Transcript Energy Notes

Energy as a tool in physics
• Energy is a very abstract notion, but it is
a very useful and quantifiable notion
• We use the law of conservation of energy
to predict behavior
– We rely on the fact that energy is not created out of nowhere
• Where did the energy we see around us
come from?
– most of what we use derives from the sun
– some derives from other, exploded stars (nuclear fission)
– ultimately, all of it was donated in the Big Bang!
Of conservation of energy
Energy cannot be
created or
destroyed
Types of Energy
All energy can be broken down into 2
categories
Kinetic Energy
Potential
Energy
Kinetic energy
An objects kinetic energy is equal to……
K=½
2
mv
Which has a bigger affect????
kinetic energy = energy due to motion
If this car had a
velocity of 24 m/s
and a mass of 1000
kg what was its
kinetic energy
when it entered
your living room?
1) 288000 joules
2) 15000 joules
3) 49000 joules
4) 391000 joules
Potential Energy
An objects Gravitational Potential energy
is equal to……
Ug = mgh
Which has a bigger affect????
Potential energy = stored energy in earth
1644 m
staircase
Calculate Spidey’s
gravitational potential
energy if he is 150 m
up a building and his
mass is 89 kg!!!
1) 288000 joules
2) 4388 joules
3) 130830 joules
4) 94224 joules
Dissipated Energy
An objects Thermal
energy is equal to……
This is energy that is hard to reclaim
from the system. Even though the
energy is not lost, it is hard to convert
into another form of energy.
An object gives off thermal energy in
the form of heat! This is where %75 of
your energy goes!!
kinetic energy = energy due to motion of
molecules, but it is extreme hard to
reclaim
Elastic and spring Potential
Energy
An objects elastic potential is equal to……
Relax! Right now you need to
understand that it is another form of
potential energy!
Think bow and arrow, rubber bands,
springs, catapults, sling shots, etc.
Potential energy = stored energy in the
spring
Mechanical vs. Nonmechanical energy
Mechanical energy – Potential and
Kinetic Energy
The total mechanical energy of a system
will be the sum of the potential and
kinetic energy
Non-Mechanical energy – everything
else! Light, Sound, Chemical, thermal.
Of conservation of energy
Energy cannot be
created or
destroyed!!
So what
happens
to an
The
point
is:
So
does anpotential
object lose
or
object’s
and
Energy
is neveras
lost…
gain
energy
itit
kinetic
energy
as
just transferred!
moves?!?
moves?!?
Doing “Work” on objects
• Work in physics is when energy is
transferred into, out of, with in the system.
• Work in the system can result in a change in
Kinetic Energy OR a change in Potential
Energy.
• Can be positive (net gain in energy) or
negative (net loss in energy)
• Work is accomplished by applying a FORCE
through a DISTANCE. Anytime there is a
Work = Force x distance
W = Fd
change in energy, you do
work!
Lets say this fat kid
pulls his wagon
with a force of 300
N. How much work
does he do on
wagon if he pulls it
for a distance of 35
meters?
A. 8.57 joules
B. 7,777 joules
C. 4,322 joules
D. 10,500 joules
W = Fd
Units
Work & Energy are scalar quantities.
Joule (J) – SI unit for Work & Energy
James Prescott Joule (1818-1889) – English
physicist; studied heat and work.
This led to the theory of conservation of energy.
Developed Kelvin scale with Lord Kelvin.