Download Mechanical Energy - Bibb County Schools

SP1.
Students will analyze
the forms and transformations
of energy.
Mechanical Energy
Energy
Energy is the ability to cause change
Anything that changes uses energy
Energy is measured in Joules (J), just like
work
One form of energy is Kinetic energy (KE)
How much KE?
How Much KE?
Kinetic Energy (KE)
Moving energy
Dependent on
– Mass
– Velocity
Mass,
Velocity,
KE
KE
Kinetic Energy (KE)
The equation for KE is
KE = 1/2mv2
Because velocity is squared, the KE will
go up very fast as you increase velocity
Example
A 1,000kg truck was traveling at 4m/s and
then accelerated to 10m/s. Find the initial
KE and the final KE.
KE = 1/2mv2
KE = (½)(1000)(4)2
– KE = 8000J
KE = (½)(1000)(10)2
– KE = 50000J
Potential Energy
Potential energy is stored energy
It comes in many forms:
– Gravitational
– Elastic
– Chemical
– Electric
– Etc.
What kind of PE?
Gravitational Potential Energy
(GPE)
Is the energy an object will use to fall.
Depends on the object’s :
– Weight (mass * gravity)
– Height from ground (or whatever it’s falling to)
Equation for GPE:
PEg = mgh
Problem
A 0.06 kg tennis ball starts to fall from a
height of 2.9m. What was its gravitational
potential energy at that height?
GPE = mgh
GPE = (0.06)(10)(2.9)
GPE = 1.74J
Elastic Potential Energy
Elastic potential energy comes when an
object is stretched or compressed.
This depends on how far the object has
been distorted.
Mechanical Energy
Mechanical energy is the energy of a system
Mechanical energy (ME) comes from two
different sources:
– Potential energy
– Kinetic energy
Mechanical energy typically does not
include nuclear, chemical, electrical and
other ancillary types of energy
Conservation of Energy
In a system, the mechanical energy is
conserved.
The amount of energy in the beginning
should equal the amount of energy at the
end*
*minus energy loss due to friction
Converting Energy
Mechanical Energy is the total of the
potential and kinetic energy in a system.
ME = PE + KE
Converting Energy
Though GPE and KE change, the total
energy remains constant.
Open vs. Closed Systems
A closed system is one where no energy is
added
An open system does add energy
Try It
The tops of the towers on the Golden
Gate Bridge are 227m above the water.
Suppose that a worker drops a 0.655 kg
wrench from the top of the tower. What
will be the kinetic energy of the wrench
when it hits the water?
Mass and Energy
Sometimes, mass can be converted into
energy
– Ex: Nuclear fission/fusion
The equation for the amount of energy
produced is E = mc2
Now it’s your turn!
Do the problems on the worksheet
ToD
Name three types of energy
transformations you’ve used today
Show your work