Work & Power
Vocabulary
Term
Definition
Work
Power
Joules
Watts
Horsepower
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Work
What is it?
(definition)
Important Idea
To do work, force must cause
displacement because a vertical
force can never cause a horizontal
displacement and a horizontal
force can never cause a vertical
displacement.
For work to be
done you need:
Measurement
Work
Force
Displacement
Symbol
Units
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List/sketch three situations where work is done:
List/sketch three situations where no work is done:
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Class Work
1. For the following situations, determine whether work was done. Place a check
mark in either the “work done” or “no work done” column.
Work Done
No Work Done
a. An ice skater pushes herself
off a wall and glides for two
meters across the ice.
b. The ice skater’s partner lifts
her up a distance of 1 meter.
c. The ice skater’s partner
carries her across the ice a
distance of 3 meters.
d. After setting her down, the
ice skater’s partner pulls her
across the ice a distance of 10
meters.
e. After skating practice, the ice
skater lifts her 20-newton gym
bag up 0.5 meter.
2. How much work is done on a 10-newton block that is lifted 5 meters off the
ground by a pulley?
Looking For
Given
Relationship
Solution
3. A woman lifts her 100-newton child up one meter and carries her for a distance of
to the child’s bedroom. How much work does the woman do when she lifts the
child up? How much work does she do while carrying her to the bedroom?
Looking For
Given
Relationship
Solution
4. You pull your sled through the snow a distance of 500 meters with a horizontal
force of 200 newtons. How much work did you do?
Looking For
Given
Relationship
Solution
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5. Because the snow suddenly gets too slushy, you decide to carry your 100-newton
sled the rest of the way home. How much work do you do when you pick up the
sled, lifting it 0.5 meter upward?
Looking For
Given
Relationship
Solution
How much work do you do to carry the sled if your house is 800 meters away?
Looking For
Given
Relationship
Solution
6. An ant sits on the back of a mouse. The mouse carries the ant across the floor for
a distance of 10 meters. Was there work done by the mouse? Explain.
7. You did 150 joules of work lifting a 120-newton backpack. How high did you lift
the backpack?
Looking For
Given
Relationship
Solution
8. A bulldozer does 30,000 joules of work to push another boulder a distance of 20
meters. How much force is applied to push the boulder?
Looking For
Given
Relationship
Solution
9. You lift a 45-newton bag of mulch 1.2 meters and carry it a distance of 10 meters
to the garden. How much work was done when carrying the bag?
Looking For
Given
Relationship
Solution
10. A 450-newton gymnast jumps upward a distance of 0.50 meters to reach the
uneven parallel bars. How much work did she do before she even began her
routine?
Looking For
Given
Relationship
Solution
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Group Work
1. Calculate the work done if you exert a force of 250 N and move a crate 4 m.
250 N
Looking For
Given
Relationship
Solution
2. Calculate the work done when Dylan applies a force of 100-N to the wall. (Hint:
the wall does not move.)
Looking For
Given
Relationship
Solution
3. Calculate the work done when a 10-N forces is applied to push a block across a
friction free surface for a displacement of 5.0 m to the right.
Looking For
Given
Relationship
Solution
4. Which requires more work – lifting a 500-N sack a vertical distance of 2 m or
lifting a 250-N sack a vertical distance of 4 m?
Looking For
Given
Relationship
Solution
Looking For
Given
Relationship
Solution
5. Calculate the work done when Jaclyn applies a 40-N force to pick up her Coach
bag a distance of 2 meters from the floor.
Looking For
Given
Relationship
Solution
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HomeWork
1. It took a 500.0-newton ballerina a force of 250 joules to lift herself upward
through the air. How high did she jump? (Hint: Solve for displacement)
Looking For
Given
Relationship
Solution
2. A people-moving conveyor-belt moves a 600-newton person a distance of 100
meters through the airport.
a. How much work was done?
Looking For
Given
Relationship
Solution
b. The same 600-newton person lifts his 100-newton carry-on bag upward a
distance of 1 meter. They travel another 10 meters by riding on the “people
mover.” How much work was done in this situation?
Looking For
Given
Relationship
Solution
3. Which person did the most work? Circle your answer:
a. John walks 1,000 meters to the store. He buys 4.448 newtons of candy and
then carries it to his friend’s house which is 500 meters away.
b. Sally lifts her 22-newton cat a distance of 0.5 meter.
c. Henry carries groceries from a car to his house. Each bag of groceries weighs
40 newtons. He has ten bags. He lifts each bag up one meter to carry it and
then walks 10 meters from his car to his house.
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Power
What is it?
