Download What is work?

STARTER
• WHAT ARE THE SIX SIMPLE MACHINES?
• WHY DO WE USE SIMPLE MACHINES?
• WHAT IS THE EFFORT FORCE AND RESISTANCE FORCE?
• WHAT IS MECHANICAL ADVANTAGE?
WHAT IS WORK?
• IN SCIENCE, THE WORD WORK HAS A DIFFERENT MEANING
THAN YOU MAY BE FAMILIAR WITH.
• THE SCIENTIFIC DEFINITION OF WORK IS: USING A FORCE TO
MOVE AN OBJECT A DISTANCE (WHEN BOTH THE FORCE AND
THE MOTION OF THE OBJECT ARE IN THE SAME DIRECTION.)
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WORK OR NOT?
• ACCORDING TO THE SCIENTIFIC
DEFINITION, WHAT IS WORK AND
WHAT IS NOT?
• A TEACHER LECTURING TO HER CLASS
• A MOUSE PUSHING A PIECE OF CHEESE
WITH ITS NOSE ACROSS THE FLOOR
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WORK AND POWER READING PASSAGE
• DIRECTIONS: READ THE PASSAGE ABOUT WORK AND POWER. AFTER READING THE PASSAGE DRAW THE
FOLLOWING TABLE. (NOT THE INFORMATION IN THE BOTTOM BLOCKS. THIS IS ONLY AN EXAMPLE)
Vocabulary Word and
Definition
Speed
The distance and
object travel in a
given time.
Formula, Triangle, Unit, Facts
Speed=distance/time
S=d/t
Unit=m/s
•How fast you move
•How slow you move
•Moving Car
EXPLAIN: WORK
The force is being
applied from the left.
The force causes the
object to move a
distance. Work is being
conducted.
EXPLAIN: WORK WORD PROBLEM
A MAN PUSHES A BOX WITH 100 NEWTONS
OF FORCE. THE BOX MOVES 5 METERS.
WHAT IS THE WORK?
F=100N
D=5M
W=?
W=F(D)
W=100N (5M)
W=500 N.M
IS WORK PRESENT?
IS WORK PRESENT?
IS WORK PRESENT?
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WHAT’S WORK?
A SCIENTIST DELIVERS A SPEECH TO AN AUDIENCE OF HIS PEERS.
A BODY BUILDER LIFTS 350 POUNDS ABOVE HIS HEAD.
A MOTHER CARRIES HER BABY FROM ROOM TO ROOM.
A FATHER PUSHES A BABY IN A CARRIAGE.
A WOMAN CARRIES A 20 KG GROCERY BAG TO HER CAR?
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WHAT’S WORK?
A SCIENTIST DELIVERS A SPEECH TO AN AUDIENCE OF HIS PEERS. NO
A BODY BUILDER LIFTS 350 POUNDS ABOVE HIS HEAD. YES
A MOTHER CARRIES HER BABY FROM ROOM TO ROOM. NO
A FATHER PUSHES A BABY IN A CARRIAGE. YES
A WOMAN CARRIES A 20 KM GROCERY BAG TO HER CAR? NO
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FORMULA FOR WORK
WORK = FORCE X DISTANCE
• THE UNIT OF FORCE IS NEWTONS
• THE UNIT OF DISTANCE IS METERS
• THE UNIT OF WORK IS NEWTON-METERS
• ONE NEWTON-METER IS EQUAL TO ONE JOULE
• SO, THE UNIT OF WORK IS A JOULE
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FORMULA FOR WORK
Variable/Measurement
Units
LEFT SIDE OF FORMULA SHEET
Work
N*m=Joule
• WORK
• DEFINITION: USING A FORCE TO MOVE AN
Force
kg*m/s=Newton
distance
meter
OBJECT A DISTANCE (WHEN BOTH THE
FORCE AND THE MOTION OF THE OBJECT
ARE IN THE SAME DIRECTION.)
• FORMUALA: W=F*D
• TRIANGLE
WORK INPUT AND OUTPUT
• WORK INPUT IS THE AMOUNT OF WORK DONE ON A MACHINE.
•INPUT FORCE X INPUT DISTANCE
• WORK OUTPUT IS THE AMOUNT OF WORK DONE BY A MACHINE.
