Download - 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
• What happens at the bottom of the fall?
• Obviously, it has stopped.
• Obviously, it has no height.
• So KE + PE = 0
• So what happened to conservation?!?
h = 0
v = 0
• 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!