(definition)
Important Idea
A car engine is an example of a machine which is given a
power rating. The power rating relates to how rapidly the car
can accelerate the car. Suppose that a 40-horsepower engine
could accelerate the car from 0 mi/hr to 60 mi/hr in 16
seconds. If this were the case, then a car with four times the
horsepower could do the same amount of work in one-fourth
the time. That is, a 160-horsepower engine could accelerate
the same car from 0 mi/hr to 60 mi/hr in 4 seconds. The point
is that for the same amount of work, power and time are
inversely proportional. The power equation suggests that a
more powerful engine can do the same amount of work in
less time.
Measurement
Power
Work
Time
Symbol
The horsepower is occasionally
used to describe the power
delivered by a machine. One
horsepower is equivalent to
approximately 750 Watts.
Units
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W
P
t
Class Work
1. A 500-N hiker is traveling up a trail. After 1800 s the hiker is 400 meters higher than his
starting point.
a. How much total work is done?
Looking For
Given
Relationship
Solution
Relationship
Solution
b. How much power does the hiker have?
Looking For
Given
2. How many watts of power are expended when 18 joules of work are done in a time
interval of 2 seconds?
Looking For
Given
Relationship
Solution
1. Find the power expended in 30-s by a woman lifting up a 50 N block as she slowly raises
it 0.80 m in the vertical direction.
Looking For
Given
Relationship
Solution
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Group Work
1. If Jaclyn does 72-Joules of work to lift Alex in 5 seconds, what is the power being used?
Looking For
Given
Relationship
Solution
2. Jenn does 1500 Joules of work to pull an object 30 meters in 4 seconds. How much
power does Jenn generate?
Looking For
Given
Relationship
Solution
3. An elevator motor does 600 Joules of work in 4 seconds. How much power does the
motor have?
Looking For
Given
Relationship
Solution
4. A 6000 Watt engine is used to move a motorcycle. If the engine is used for 30 seconds.
How much work is done?
Looking For
Given
Relationship
Solution
5. A weightlifter does 200 Joules of work on a bench press. How much power does he
demonstrate if he lifts up the barbell in 4 seconds?
Looking For
Given
Relationship
Solution
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HomeWork
1. Marissa does 3.2 J of work to lower the window shade in her bedroom a distance
of 0.8 m. (a) How much force must Marissa exert on the window shade?
Looking For
Given
Relationship
Solution
(b) If it takes Marissa 2.5 seconds to lower the shade how much power does she
generate?
Looking For
Given
Relationship
Solution
2. Katie, a 300-N child, climbs a tree to rescue her cat who is afraid to jump 8.0 m to
the ground.
(a) How much work does Katie do in order to reach the cat?
Looking For
Given
Relationship
Solution
(b) How much power does she generate if it takes Katie 10 seconds?
Looking For
Given
Relationship
Solution
3. On his way off to play college football, Dan drags his suitcase 15.0-m from the
door of his house to the car at a constant speed with a horizontal force of 95.0 N.
How much work does Dan do to overcome the force of friction?
Looking For
Given
Relationship
Solution
4. Hercules lifts a 2000 N barbell 2.00 m off the ground in 1.00 s. Atlas lifts the
same 2000 N barbell off the ground in 3.00 s. Which weightlifter does more
work? Calculate which man is more powerful?
Looking For
Given
Relationship
Solution
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Lab: Computing Personal Power
Introduction:
Power is the rate at which work is done and the basic unit of work is the watt. One
watt is the performance of 1 Joule of work per second. The unit of power in our system,
one horsepower, is equivalent to 746 watts. In this lab, you will investigate how much
power human beings can put out when going up a hill or stairs. You will time people as
they do work to run up a flight of stairs and then calculate their power. Power equals the
work done by the runner divided by the time it takes to gain the potential energy (P =
W/t). You will draw two graphs and investigate the reasons for the shape of the graphs.
Research Question:
Do you think that you will do more walk walking up a flight of stairs or running up the
same stairs?
Procedure:
1.
2.
3.
4.
5.
Gathering Data
You will be working in groups of two.
Find your weight, in pounds, using the scale at the front of the
room. Weight in lbs. = __________ lbs.
Convert your weight into Newtons by multiplying by 4.42 N/lb.
____________ x 4.42 N/lb = __________ Newtons
Measure the height of a single step. Count the number of steps
and multiply the two to find the total height of the steps you will
climb.
(Height of a single step) x (# of steps) = ______________
Time yourself three times each walking and hurrying up the stairs.
