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 = F x D

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

THE ENERGY

OF MOTION

POTENTIAL ENERGY

STORED

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.?

5kg X 9.8m/s2 X 1.5m

73.5N·m or 73.5J

G.P.E. = m•g•h

ELASTIC POTENTIAL ENERGY

THE ENERGY

ASSOCIATED

WITH OBJECTS

THAT CAN BE

STRETCHED

OR

COMPRESSED

POTENTIAL ENERGY

KINETIC ENERGY =

MASS X VELOCITY2

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

                                                                                         

Image source: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/energy/ce.html

Kinetic vs. Potential

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)

ENERGY

THE ENERGY OF MOTION

STORED ENERGY: THE ENERGY OF

SHAPE OR POSITION

ELASTIC = ENERGY ASSOCIATED

W/OBJECTS THAT CAN BE STRETCHED

OR COMPRESSED

GRAVITATIONAL = ENERGY THAT’S DEPENDENT ON

HEIGHT

IF YOU INCREASE MASS OR

VELOCITY, YOU INCREASE KINETIC ENERGY

K.E. = ½ MV2

G.P.E. = MGH

KINETIC = ROCK ROLLING DOWN A HILL

POTENTIAL = ROCK SITTING ON TOP OF THE HILL

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

POWER =

ENERGYTIME

YOU CAN MEASURE ENERGY USED BY

DIVIDING THE ENERGY BY 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:

SPEEDINCREASING SPEED

REQUIRES MORE

ENERGY

HOW ENERGY RELATES TO:

MOMENTUMAN OBJECT THAT

HAS MOMENTUM

HAS KINETIC ENERGY

HOW ENERGY RELATES TO:

FORCEA 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:

POWERSINCE 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:

WORKWORK 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.