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WORK

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**THE USE OF FORCE TO MOVE AN OBJECT SOME DISTANCE**

DEFINITION OF WORK THE USE OF FORCE TO MOVE AN OBJECT SOME DISTANCE

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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.

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**FORCE (N) X DISTANCE (m)**

FORMULA FOR WORK WORK = FORCE (N) X DISTANCE (m)

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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.

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**SAMPLE WORK PROBLEM W= F X D W = 900 N X 100 m**

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

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**W = F x D FORCE (20N) X DISTANCE (30m)**

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 = F x D FORCE (20N) X DISTANCE (30m) 20N X 30m = 600J

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**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

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ENERGY

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**THE ABILITY TO DO WORK OR CAUSE A CHANGE**

ENERGY THE ABILITY TO DO WORK OR CAUSE A CHANGE

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ENERGY & WORK IF WORK IS THE USE OF FORCE TO MOVE AN OBJECT SOME DISTANCE, THEN WORK IS ALSO THE TRANSFER OF ENERGY

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**IF AN OBJECT DOES WORK, IT USES ENERGY.**

SO… BECAUSE WORK & ENERGY ARE DIRECTLY RELATED, BOTH ARE MEASURED IN JOULES

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KINETIC ENERGY POTENTIAL ENERGY THE ENERGY OF MOTION STORED ENERGY

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**GRAVITATIONAL POTENTIAL ENERGY**

POTENTIAL ENERGY THAT’S DEPENDENT ON HEIGHT

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**GRAVITATIONAL POTENTIAL ENERGY (G. P. E**

GRAVITATIONAL POTENTIAL ENERGY (G.P.E.) = m•g•h MASS X ACCELERATION DUE TO GRAVITY (9.8m/s2) X HEIGHT ABOVE THE GROUND

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**G.P.E. CALCULATION G.P.E. = m•g•h 5kg X 9.8m/s2 X 1.5m**

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

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**ELASTIC POTENTIAL ENERGY**

THE ENERGY ASSOCIATED WITH OBJECTS THAT CAN BE STRETCHED OR COMPRESSED

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POTENTIAL ENERGY

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KINETIC ENERGY = MASS X VELOCITY2 2 OR… ½ MV2

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**IF YOU INCREASE MASS OR VELOCITY, YOU INCREASE KINETIC ENERGY**

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**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

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MECHANICAL ENERGY THE ENERGY POSESSED BY AN OBJECT DUE TO ITS MOTION OR POSITION OR… AN OBJECT’S COMBINED POTENTIAL AND KINETIC ENERGY

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**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.

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**MECHANICAL ENERGY = POTENTIAL ENERGY + KINETIC ENERGY**

OR… ME = PE + KE

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**THE LAW OF CONSERVATION OF ENERGY**

ENERGY CAN BE NEITHER CREATED NOR DESTROYED, IT CAN ONLY BE CHANGED FROM ONE FORM TO ANOTHER

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**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

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**CHANGES IN THE FORMS OF ENERGY**

ENERGY CONVERSION CHANGES IN THE FORMS OF ENERGY ONE OF THE MOST COMMON IS CHANGING FROM: KINETIC TO POTENTIAL OR POTENTIAL TO KINETIC

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**Potential Energy and Conservation of Energy **

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Kinetic vs. Potential Image source:

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**ALL FORMS OF ENERGY CAN BE CONVERTED TO OTHER FORMS:**

OTHER CONVERSIONS ALL FORMS OF ENERGY CAN BE CONVERTED TO OTHER FORMS: EXAMPLES:

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LOSING ENERGY IN MOST ENERGY TRANSFORMATIONS, SOME OF THE ENERGY IS TRANSFORMED INTO HEAT DUE TO FRICTION (INCLUDING AIR RESISTANCE)

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**G.P.E. = MGH ENERGY THE ENERGY OF MOTION**

STORED ENERGY: THE ENERGY OF SHAPE OR POSITION THE ENERGY OF MOTION IF YOU INCREASE MASS OR VELOCITY, YOU INCREASE KINETIC ENERGY ELASTIC = ENERGY ASSOCIATED W/OBJECTS THAT CAN BE STRETCHED OR COMPRESSED GRAVITATIONAL = ENERGY THAT’S DEPENDENT ON HEIGHT ENERGY KINETIC = ROCK ROLLING DOWN A HILL POTENTIAL = ROCK SITTING ON TOP OF THE HILL K.E. = ½ MV2 G.P.E. = MGH

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OTHER FORMS OF ENERGY THERMAL CHEMICAL NUCLEAR ELECTROMAGNETIC

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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)

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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.

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**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)

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**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.

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**MORE ENERGY CONVERSIONS**

IT IS POSSIBLE TO TRANSFER, OR CONVERT ONE FORM OF ENERGY TO OTHER FORMS: EXAMPLES: RUBBING HANDS TOGETHER

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POWER

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**THE RATE AT WHICH YOU DO WORK**

DEFINITION OF POWER THE RATE AT WHICH YOU DO WORK

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**FORCE X DISTANCE TIMEXX**

FORMULA FOR POWER WORK TIME FORCE X DISTANCE TIMEXX

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**WATT 1 WATT IS EQUAL TO 1 JOULE PER SECOND**

SI UNIT OF POWER WATT 1 WATT IS EQUAL TO 1 JOULE PER SECOND 1 w = 1 J/s

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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

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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

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**AN AIRPORT CONVEYOR BELT DOES 1200J OF WORK TO MOVE A SUITCASE IN 20S**

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

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**P = FORCE X DISTANCE / TIME**

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

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**CALCULATING POWER FROM ENERGY**

YOU CAN MEASURE ENERGY USED BY DIVIDING THE ENERGY BY TIME POWER = ENERGY TIME

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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

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**INCREASING SPEED REQUIRES MORE ENERGY**

HOW ENERGY RELATES TO: SPEED INCREASING SPEED REQUIRES MORE ENERGY

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**AN OBJECT THAT HAS MOMENTUM HAS KINETIC ENERGY**

HOW ENERGY RELATES TO: MOMENTUM AN OBJECT THAT HAS MOMENTUM HAS KINETIC ENERGY

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**FORCE HOW ENERGY RELATES TO:**

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

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**POWER HOW ENERGY RELATES TO:**

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.

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**WORK HOW ENERGY RELATES TO:**

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.

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