Transcript Calculating Gravitational Potential Energy

Work and Energy
Vocabulary
 Work: The use of force to move an object over a
distance (Joule or J)
 Joule: The unit used to measure work; one joule of
work is done when a force of one newton moves an
object one meter (Joule = Newton ● meter)
 Power: the rate at which work is done (Watts)
 Watt: the unit used to measure power; one watt is equal
to one joule of work done in one second
 Horsepower: the unit of measurement for engines and
motors; one horsepower equals 745 watts.
WORK
 Force is necessary to do work!
 Work is done only when an object that is being
pushed or pulled actually moves.
 The work done by a force is related to the size of
the force and the distance over which the force is
applied.
 Work= (Force) ● (distance) W= Fd
Units: Work (Joules), Force (Newtons),
Distance (Meters)
Calculating Work (W=Fd)
 How much work is done if a person lifts a
barbell weighing 500 N to a height of 2
meters?
I
F
S
M
U
Calculating Work
 I: Force= 500 N, distance= 2 m, Work=?
 F: Work= Force●distance (W=Fd)
 S: W= 500 N ● 2 m
 M: 1,000
 U: 1,000 J
 (remember the units for work are joules, units for
force are Newtons, and units for distance are
meters)
Work
 Objects that are moving can
do work
 Examples:
 Bowling balls do work on the
pins they hit in a bowling alley
 Baseball bat does work on the
ball
 Gravity does work on falling
water
Energy is transferred when
work is done
 Energy is the ability to do work or cause
change
 When you do work on an object, some of your
energy is transferred to the object.
 Think of work as a transfer of energy
 Work and energy are both measured in the
same unit, the joule
Calculating Gravitational
Potential Energy
 Potential energy (stored energy) that is
caused by gravity (downward force) is
gravitational potential energy. (GPE)
 Gravitational potential energy=
mass ● gravitational acceleration ● height
(GPE = mgh)
Calculating Gravitational
Potential Energy (GPE=mgh)
 What is the gravitational potential energy of a girl who has a
mass of 40 kg and is standing on the edge of a diving board
that is 5 meters above the water?
 * remember gravitational acceleration is a constant (does
not change) so…
 g= 9.8 m/s2
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I
F
S
M
U
Calculating Gravitational
Potential Energy
 I: mass= 40 kg, g= 9.8 m/s2 , height =5 m, GPE=?
 F: GPE = mgh
 S: GPE= 40 kg ● 9.8 m/s2 ● 5 m
 M: 1960 (kg ● m/s2 ) ● m
 U: 1960 Joules
 Remember the unit (Joules = N ● m)
Calculating Kinetic Energy
KE= mv2
2
 Kinetic energy is the energy of motion
 Kinetic energy= mass ● velocity2
2
Calculating Kinetic Energy
KE= mv2
2
 What is the kinetic energy of a girl who
has a mass of 40 kg and a velocity of
3 m/s
I
F
S
M
U
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Calculating Kinetic Energy
KE= mv2
2
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I: mass= 40 kg, velocity = 3 m/s , KE=?
F: KE= mv2
2
S: KE= (40kg ● (3m/s )2
2
■ M: (40kg ● 9 m2/s2 )
2
360
2
180 ( kg ● m2/s2 )
(kg ● m/s2 ) ● m
N●m
180 N ● m
■U:
180 J
(Remember N ● m = J)
Calculating Mechanical
Energy ( ME= PE + KE)
 Mechanical energy is the object’s
combined potential and kinetic energy
 Mechanical energy= potential energy +
kinetic energy
 ME = PE + KE
Calculating Mechanical
Energy ( ME= PE + KE)
 The potential energy of a girl on a swing just
before she is released is 180 J and her kinetic
energy after she has been released is 300 J.
What is her mechanical energy (total energy) ?
 I
 F
 S
 M
 U
Calculating Mechanical
Energy ( ME= PE + KE)
 I: PE= 180 J, KE= 300 J, ME=?
 F: ME= PE + KE
 S: ME= 180 J + 300 J
 M: 480
 U: 480 J
Law of conservation of
energy
 Energy can not be created or destroyed it
is just transferred or transformed
 No matter how much energy is transferred
all of the energy is still present somewhere
in one form or another
 Mechanical energy is the combination of
both kinetic and potential energy.
Power
 Power is the rate at
which work is done
 Power = Work
Time
 The unit of
measurement for
power is watt (W)
P= W/t
Calculating Power from
work ( P= W/t)
 An Antarctic explorer uses 6000 J of
work to pull his sled for 60 Seconds.
What power does he need?
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M
U
Calculating Power from
work ( P= W/t)
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I:
F:
S:
M:
U:
Work= 6000 J, time= 60 s , P=?
P= W/t
P= 6000J/ 60 s
P=100 J/s ( J/s =Watts)
P=100 W
Power can be calculated
from energy (P=E/t)
 Power = Energy/time
 A light bulb used 600 J of energy in 6 s.
What is the power of the light bulb?
I
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M
U
Power can be calculated
from energy (P=E/t)
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I: Energy= 600 J, Time= 6 s, P=?
F: P=E/t
S: P= 600 J/6 s
M: P= 100 J/s
U: P= 100 Watts