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WORK
DEFINITION OF WORK
THE USE OF FORCE
TO MOVE AN OBJECT
SOME DISTANCE
WORK IS DONE ONLY WHEN:
• THERE HAS BEEN MOVEMENT
OVER SOME DISTANCE AND
• THE DISTANCE THE OBJECT MOVED
WAS IN THE SAME DIRECTION AS
THE FORCE APPLIED.
FORMULA FOR WORK
WORK =
FORCE (N) X DISTANCE (m)
UNITS FOR WORK
• NEWTON-METER = JOULE
• A FORCE OF 1 NEWTON EXERTED
ON AN OBJECT THAT MOVES
1 METER DOES 1 NEWTON-METER
OR (1 JOULE) OF WORK.
SAMPLE WORK PROBLEM
• A MOUNTAIN CLIMBER EXERTS A FORCE OF
900-N TO SCALE A 100-m CLIFF. HOW MUCH
WORK IS DONE BY THE MOUNTAIN CLIMBER?
W= F X D
W = 900 N X 100 m
90,000 n-m OR JOULES OF WORK
WORK PROBLEMS
• JANICE PULLS A WAGON FOR 30 METERS. IF
SHE USES A FORCE OF 20N TO PULL, HOW
MUCH WORK DOES SHE DO?
W=FxD
FORCE (20N) X DISTANCE (30m)
20N X 30m = 600J
• HOW MUCH WORK IS DONE IN
LIFTING A 12N HAMMER FROM THE
FLOOR TO A HEIGHT OF 2m?
W = 12N X 2m
= 24J of work is done
ENERGY
ENERGY
THE ABILITY
TO DO WORK OR
CAUSE A CHANGE
ENERGY & WORK
IF WORK IS THE USE OF FORCE
TO MOVE AN OBJECT SOME
DISTANCE, THEN WORK IS
ALSO THE TRANSFER OF ENERGY
IF AN OBJECT DOES WORK,
IT USES ENERGY.
SO…
BECAUSE WORK & ENERGY ARE
DIRECTLY RELATED, BOTH ARE
MEASURED IN JOULES
KINETIC ENERGY
POTENTIAL ENERGY
THE ENERGY
STORED
OF MOTION
ENERGY
GRAVITATIONAL POTENTIAL ENERGY
POTENTIAL
ENERGY THAT’S
DEPENDENT ON
HEIGHT
GRAVITATIONAL POTENTIAL ENERGY
(G.P.E.) = m•g•h
MASS X ACCELERATION
DUE TO GRAVITY (9.8m/s2)
X HEIGHT ABOVE THE GROUND
G.P.E. CALCULATION
A BOX WITH A MASS OF 5kg IS SITTING ON A
CABINET THAT IS 1.5m high. What is the G.P.E.?
G.P.E. = m•g•h
5kg X 9.8m/s2 X 1.5m
73.5N·m or 73.5J
ELASTIC POTENTIAL ENERGY
THE ENERGY
ASSOCIATED
WITH OBJECTS
THAT CAN BE
STRETCHED
OR
COMPRESSED
POTENTIAL ENERGY
KINETIC ENERGY =
MASS X
2
VELOCITY
2
OR…
½ MV2
SO…
IF YOU INCREASE
MASS OR VELOCITY,
YOU INCREASE
KINETIC ENERGY
KINETIC ENERGY PROBLEM
• A 50kg ROCK IS TRAVELING 5 METERS
PER SECOND. WHAT IS THE KINETIC
ENERGY OF THE OBJECT?
K.E. = ½ MV2
½ X 50kg X (5m/s)2 =
25kg X 25m2/s2 =
625J
MECHANICAL ENERGY
THE ENERGY POSESSED BY AN OBJECT
DUE TO ITS MOTION OR POSITION
OR…
AN OBJECT’S COMBINED
POTENTIAL AND KINETIC ENERGY
MECHANICAL ENERGY
• THE ENERGY ASSOCIATED WITH MOTION
• SUM OF KINETIC AND POTENTIAL ENERGY
• EXAMPLES: WATER, WIND, MOVING VEHICLE,
WALKING, HITTING A BALL, SOUND, BLOOD
FLOW.
MECHANICAL ENERGY
MECHANICAL ENERGY =
POTENTIAL ENERGY + KINETIC ENERGY
OR…
ME = PE + KE
THE LAW OF CONSERVATION OF ENERGY
ENERGY CAN BE NEITHER
CREATED NOR DESTROYED,
IT CAN ONLY BE CHANGED
FROM ONE FORM TO ANOTHER
MANY TIMES A SERIES OF ENERGY
CONVERSIONS TAKE PLACE IN DOING A
SPECIFIC JOB.
