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WORK, ENERGY, POWER
Measures of Effort & Motion;
Conservation Laws
https://www.youtube.com/watch?v=k60jGJfV8oU
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WHAT IS WORK?
• Pushing a car?
• Picking up a weight?
• Holding a weight up?
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WHAT IS WORK?
• You do work on an object when
you move it.
• The rate at which you do work is
your power output.
• When you do work on an object,
you transfer energy from one
object to another.
Work Done by a Constant Force
The definition of work, when the force is
parallel to the displacement:
WORK
• work = force  displacement
W=F·d
• Work can be done by you, as
well as on you
• Work is a measure of expended
energy
UNITS OF WORK
Work = F x d = F x cosθ x d
x
x
What are the SI units for Work?
A m/s^2
B kg * m^2 / s^2
C N * m/s^2
UNITS OF WORK
Work = F x d = F x cosθ x d
x
x
What are the SI units for Work?
A m/s^2
B kg * m^2 / s^2
C N * m/s^2
WORK IS MEASURED IN JOULES
• Joules are the unit of energy
• One joule of work is done
when a force of 1 N acts on
a system over a
displacement of 1m
W = F•D
• What is the work if:
• The force on the box is 10 Newtons
• The distance pushed is 5 meters
W = F•D
• What is the work if:
• The force on the box is 10 Newtons
• The distance pushed is 5 meters
• W = Fx x dx
• W = 10 x 5 = 50 J
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HOW MUCH WORK IS DONE?
• A friend’s car is stuck on the
ice. You push down on the
car to provide more friction for
the tires (Ff = FN) allowing the
car’s tires to propel it forward
5 meters onto less slippery
ground. How much work did
you do?
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WORK DONE?
• A friend’s car is stuck on the ice. You
push down on the car to provide more
friction for the tires (Ff = FN) allowing
the car’s tires to propel it forward 5
meters onto less slippery ground. How
much work did you do?
• W = F x cosθ x d
• W = F (cos 90°) d
• W = F (0) d
•W = 0 J
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Negative, Positive, Zero Work
What is the work if:
•The force on the box is 8 Newtons
•The distance pushed is 2 meters
•The angle is 30 degrees
• Only the force in the direction of the
displacement “counts”
• Only a component of the force is doing
work
• W = Fx x dx = F x cosθ x d
WORK = F X COS X D
• W = Fx x dx = F x cosθ x d
• What is the work if: The force on the
box is 8 Newtons; the distance pushed
is 2 meters; The angle is 30 degrees
• W = 8 x cos(30) x 2 = 13.86 J
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https://www.youtube.com/watch?v=azbpj0pwl0k
5/12/2017
WORK
• Machines can make work
easy (ramps, levers)
• Apply less force over larger
distance for same work
WORKING AT AN ADVANTAGE
Work = Force  Distance
Work = Force  Distance
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RAMPS
Larger Force
Short Distance
M
Small Force
Long Distance
Whiteboard Team Challenge

Grab a whiteboard and get ready to play…
“You
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better work!”
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1) To Work or Not to Work
Is it possible to do work on an
1) yes
object that remains at rest?
2) no
1) To Work or Not to Work
Is it possible to do work on an
1) yes
object that remains at rest?
2) no
Work requires that a force acts over a distance.
If an object does not move at all, there is no
displacement, and therefore no work done.
2) Friction and Work I
A box is being pulled
across a rough floor
1) friction does no work at all
at a constant speed.
2) friction does negative work
What can you say
3) friction does positive work
about the work done
by friction?
2) Friction and Work I
A box is being pulled
across a rough floor
1) friction does no work at all
at a constant speed.
2) friction does negative work
What can you say
3) friction does positive work
about the work done
by friction?
Friction acts in the opposite
N displacement
direction to the displacement, so
the work is negative. Or using the
Pull
f
definition of work: W = F d cos 
since  = 180o, then W < 0.
mg
3) Friction and Work II
Can friction ever
do positive work?
1) yes
2) no
3) Friction and Work II
Can friction ever
do positive work?
1) yes
2) no
Consider the case of a box on the back of a pickup truck.
If the box moves along with the truck, then it is actually
the force of friction that is making the box move.
4) Play Ball!
In a baseball game, the
catcher stops a 90-mph
1) catcher has done positive work
pitch. What can you say
2) catcher has done negative work
about the work done by
3) catcher has done zero work
the catcher on the ball?
4) Play Ball!
In a baseball game, the
catcher stops a 90-mph
1) catcher has done positive work
pitch. What can you say
2) catcher has done negative work
about the work done by
3) catcher has done zero work
the catcher on the ball?
The force exerted by the catcher is opposite in direction to the
displacement of the ball, so the work is negative. Or using the
definition of work (W = F d cos  ), since  = 180o, then W < 0.
Note that because the work done on the ball is negative, its
speed decreases.
Follow-up: What about the work done by the ball on the catcher?
5) Tension and Work
A ball tied to a string is
being whirled around in
a circle. What can you
say about the work
done by tension?
1) tension does no work at all
2) tension does negative work
3) tension does positive work
5) Tension and Work
A ball tied to a string is
being whirled around in
a circle. What can you
say about the work
1) tension does no work at all
2) tension does negative work
3) tension does positive work
done by tension?
No work is done because the force
acts in a perpendicular direction to
the displacement. Or using the
definition of work: W = F d cos 
since  = 90o, then W = 0.
T
v
Follow-up: Is there a force in the direction of the velocity?