Download Energy - Schurz High School

Energy
1.31.14
Two groups of Energy

Potential Energy (PE)

Energy is stored and ready to do work
 Elastic
PE – a spring, a rubberband, a bow
 Chemical PE – batteries, gasoline
 Nuclear PE – the sun, bombs
 Gravitational PE – any object elevated

Kinetic Energy (KE)

Object is in motion
What’s “Mechanical” Energy?


Mechanical energy refers to objects in
motion or that have the potential to
move. It is the sum of potential and
kinetic energy.
Mechanical Energy is the property of
an object that enables it to do ‘work’.
Work is the Energy of a force
Work is the product of the force on an object
and the distance that object moved (in the
same direction as the force).
W  Fd
Work is done to either:
 to change the speed of an object or
 against another existing force, such as lifting
against gravity.
SI Metric Units of Energy
Energy is measured in Joules (J). This includes
Work, Potential Energy, or Kinetic Energy.
1 Joule = 1 Newton x 1 meter
1 Newton x 1 meter is also called a “Newtonmeter”.
Example problem
think!
You lift a 100-N boulder 2m.
Q. How much work is done on the boulder?
Answer:
W = Fd = 100 N x 2 m = 200 N·m = 200 J
Potential Energy:
Gravity exerts a force on this boulder. In order to move it, work must be
done (against the existing force of gravity). Which example requires more
work?
•
The boulder is lifted with 100 N of force.
•
The boulder is pushed up the 4-m incline with 50 N of force.
•
The boulder is lifted with 100 N of force up each 0.5-m stair.
Trick question! Each example requires the same amount of work.
Potential Energy
PE  mgh

But, it’s also equal and opposite to Work!!!!!!

Because we would have to do work against gravity to lift
something to get it elevated to store PE
Therefore, PE converts to Work!
So, what is the gravitational potential energy of the
boulder in the lifted position?
Answer: it is still 200 J!
Why? Because PE is equal and opposite to work!

W  Fd
and
d h
So,
W  Fh
and
ag
So,
F  mg
Then,
F  ma
Therefore,
W  mgh  PE
W is equal and opposite to PE
Kinetic Energy
KE  mv
1
2
2
But it’s ALSO equal to the work required to bring
something to its final motion or to rest because it
is a conversion of potential energy.
…and potential energy is also equal to work and
measured in Joules, and work is equal to force
multiplied by distance. Therefore:

W  Fd  mgh  PE  KE  mv
1
2
2
The Law of Conservation of Energy




We have discovered a natural law of the
universe that all energy is conserved.
Energy is not created or destroyed.
All the energy that was here in the beginning is
still here, and will be here until the end.
Energy simply converts from one form to
another. For example, PE converts to KE.
PE   KE
Forms of Energy
Thermal
 Chemical
 Electric
 Radiant
(electromagnetic)
 Nuclear
 Magnetic

Elastic
 Sound
 Mechanical
 Luminous
 Mass (E=mc2)
 Dark (theoretical)

Can you categorize each of these as Potential or Kinetic?