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ENERGY
ENERGY
Basic Ideas about Energy:
•It is the property of an object that gives it the ability
to cause change
ENERGY
•It can be neither created nor destroyed, instead it is
transferred from one type to another: thermal,
chemical, nuclear, electrical, etc
•Units for ENERGY are kg•m2/s2 = Joule (J)
•It takes energy to get something done
•i.e, Energy is converted when Work takes place
POTENTIAL ENERGY
Potential Energy = the stored energy of an object as
a result of the position
We are going to
learn about two
types of
PE…gravitational
potential energy and
elastic potential
energy!
ENERGY
Gravitational Potential Energy (GPE)
•This type of PE is when an object’s stored energy
is the result of its vertical position (height)
ENERGY
•It is dependent on mass and height
•Formula is Peg = mgh
Energy is a
scalar quantity,
so direction is
unimportant!
Peg = mgh
Peg = (75)(9.8)(4)
Peg = 2,940 J
EXAMPLE:
A 2 kg book sits on a
bookshelf 2.1 m
high. What is the
potential energy of
the book relative to
the floor?
ENERGY
Peg = mgh
Peg = (2)(9.8)(2.1)
Peg = 41 J
Elastic Potential Energy
•Stored energy in elastic
materials as the result of
their stretching or
compression
ENERGY
•Ex: springs, bungee cords,
bow and arrows
Elastic Potential Energy
PEs = ½kx2
F = -kx
n this equation, the
orce used to stretch or
compress the object (F)
s equal to the amount
of deformation/how
ar stretched or
compressed (x)
multiplied by the
spring constant (k)
ENERGY
k = spring constant
x = amount of
deformation
PEs Example
The staples inside a stapler are kept in place by a
spring with a relaxed length of 0.115 m. If the spring
constant is 51 N/m, how much potential energy is
stored in the spring when the length is stretched to
0.150 m?
ENERGY
PEs = ½kx2
PEs = ½(51)(0.150-0.115)2
PEs = ½(51)(0.035)2
PEs = .031 J
Kinetic Energy
•Kinetic energy is the
energy of motion
ENERGY
•It is dependent on
mass and velocity
•Formula is KE = ½mv2
KE Example
Determine the kinetic energy of a 625 kg rollercoaster
cart that is moving with a speed of 18 m/s.
ENERGY
KE = ½mv2
KE = ½(625)(18)2
KE = ½(625)(324)
KE = 1.05 x 105 J
Relationship Between PE and KE
•As an object moves, the amount of PE and KE may
shift back and forth
ENERGY
Graphing
•In a Force (F) vs. Deformation (x) graph, the slope of
the line is the Spring Constant (k)
ENERGY
You can reason this out
since slope = y2-y1/x2-x1
(which is the same as
dividing Force by
Deformation) and the
rearranged equation for
F = -kx would then be
k = F/x