Download Potential and Kinetic Energy

Potential and Kinetic Energy
What is Energy
Energy
–is the ability to do WORK!!!
Two types of energy
–Potential Energy
–Kinetic Energy
Potential and Kinetic Energy
Animation
http://www.teachersdomain.org/resource/h
ew06.sci.phys.maf.rollercoaster/
Potential Energy
1. Energy at rest
2. Stored Energy
3. Higher the object - greater the
potential energy
Potential Energy
Which boulder
has the least
amount of
potential energy
and Why?
Types of Potential Energy
Elastic potential
energy
– Energy stored
by things that
stretch or
compress
Gravitational
potential energy
(GPE)
– The amount of
GPE an object
has depends on
its mass, the
acceleration due
to gravity, and
its height above
ground.
Quick Write: Elastic Potential
Energy
Explain how this
rubber band has
elastic potential
energy!
Quick Write: Potential Energy
When is the spring
having potential
energy?
Brain Pop Movie: Potential Energy
http://glencoe.mcgraw-
hill.com/sites/0078802482/student_view0/
brainpop_movies.html
Calculating potential energy
PEg = m*g*h or PEg =Fw*h
Units
PEg = gravitational potential energy
m = mass
g = acceleration due to gravity
h = height
J or Nm
kg
m/sec2
m
Fw=Force of Weight
N
NOTE: Units for kinetic and potential energy are J
(joules). 1 joule = 1 kg × m2/sec2 = 1 Nm
FLASH CARD
PE g = m g h
PE
g
=F
w
h
PE g
m
g
h
Gravitation Potential Energy
Units
PE - gravitational potential energy
g
Fw - weight
N
h - height
m
m - mass
g - acceleration due to gravity
J
kg
m/sec2
Potential Energy Problems
1.
A weight lifter lifts a 120 kg mass from the
floor to a position above his head, 2.5 meters
above the floor. What is the potential energy
expressed by the weight?
PE= m g
h
PE = (120)(9.8)(2.5)
PE= 2940 J
The potential energy of the weight is 2940 J.
2. What is the potential energy of a
90 N boat before it falls from the top
of a 30 m waterfall?
PE = ?
Fw = 90 N
h = 30 m
PE = Fw*h = (90 N) (30 m)
PE = 2700 N • m = 2700J
If the acceleration on a new planet is
5.2 m/s2 and a meteor is sitting on a
hilltop at 100 m with 650 J of PE,
then what is the mass of the
meteor?
Kinetic Energy
1. Energy of motion
2. The amount of kinetic energy an object
has depends on its mass and its velocity
Calculating Kinetic Energy
KE = ½ mv2
KE = kinetic energy
m = mass
v = velocity
Units
J or Nm
kg
m/sec
Example:
What is the kinetic energy of a 120 kg
object moving at 30 m/sec?
KE = ½ mv2
m = 120 kg v = 30 m/sec KE=?
KE = ½ (120 kg)(30 m/sec)2
KE = (60 kg)(900 m2/sec2)
= 54000 kg m2/sec2 = 54000 J
Quick Writes: Kinetic Energy
Which one has
the highest KE?
Which one has
the least KE?
80 km/h
50 km/h
80 km/h
FLASH CARD

2
KE =  m v
2 KE
m=
2
v
v =
2KE
m
Kinetic Energy
Units
KE - kinetic energy
J
v - velocity
m/sec
m - mass
kg
Brain Pop Movie: Kinetic Energy
http://glencoe.mcgraw-
hill.com/sites/0078802482/student_view0/
brainpop_movies.html
Potential and Kinetic Energy Practice Problems
1. A 100kg rock sets at the top of a 50 m cliff.
What is the rocks potential energy?
2. A 70 kg rock is falling at 30 m/sec. What is
its kinetic energy?
3. A 315 kg object sets at the top of a 50 m
building. What is its potential energy?
4. A car has 3600 J of kinetic energy and is
going 3 m/sec. What is its mass?
Work
1. Transfer of energy through
motion.
2. The force must act in the same
direction as the motion.
3. Force exerted through a distance
Example 1: Lifting a log is work.
Log moves in the direction the force is
applied
Example 2: Carrying a log is not work.
Log is held up, but moves forward,
therefore no work is done on the log.
Calculating Work
W = F*d
W = work
F = force
d = distance
Units
J
N
m
Example:
How much work is done when 87 kg mass is
lifted 1.2 m?
W=Fd
d = 1.2 m FW = mg
m = 87 kg g = 9.8 m/sec2
W = mgd
W = (87 kg)(9.8 m/sec2)(1.2 m)
= 1023 J
FLASH CARD
W=Fd
W
F
d
Work
Units
W - work
J
F - force
N
d - distance
m
Work Practice Problems
1.You lift 19.0 kg of books 1.3 m. How much
work did you do?
2.A carpenter lifts a 80 kg beam 1.4 m and
carries it 9 m. How much work is done
during the lift? How much work is done on
the beam during the carry?
3.If 19.6 J are used to move 16 N object,
how far was the object moved?
4.If 1040 J are used to move a 76 N object,
how far was it moved?
5.If 763 J are used to move an object 20 m,
how much force was used?
6.If 70043 J are needed to move an object 73
m, how much force was needed?