Weight
Pounds
Newtons
Time (s)
Trial
Walking
Hurrying
1
2
3
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Average
Calculations
1. Calculate the work done when climbing the stairs slow and steady.
Work = Force x Distance
2. Calculate the work done when climbing the stairs rapidly.
3. Compute the power, in watts, that you generated in walking up the stairs.
Pay attention to units! Power = Work/Time
Work Done
Trial
Walking
Joules
Hurrying
Joules
1
2
3
Average
Power
Trial
Walking
Watts (J/s)
Hurrying
Watts (J/s)
1
2
3
Average
4. Compute the power, in horsepower, that you generated in walking up the
stairs.
(1 horsepower =746 Watts).
Power
Trial
Walking
Walking
Watts (J/s)
Horsepower
1
2
3
Average
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Analysis Questions
1. Did you do more work walking or hurrying up the flight of stairs? Why?
2. Did you generate more power walking or hurrying up the flight of stairs? Why?
3. A big person and a small person run up the stairs in the same time. Which person
has a larger force acting upon them? Which of them does the most work? Which
of them develops the most power? Explain.
4. What three things can be done to increase the power you develop while climbing
the flight of stairs?
5. Compare and contrast your data with those of other groups in your class.
Collaborate with each group and get each person’s weight, time and power they
generated while hurrying up the flight of stairs. According to the data you filled
out why were the fastest climbers not necessarily the ones who developed the
most power hurrying up the flight of stairs?
Weight (Newtons)
Time (seconds)
Power (Watts)
6. If you were designing a stair-climbing machine for the local health club, what
information would you need to collect? You decide that you will design a stairclimbing machine with the ability to calculate the power developed. What
information would you have the machine collect in order to let the climber know
how much power he or she developed?
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Rubric – Personal Power
General Physics
Research Question
(1 pt.)
Hypothesis
(2 points)
Sketch & Description
(2 pts.)
Data Tables
(5 pts.)
Analysis
(15 pts.)
Total =
Student states the research question
of the lab as a statement.
Student states hypothesis and
explains their prediction.
Student sketches materials used in
lab and a brief description of what
they did.
Student creates data tables using
Excel or similar program.
Student answers analysis questions
accurately and in complete sentences.
/25 points
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The Distance of a Hamburger
Purpose: In this activity you will be investigating how much chemical energy is in the
food that you eat.
Part I: Let’s Do Lunch
1)
2)
3)
4)
Use the menu to select what you would like to eat for either breakfast or lunch.
Money is no object.
Select as many portions that you can realistically eat.
List the items in the table below. (You do not have to fill in every space).
Part II: How High Would 1 Calorie Lift you?
1) To compute the force that one-calorie of energy would have to exert in order to
lift you, your weight in pounds must be converted into Newtons. This is
accomplished by multiplying your weight in pounds using the conversion factor
4.45N/pound.
Your weight (F g ) in Newtons = Your weight (F g ) in Pounds x 4.45N/pounds
=(
pounds) (4.45 N/pound)
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2) One Calorie can perform 4186 J of work. Since Work = F.d, the distance you will
be moved is going to be the work done by one Calorie (4186 J) divided by your
weight (F g ) in Newtons. W=Fxd therefore d=W/F. How high will one calorie lift
you?
Distance lifted = 4186 J________ = _4186 (N-m)___ = _______________ =
Your weight (N)
Your weight (N)
Part III: How Many Calories Would You Consume?
1) Copy down your meal items into the table below.
2) Take the second menu sheet and record the number of calories for each item you
chose.
3) Add the total number of calories that you would have consumed.
Menu Item
Calories
Total Calories
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Part IV: How Much Energy Did You Consume?
1) Multiply the number of Calories by 4186 J/Cal. This tells you the total amount
of energy that you consumed convert Calories into Joules by
Energy = Total Calories in your meal x 4186 J/Cal =
____________x 4186 J/Cal = ___________J
2) Calculate the distance that the calories in your meal would lift you.
Distance lifted =
? J of energy__ =
Your weight (N)
______ = _______________ =
Your weight (N)
3) There are 1609 meters in a mile. What distance would your lunch lift you?
Distance lifted (miles) = distance (m) = ___________________________ =
1609 m/mile
Part V: How Fast Did You Eat Your Meal?
1) Approximate how much time it would take you to eat your meal?
Time Taken = (
minutes) x _60 seconds = __________seconds
minute
2) Calculate how much power you generate while you eat your meal.
Power = Your Weight (N) x Distance Lifted (m) = _______________ =
Time (s)
3) A typical automobile engine produces 25,000 Watts of power while cruising.
How does this compare to the power you would generate while eating your
McDonald’s meal?
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