Transcript The nature of Energy

The Nature of
Energy
Energy
The ability to cause change or the
ability to do work
Joule – the SI unit used to measure
energy
Energy
 Two main types of energy:
Kinetic
Potential
 Other types of energy include…
 Thermal
 Light
 Sound
 Electrical
 Chemical
 Nuclear
Kinetic energy
Energy in the form of motion
Kinetic energy = ½ x mass x velocity2
KE =
2
½mv
Work - Energy Theorem
 Net work = change in KE
Wnet = ΔKE
(“change in” means final minus initial)
Wnet = KEf – KEi
When you combine the kinetic energy
formula and the work energy
theorem…you get
Wnet = (½ mv2)f – (½ mv2)i
Potential Energy
Energy stored in a motionless
object, giving it the potential to
cause change
3 Types of Potential Energy
 Elastic Potential Energy – Energy stored by
things that stretch or compress (springs,
elastic, rubberbands)
 Chemical Potential Energy - energy stored in
chemical bonds between atoms (Snickers bar,
food, even gasoline)
 Gravitational Potential Energy – energy stored
by things that are above Earth (book sitting on
shelf, person standing on a cliff, rollercoaster
perched at the top of a hill)
Gravitational Potential Energy
GPE = mass x acceleration due to
gravity x height
GPE = mgh
Be aware that U is the general
variable used for all types of
potential energy in upper level
physics…different types are denoted
by subscripts on the U.
Ug = mgh
Mechanical Energy
 The energy an object has because of its motion or position; The
sum of the kinetic energy and all forms of potential energy in a
system; for now we will focus only on GPE
 ME = KE +∑PE (real formula)
 ME – KE + GPE (current use of formula)
NOTE: This is NOT the total energy. Remember that other
types also exist in the world…
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Thermal
Light
Sound
Electrical
Chemical
Nuclear
Law of Conservation of Energy
Energy may change from one
form to another, but the total
amount of energy in a system
does not change
MEi = MEf or…
KEi + PEi = KEf + PEf
Law of Conservation of Energy (example)
 Fuels store energy in the form of chemical
potential energy.
 This energy is transformed into heat energy,
which is then transformed into kinetic energy.
Ex: 1…gasoline put into your car (chem. Pot. Energy)…it is
heated/combusted and a chemical reaction occurs so that the
energy stored in the bonds can be released in the form of heat.
Then, the heat energy is used to move parts like pistons, etc
(kinetic energy) …which eventually moves your car!
Ex: 2…Candy bar has chemical potential energy…you eat it…your
body digests it, meaning it breaks down the food – even
breaking the bonds and releasing energy. Your body uses the
energy to move (kinetic energy)!
Example Problem 1
A 50 kg boy and his 100 kg father went
jogging. Both ran at a rate of 5 m/s. Who
had more kinetic energy? Show your work
and explain.
Example Problem - answer
KE = ½mv2
Boy…
KE = ½(50 kg)(5 m/s)2
KE = 625 J
Dad…
KE = ½(100 kg)(5 m/s)2
KE = 1250 J
had more Kinetic energy because his mass was
Dad
greater.
Example Problem 2
What is the potential energy of a 10 N
book that is placed on a shelf that is 2.5
meters high?
Example Problem 2 - answer
GPE = mgh
GPE = (10 N) (2.5m)
GPE = 25 J
Remember that weight = mg
and that the force provided is
weight.
NOTE: you may want to change
your variable for weight to Fg.
Rollercoaster Example…