Transcript IS IT WORK?

“Work”
Feb. 7 & 10, 2014
Do Now
Take one and complete
Tonight’s Read & Study 8.1;
HW
Complete #1-4 p. 119
“A” DAY STUDENTS: any
work from today’s class that
is not finished becomes HW
HW Due
Ch. 8.1 – 8.6 Vocab – please place in In Box
Go to your test seats…
Open Notes quiz on Work Power and Energy
Use your Movie Notes and your HW from last
night
12 questions – 2 points each
Timed – 10 minutes to complete.
Good Luck!
NO TALKING during the quiz.
When done, back to test seats and finish DO
NOW, and/or start tonight’s HW
Ch. 8 Overview
Earlier in the year, we utilized Newton's laws to
analyze the motion of objects.
• Force and mass information were used to
determine the acceleration of an object.
• Acceleration information was used to
determine information about the velocity or
displacement of an object after a given
period of time.
• In this manner, Newton's laws serve as a useful
model for analyzing motion and making
predictions about the final state of an object's
motion.
Ch. 8 Overview
• In this unit, an entirely different model will be used to
analyze the motion of objects.
• Motion will be approached from the perspective of work
and energy.
• The affect that work has upon the energy of an object
(or system of objects) will be investigated; the resulting
velocity and/or height of the object can then be
predicted from energy information.
• In order to understand this work-energy approach to the
analysis of motion, it is important to first have a solid
understanding of a few basic terms.
• Today’s term is WORK!
IS IT WORK?
A teacher applies a force to a wall and becomes
exhausted.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A book falls off a table and free falls to the ground.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A waiter carries a tray full of meals above his head
by one arm straight across the room at constant
speed.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A rocket accelerates through space.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
Bruno Mars sings during the halftime show
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
DEMO!
I am going to show you a scenario,
explaining when work IS done, and NOT
done.
It is your job to create a working definition
of what WORK is.
*Does anyone wish to share their
definition?
*
Every case in which work is done
involves 3 criteria:
When a force acts upon an object to cause a
displacement of the object, it is said that
work was done upon the object.
There are three key ingredients to work:
1. force,
2. displacement,
3. and cause.
In order for a force to qualify as having done work on an
object, there must be a displacement and the force
must cause the displacement.
When talking about work, you MUST specify the OBJECT on
which work is done!!!!
KEY POINT: **If we do work on an object, we change the
object’s ENERGY.**
Work generally falls into 2 categories….
Work done against another force,
Work done to change the speed of
like….
an object, like….
• Lifting an object against
• Hitting the gas pedal to speed
GRAVITY
• Pushing an object against
FRICTION
• Foiling the other team’s
EFFORTS in a game of tug-ofwar
up a car,
• Or hitting the brakes to slow it
down
NEGATIVE WORK
***On occasion, a force acts upon a moving object to
hinder a displacement.
Examples might include a car skidding to a stop on a
roadway surface or a baseball runner sliding to a stop
on the infield dirt.
In such instances, the force acts in the direction
opposite the objects motion in order to slow it down.
The force doesn't cause the displacement but rather
hinders it.
These situations involve what is commonly
called negative work.
??? Units =
N m
The Unit we use to measure work is not N m, but the
JOULE (J) named after this guy 
James Prescott Joule, English physicist and brewery
owner. He and his brother were FASCINATED with
electricity and used to experiment by giving electrical
shocks to each other, friends, & family. (People were not
as knowledgeable in certain fields then as we are
now….Don’t try this at home !!!!!)
STOP – check your units – do you need any DA?
Work = F d
Work = (60 N) (4m)
Work = 240 J
Work
Fd
"J"
Δ energy
Impulse
Force
Motion
Δ
Ft
"Ns"
Δ momentum
Your Role Now…
In groups, create 1 or 2 mini skits to present to
the class. You all must participate in some way,
whether it is in the skit or as a narrator. Your
skit(s) must demonstrate and narrate the
following:
1. One scenario must show an example in which it
may appear work is being done (to the non
physics student!) but it is not
2. The other scenario must show an example in
which work is being done.
Your Role Now…
EVERYONE must participate in some way!
1.
2.
Looks like work is being done… but is NOT work
Work IS being done
You MUST specify:
the FORCE doing the work,
the DISPLACEMENT caused by the work,
and the OBJECT on which work is being done,
along with any other relevant information, such as…
is the force working against another force? If so, what?
Is the force changing the object’s velocity? How so?
Your choice if you want 2 separate skits, or one continued
scene, to demonstrate #1-2
In Your Notes
Fill in your chart based on what you see!
Math…
1.Complete #1-2, and check in with a
teacher
2.Complete #3 – check in for help if
needed
3.Complete the remainder of the page and
check online by visiting:
http://tinyurl.com/FreeBodyWork14
4.Stopping with 5-10 minutes left for an
exit
Exit…
1.Finish the Do Now (rest of chart & #3)
HW is to…
*
*
*
Read & Study 8.1
#1-4 on p. 119
Finish the notes
IS IT WORK?
A teacher applies a force to a wall and becomes
exhausted.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A teacher applies a force to a wall and becomes
exhausted.
No.
This is not an example of work. The wall is not
displaced. A force must cause a displacement in
order for work to be done.
IS IT WORK?
A book falls off a table and free falls to the ground.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A book falls off a table and free falls to the ground.
Yes.
This is an example of work. There is a force
(gravity) which acts on the book which causes it to
be displaced in a downward direction (i.e., "fall").
IS IT WORK?
A waiter carries a tray full of meals above his head
by one arm straight across the room at constant
speed.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A waiter carries a tray full of meals above his head
by one arm straight across the room at constant
speed.
No.
This is not an example of work. There is a force
(the waiter pushes up on the tray) and there is a
displacement (the tray is moved horizontally across
the room). Yet the force does not cause the
displacement. To cause a displacement, there must
be a component of force in the direction of the
displacement.
IS IT WORK?
A rocket accelerates through space.
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
A rocket accelerates through space.
Yes.
This is an example of work. There is a force (the
expelled gases push on the rocket) which causes
the rocket to be displaced through space.
IS IT WORK?
Bruno Mars sings during the halftime show
Show me your answer (hands up)
1. YES
2. NO
3. NOT QUITE SURE…& that’s OK!
IS IT WORK?
Bruno Mars sings during the halftime show
No.
Nothing mentioned about him MOVING!!!
(Unless you spin it so that his vocal cords are doing
work…. )