•OUTPUT FORCE X OUTPUT DISTANCE
Wout = Win
Fout x Dout = Fin x Din
10N x 3m = 2N x 15m
Dout
Din 15 m
3m
10 N
Fin
EFFICIENCY
WE SAID THAT THE INPUT FORCE TIMES THE DISTANCE EQUALS THE
OUTPUT FORCE TIMES DISTANCE, OR:
INPUT FORCE X DISTANCE = OUTPUT FORCE X DISTANCE
HOWEVER, SOME OUTPUT FORCE IS LOST DUE TO FRICTION.
THE COMPARISON OF WORK INPUT TO WORK OUTPUT IS CALLED
EFFICIENCY.
NO MACHINE HAS 100 PERCENT EFFICIENCY DUE TO FRICTION.

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POWER
POWER IS THE RATE AT WHICH WORK IS DONE.
POWER = WORK*/TIME
*(FORCE X DISTANCE)
THE UNIT OF POWER IS THE WATT.
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POWER
HORSEPOWER
MORE OR LESS AMOUNT OF WORK
1. YOU USE MORE FORCE TO MOVE AN OBJECT.
2. YOU LIFT A 20N OBJECT FASTER.
3. YOU RAISE AN OBJECT A SHORTER HEIGHT.
4. YOU MOVE A LIGHTER OBJECT.
5. YOU MOVE AN OBJECT FARTHER.
IS THE PERSON DOING WORK?
1. WHEN PUSHING A 1000N CAR 20 METERS?
2. WHEN LIFTING A ROCK OFF THE GROUND?
3. WHEN HOLDING A BOOK IN THEIR HANDS?
4. WHEN PUSHING HARD AGAINST A BRICK WALL?
5. WHEN WALKING UP THE STAIRS?
POWER
LEFT SIDE
• POWER
• DEFINITION: THE RATE AT WHICH
WORK IS DONE
• FORMULA: WORK/TIME
• TRIANGLE
Variable/Measurement
Units
Power
J/s=Watt
Work
N*m=Joule
Time
seconds
CHECK FOR UNDERSTANDING
• TWO PHYSICS STUDENTS, BEN AND BONNIE, ARE IN THE WEIGHTLIFTING ROOM.
BONNIE LIFTS THE 50 KG BARBELL OVER HER HEAD (APPROXIMATELY .60 M) 10
TIMES IN ONE MINUTE; BEN LIFTS THE 50 KG BARBELL THE SAME DISTANCE OVER
HIS HEAD 10 TIMES IN 10 SECONDS.
• WHICH STUDENT DOES THE MOST WORK?
• WHICH STUDENT DELIVERS THE MOST POWER?
• EXPLAIN YOUR ANSWERS.
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BEN AND BONNIE DO THE
SAME AMOUNT OF WORK; THEY
APPLY THE SAME FORCE TO LIFT
THE SAME BARBELL THE SAME
DISTANCE ABOVE THEIR HEADS.
YET, BEN IS THE MOST
POWERFUL SINCE HE DOES THE
SAME WORK IN LESS TIME.
POWER AND TIME ARE
INVERSELY PROPORTIONAL.
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PRACTICE QUESTIONS
1. EXPLAIN WHO IS DOING MORE WORK AND WHY: A BRICKLAYER CARRYING
BRICKS AND PLACING THEM ON THE WALL OF A BUILDING BEING CONSTRUCTED,
OR A PROJECT SUPERVISOR OBSERVING AND RECORDING THE PROGRESS OF
THE WORKERS FROM AN OBSERVATION BOOTH.
2. HOW MUCH WORK IS DONE IN PUSHING AN OBJECT 7.0 M ACROSS A FLOOR
WITH A FORCE OF 50 N AND THEN PUSHING IT BACK TO ITS ORIGINAL
POSITION? HOW MUCH POWER IS USED IF THIS WORK IS DONE IN 20 SEC?
3. USING A SINGLE FIXED PULLEY, HOW HEAVY A LOAD COULD YOU LIFT?
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PRACTICE QUESTIONS
4. GIVE AN EXAMPLE OF A MACHINE IN WHICH FRICTION IS BOTH AN ADVANTAGE
AND A DISADVANTAGE.
