Transcript The nature of Energy

The Nature of
Energy
Power,
by definition, is
the time rate of doing work;
or the time rate transfer of energy.
P=W/t
Power is a scalar quantity.
The SI unit of power
is the Watt,
named in honor of
James Watt.
One Watt, W, of power
is the power achieved
when 1.0 J of work is done or
1.0 J of energy is transferred
in a time of 1.0 s.
Work is only done by a force on an
object if the force causes the object
to move in the direction of the force.
Objects that are at rest may
have many forces acting on them,
but no work is done
if there is no movement.
Work,
by definition, is
the product of the force exerted on
an object and the distance the object
moves in the direction of the force.
W = F·d
Work is a scalar quantity.
The SI unit of work
is the Joule,
named in honor of
James Prescott Joule.
One Joule, J, of work
is the work done when
1.0 N of force is applied
through a distance of 1.0 m.
If the force and displacement are not
in the exact same direction, then
work = Fd(cosq),
where q is the angle between the force
direction and displacement direction.
F =40 N
35
d = 3.0 m
The work done in moving the block 3.0 m
to the right by the 40 N force at an angle
of 35 to the horizontal is ...
W = Fd(cos q) = (40N)(3.0 m)(cos 35) = 98
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
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.
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
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)!
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)
Potential Energy
energy of position or condition
elastic potential energy
PEe =
2
½ kx
k – elastic constant in N/m
x - elongation or compression in m
PEe – elastic potential energy in J
Click here to investigate elastic constants.
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
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.