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Lecture on Energy
Energy:
-what is it?
-where does it come from?
-where does it go?
-how do you measure it?
A. energy is “the ability to do work”:
to change things:
in terms of position or
speed or
temperature etc.
B. so, work is related to energy:
work = force x distance
W= F x D
C. power is also related to energy:
rate (time) work is done:
power = work
time
The energy-work-power triangle:
Energy
Power
Work
D. the unit for energy is the Joule (J), and for work too
1. 1 joule = 1 Newton x 1 meter:
energy = force x distance (same as work!!)
2. so a joule is a function of the amount of
force applied for a motion.
Need motion to occur for energy to be used
E. E = MC2: more mass = more energy
F. energy is released in thermonuclear blasts, so
energy and mass are related: different forms of the
same thing
G. four forms of energy:
1. mechanical: due to a motion or a position
examples: both kinetic and potential energy
2. chemical: stored in the bonds between
atoms: food, gasoline, battery acid
3. nuclear: stored in the nucleus of all
atoms, due to both their strong and weak
nuclear forces:
the sun is a massive nuclear rxn, releasing
many different types of energy: nuclear,
heat, light etc
4. radiant: pure energy from the
electromagnetic spectrum:
-long waves: radio, micro
-short waves: x-ray, gamma
-in between waves: IR, light (roygbiv),
ultra violet (uv)
H. law of conservation of energy: energy is not created
or destroyed, only transferred, often as heat: rub
hands, feel brakes/tires, paper on fire
I. energy sources for Earth:
1. sun is the biggest one for Earth/humans, so
much so we fight wars over it:
-fossil fuels: what are they, how
produced?
-photosynthesis: what is it, what does it
produce
-water cycle: what is it/what drives it?
-wind energy: how is it produced?
2. atomic: nuclear?
3. geothermal: fission w/in Earth?
J. two categories of energy
1. potential energy: stored energy
a. by position: ?
b. as matter: mass: E = MC2
c. formula: Ep= mgh
m= mass, g= accel of gravity, h= height
Ep= mgh
work = force x distance
force = mass x accel
so, potential energy is the same as work!:
work = force x distance
A rock, having a mass of 15.3 kg, rests at the top
of a cliff, 123, 000 cm above the ground. What
is the potential energy of the rock? Show your
work:
2. kinetic energy: energy of motion
a. depends on the mass moving, and
b. depends on the velocity
c. formula: Ek = 1MV2
2
d. note: the faster (velocity) an object
is, has a huge effect because it is
squared!!
(always square the velocity first: order
of operations)
Jean shoots a 0.251 kilogram arrow at a tree with
a kinetic energy of 10.0 J. What is the velocity
with which the arrow hits the tree?
PE = 100%, KE = 0%
PE & KE = 50%
KE = 100%, PE = 0%