Transcript Types of Energy - reynardearthsci

Types of Energy
Energy is the ability to do work!
• Energy is measured by
the amount of work it
can do.
Energy comes in different
forms…
• Potential energy
(PE):
– Energy possessed by
an object due to its
position
– Sometimes referred
to as “stored energy”
Gravitational Potential Energy
• If an object, originally
at rest on Earth’s
surface, is lifted to
some height, work is
done against the
gravitational force.
• The work done in lifting
the object is equal to
the objects
gravitational potential
energy.
work done = gravitational potential energy
W = ΔPE
W = Fd
Fg
w
W = mgd
h
ΔPE = mgh
mg
Knowing that the potential energy at the top of the tall
platform is 50 J, what is the potential energy at the other
positions shown on the stair steps and the incline?
Path doesn’t matter…
Remember that the changes in an object's potential energy
only depend on comparing its starting position and its ending
position, not on whether it does or does not pass through
various points in-between.
Try this…
• How much potential energy is gained by
an object with a mass of 2 kg that is
lifted from the floor to the top of a .8 m
high table?
ΔPE = mgh
ΔPE = (2kg)(10m/s2)(.8m)
ΔPE =16 J
Try this…
• King Kong is on top of the Empire State
Building 426 m above the surface of the
Earth. What is his gravitational potential
energy relative to the ground? Let’s say
his mass is 1000 kg (a metric ton).
ΔPE = mgh
ΔPE = (1000kg)(10m/s2)(426m)
ΔPE = 4,260,000 J
Draw how the graph would look that
represents this relationship- PE vs. h
ΔPE = mgh
PE = mg
h
PE
What if m = .1 kg
h
Elastic Potential Energy
• Energy can be
stored in a
spring and is
measured as
the work
required to
stretch or
compress it.
Remember Hooke’s Law…
• The compression or
elongation of a spring
is directly
proportional to the
applied force.
Fs = kx
Spring constant
The larger the k, the stiffer the spring.
x
F
What’s the spring constant of this spring?
F = kx
k=F
x
= 25 N = 50 N/m
.50 m
Potential Energy of a Spring
W = PEs
W = Fd
½ kx
x
PEs = ½ kx2
What would the graph look like that shows
this relationship- PE vs. x?
PE
PE
x
PE
x
PE
x
PEs = ½ kx2
What if we made k = 2 N/m
PEs = x2
x
Elastic potential energy can be stored in
rubber bands, bungee chords, trampolines,
springs, an arrow drawn into a bow, etc.
Try this…
• A force of 50 N is needed to compress a
spring a distance of 1 m. What is the
potential energy stored in the
compressed spring?
Fs = kx
Fs = kx
x
x
k = Fs
x
PEs = ½ kx2
k = 50 N
1m
PEs = ½ (50)(1m)2
k = 50 N/m
PEs = 25 J
Try this…
• When a spring is stretched .2 m from its
equilibrium position, it possesses a
potential energy of 10 J. What is the
spring constant for the spring?
PEs = ½ kx2
k = 2PE
x2
k = 500 N/m
Kinetic Energy
• When a moving
object strikes
another object and
displaces it, the
moving object
exerts a force on
the second object
and does work on
it.
Kinetic Energy- the energy an object
possesses due to its motion.
W = ΔKE
W = Fd
ma
W = ΔKE = m v v t
t2
vt
v
t
from restv
2
ΔKE = ½ mv2
Try this…
• What is the kinetic energy of a 980 kg
race car traveling at 90 m/s?
ΔKE = ½ mv2
ΔKE = ½ (980kg)(90m/s)2
ΔKE = 3,969,000 J
Try this…
• Determine the kinetic energy of a 625kg roller coaster car that is moving with
a speed of 18.3 m/s.
ΔKE = ½ mv2
ΔKE = ½ (625kg)(18.3m/s)2
ΔKE = 104,653 J
Try this…
• A platform diver for the Circus has a
kinetic energy of 12 000 J just prior to
hitting the bucket of water . If the divers
mass is 40 kg, then what is her speed?
ΔKE = ½ mv2
v2 = 2KE
m
v = 25 m/s