5. WHY IS IT NOT POSSIBLE TO HAVE A MACHINE WITH 100% EFFICIENCY?
6. WHAT IS EFFORT FORCE? WHAT IS WORK INPUT? EXPLAIN THE RELATIONSHIP
BETWEEN EFFORT FORCE, EFFORT DISTANCE, AND WORK INPUT.
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PRACTICE QUESTIONS
1. EXPLAIN WHO IS DOING MORE WORK AND WHY: A BRICKLAYER CARRYING BRICKS AND
PLACING THEM ON THE WALL OF A BUILDING BEING CONSTRUCTED, OR A PROJECT
SUPERVISOR OBSERVING AND RECORDING THE PROGRESS OF THE WORKERS FROM AN
OBSERVATION BOOTH. WORK IS DEFINED AS A FORCE APPLIED TO AN OBJECT, MOVING THAT
OBJECT A DISTANCE IN THE DIRECTION OF THE APPLIED FORCE. THE BRICKLAYER IS DOING
MORE WORK.
2. HOW MUCH WORK IS DONE IN PUSHING AN OBJECT 7.0 M ACROSS A FLOOR WITH A FORCE
OF 50 N AND THEN PUSHING IT BACK TO ITS ORIGINAL POSITION? HOW MUCH POWER IS
USED IF THIS WORK IS DONE IN 20 SEC? WORK = 7 M X 50 N X 2 = 700 N-M OR J; POWER =
700 N-M/20 SEC = 35 W
3. USING A SINGLE FIXED PULLEY, HOW HEAVY A LOAD COULD YOU LIFT?SINCE A FIXED PULLEY
HAS A MECHANICAL ADVANTAGE OF ONE, IT WILL ONLY CHANGE THE DIRECTION OF THE
FORCE APPLIED TO IT. YOU WOULD BE ABLE TO LIFT A LOAD EQUAL TO YOUR OWN WEIGHT,
MINUS THE NEGATIVE EFFECTS OF FRICTION.
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PRACTICE QUESTIONS
4. GIVE AN EXAMPLE OF A MACHINE IN WHICH FRICTION IS BOTH AN ADVANTAGE AND A
DISADVANTAGE. ONE ANSWER MIGHT BE THE USE OF A CAR JACK. ADVANTAGE OF FRICTION:
IT ALLOWS A CAR TO BE RAISED TO A DESIRED HEIGHT WITHOUT SLIPPING. DISADVANTAGE
OF FRICTION: IT REDUCES EFFICIENCY.
5. WHY IS IT NOT POSSIBLE TO HAVE A MACHINE WITH 100% EFFICIENCY? FRICTION LOWERS
THE EFFICIENCY OF A MACHINE. WORK OUTPUT IS ALWAYS LESS THAN WORK INPUT, SO AN
ACTUAL MACHINE CANNOT BE 100% EFFICIENT.
6. WHAT IS EFFORT FORCE? WHAT IS WORK INPUT? EXPLAIN THE RELATIONSHIP BETWEEN
EFFORT FORCE, EFFORT DISTANCE, AND WORK INPUT. THE EFFORT FORCE IS THE FORCE
APPLIED TO A MACHINE. WORK INPUT IS THE WORK DONE ON A MACHINE. THE WORK INPUT
OF A MACHINE IS EQUAL TO THE EFFORT FORCE TIMES THE DISTANCE OVER WHICH THE
EFFORT FORCE IS EXERTED.
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KWFL: WORK AND POWER
1.
2.
3.
4.
YOU MOVE A 20 NEWTON OBJECT 5 METERS. HOW MUCH WORK DID YOU DO?
YOU CARRY A 20 NEWTON BAG OF DOG FOOD UP A 6 METER FLIGHT OF STAIRS. HOW MUCH WORK WAS
DONE?
YOU DO 15 JOULE OF WORK IN 3 SECONDS. HOW MUCH POWER DO YOU USE?
YOU DO 25 JOULE OF WORK IN 5 SECONDS. HOW MUCH POWER DO YOU USE?
EXIT SLIP: WORK
DRAW A REAL LIFE EXAMPLE OF WORK. DRAW AND LABEL ARROWS
TO SHOW THE FORCE AND DISTANCE.