!
• In a closed system, no energy enters or
leaves.
• For example: no work is done by an
external force.
Conservation of Energy
• In closed systems, all energy is
conserved.
• This includes Potential Energy, and
Kinetic Energy.
• E = PE + KE
• In a closed system, this total E will not
change.
• Kinetic Energy can take many forms:
– Translational KE (1/2 mv2)
– Vibrational
– Rotational
• Potential Energy can also take many
forms:
– Gravitational Potential
– Spring Potential
• No matter what type of potential or
kinetic energy we use, E is always
conserved.
• EX: Throwing a ball of mass m.
• Immediately after the ball is thrown:
– h = 0, --> PE = mgh = 0
– All the energy is kinetic energy.
• When the ball reaches the top of its
climb:
– It temporarily stops, so KE = 0
– It has a height, h, so PE = mgh.
h=h
v=0
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What happens at the bottom of the fall?
Obviously, it has stopped.
Obviously, it has no height.
So KE + PE = 0
v=0
h=0
• So what happened to conservation?!?
• When the ball hits the ground:
– The ground exerts a force on the ball.
– The ball and the ground heat up.
– The ball makes a THUD.
• These are all manifestations of energy.
• But since the energy is no longer in the
ball, we can no longer say it is
conserved!