TURN TO PAGE 127 AND DESCRIBE
THE ENERGY CONVERSIONS
TAKING PLACE IN THE PICTURE
ENERGY CONVERSION
CHANGES IN THE FORMS OF ENERGY
ONE OF THE MOST COMMON IS
CHANGING FROM:
KINETIC TO POTENTIAL OR
POTENTIAL TO KINETIC
Potential Energy and Conservation of Energy
Kinetic vs. Potential
Image source: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/energy/ce.html
OTHER CONVERSIONS
ALL FORMS OF ENERGY CAN BE
CONVERTED TO OTHER FORMS:
EXAMPLES:
LOSING ENERGY
IN MOST ENERGY
TRANSFORMATIONS, SOME OF THE
ENERGY IS TRANSFORMED
INTO HEAT DUE TO FRICTION
(INCLUDING AIR RESISTANCE)
THE ENERGY OF
MOTION
IF YOU
INCREASE
MASS OR
VELOCITY, YOU
INCREASE
KINETIC
ENERGY
STORED ENERGY:
THE ENERGY OF
SHAPE OR
POSITION
ENERGY
K.E. = ½ MV2
G.P.E. = MGH
KINETIC = ROCK
ROLLING DOWN
A HILL
POTENTIAL = ROCK
SITTING ON TOP
OF THE HILL
ELASTIC = ENERGY
ASSOCIATED
W/OBJECTS THAT
CAN BE STRETCHED
OR COMPRESSED
GRAVITATIONAL =
ENERGY THAT’S
DEPENDENT ON
HEIGHT
OTHER FORMS OF ENERGY
• THERMAL
• CHEMICAL
• NUCLEAR
• ELECTROMAGNETIC
THERMAL ENERGY
• THE ENERGY AN OBJECT HAS DUE TO THE
MOTION OF ITS MOLECULES
• THE FASTER ATOMS MOVE THE MORE
THERMAL ENERGY THE OBJECT HAS
• OFTEN RESULTS FROM FRICTION
• EXAMPLES: ANY CHANGE IN TEMPERATURE
OR PHASE OF MATTER (I.E. WATER BOILING)
CHEMICAL ENERGY
• THE ENERGY STORED IN CHEMICAL BONDS THAT
HOLD CHEMICAL COMPOUNDS TOGETHER
• USUALLY, WHEN BONDS ARE BROKEN, THEN
ENERGY IS RELEASED OR ABSORBED
• EXAMPLES: BURNING FUEL (COAL OR WOOD),
FOOD, FIREWORKS.
NUCLEAR ENERGY
• POTENTIAL ENERGY STORED IN THE
NUCLEUS OF AN ATOM.
• HEAT & LIGHT ENERGY ARE RELEASED WHEN
NUCLEI FUSE (FUSION) OR WHEN THE
NUCLEUS OF AN ATOM SPLITS (FISSION).
• EX: SUN (FUSION), NUCLEAR POWER (FISSION)
ELECTROMAGNETIC ENERGY
• THE ENERGY ASSOCIATED WITH ELECTRICAL
AND MAGNETIC INTERACTIONS
• ELECTRICAL ENERGY: POWER LINES w/ELECTRICTY,
HOME WIRING.
• RADIANT ENERGY: ENERGY CARRIED BY LIGHT,
INFRARED WAVES, X-RAYS.
MORE ENERGY CONVERSIONS
IT IS POSSIBLE TO TRANSFER,
OR CONVERT ONE FORM OF
ENERGY TO OTHER FORMS:
EXAMPLES: RUBBING HANDS TOGETHER
POWER
DEFINITION OF POWER
THE RATE AT WHICH
YOU DO WORK
FORMULA FOR POWER
WORK
TIME
FORCE X DISTANCE
TIMEXX
SI UNIT OF POWER
WATT
1 WATT IS EQUAL TO
1 JOULE PER SECOND
1 w = 1 J/s
SAMPLE POWER PROBLEMS
• A SMALL MOTOR DOES 4000 J OF
WORK IN 20 SECONDS. WHAT’S THE
POWER OF THE MOTOR IN WATTS?
P = W/t or F•d/t
4000J / 20s
= 200w OF POWER
POWER PROBLEMS
• AN ELECTRICAL CHARGER USES 144J IN
30S TO CHARGE A MOBILE PHONE.
HOW MUCH POWER DID THE CHARGER
USE?
P = W/t
144J / 30s
4.8 w OF POWER
• AN AIRPORT CONVEYOR BELT DOES 1200J
OF WORK TO MOVE A SUITCASE IN 20S.
WHAT’S THE POWER OF THE CONVEYOR
BELT?
P=W/t
1200J / 20s
60 w OF POWER
• A MACHINE MOVES AN OBJECT WITH A
FORCE OF 50N A DISTANCE OF 28 M IN 70
S. HOW MUCH POWER DID IT TAKE?
P = FORCE X DISTANCE / TIME
50N X 28m = 1400N-m / 70s
= 20w OF POWER USED
CALCULATING POWER FROM ENERGY
YOU CAN MEASURE ENERGY USED BY
DIVIDING THE ENERGY BY TIME
POWER =
ENERGY
TIME
SAMPLE POWER PROBLEMS
• A LIGHT BULB USED 600 J OF ENERGY
IN 6 SECONDS, WHAT IS THE POWER
OF THE LIGHT BULB?
P = E/t
600J / 6s
= 100 J/s OR WATTS OF POWER
HOW ENERGY RELATES TO:
SPEED
INCREASING SPEED
REQUIRES MORE
ENERGY
HOW ENERGY RELATES TO:
MOMENTUM
AN OBJECT THAT
HAS MOMENTUM
HAS KINETIC ENERGY
HOW ENERGY RELATES TO:
FORCE
A FORCE IS REQUIRED TO CHANGE THE
MOTION OF AN OBJECT. THIS IS
WORK, & IF IT’S DOING WORK IT
CHANGES THE ENERGY OF THE OBJECT
HOW ENERGY RELATES TO:
POWER
SINCE POWER IS THE RATE AT
WHICH WORK IS DONE, IT MUST
ALSO BE THE RATE AT WHICH
ENERGY IS CONSUMED BECAUSE
IT TAKES ENERGY TO DO WORK.
HOW ENERGY RELATES TO:
WORK
WORK DONE ON A MACHINE MEANS THAT
ENERGY GOES INTO THE MACHINE.
BECAUSE ENERGY IS CONSERVED,
WORK IS CONSERVED. FRICTION ENERGY IS
NOT LOST BUT CONVERTED TO HEAT ENERGY.