Transcript Slide 1

Lecture # 3
Cassandra Paul
Physics
Summer Session II 2008
Today
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Work
Mechanical Energy Systems
Force
Graphing
Heat and Work
Let’s Quickly review heat so we can differentiate it
from work…
Ice-cube
00 C
Water
00 C
An ice-cube sits in a bath of water. Water and ice can
exchange heat with each other but not with the
environment. What is the direction of heat transfer?
A) From ice-cube to water
B) From water to icecube
C) Impossible to tell
D) Neither of above
No Heat Transfer!
Ice
Water
Temperature (K)
Temperature (K)
gas
gas
liquid
liquid
solid
solid
Energy added (J)
Energy added (J)
Heat Transfer
Heat Transfer can only happen if there is a
Temperature difference.
Heat
is a transfer of energy (a process)
that takes place from a hot object to a cold one
because the objects are at different temperatures.
Low temp
High temp
Energy leaves hot objects in the form of heat
Energy enters cold objects in the form of heat
Work
Work is done whenever a force is exerted.
Work
is a transfer of energy (process) that takes place
from a physical system to another physical system due to an
interaction that involves “Force”.
Baseball
KE
Work
Speed
The pitcher’s hand “pushed” the baseball.
The pitcher’s hand exerted force on the baseball.
As a result, the baseball started moving (its KE increased).
What are some examples of when
work is done?
Pushing
Lifting
Even Falling?
Work changes Mechanical Energies
• Energy specifically due to motion of ‘everyday
things.’
• Kinetic Energy (Translational)
• Gravitational Potential Energy
• Spring Potential Energy
Sweet! New bubbles to put in my energy interaction diagrams!!!!
KEtrans
Speed
PEgravity
Height
PEspring
Displacement
X
from Equilibrium
Work is done when there is Force
To be more precise, we need the concept of
“Force” : “Push” or “Pull”
An overall push (or pull!) in the direction the object is travelling
has the effect of speeding it up.
Consider a block being pushed by you on a level surface with no
friction:
Block is already moving, you
push in same
direction:
direction of Force
direction of
travel
KE
Work
Speed
W=ΔKE=Fd
Consider a block being pushed by you on a level surface with no
friction:
Block is already moving, you
push in same
direction:
direction of Force
direction of
travel
KE
Work
Speed
W=ΔKE=Fd
What does this d mean?
A)The distance the block travels
B)The distance the force is exerted over
Force
Properties of forces
Force is a vector quantity
i.e. Forces have both magnitude and direction
Force is the agent of interaction of TWO objects
e.g. The pitcher’s hand and the baseball
The two forces involved in an interaction are opposite and
equal
(Newton’s Third Law)
Fhand on the baseball = - Fbaseball on the hand
More on this in 7B!
Force
Properties of forces
Force is a vector quantity
i.e. Forces have both magnitude and direction
Force is the agent of interaction of TWO objects
e.g. The pitcher’s hand and the baseball
The two forces involved in an interaction are opposite and
equal
(Newton’s Third Law)
Contact force vs non contact force
Fgravitational
Gravity is a force, therefore you can
model a ball falling as an open system!
Ignoring Friction, find the final
speed of a ball just before it hits
the floor after it falls from a height
of 10 meters to the floor.
Ignoring Friction, find the amount
of work done by gravity on the ball
as it falls from a height of 10
meters to the floor.
Work
KEtrans
Speed
PEgravity
Height
KEtrans
Speed
+
ΔKE +ΔPE= 0
7A way convention…
+
ΔKE = W
…but nothing wrong with this way too!
Work can enter or leave a system
Example: A book is initially at rest, you slide the
book across the table to your friend. It stops
right in front of your friend.
System: Book
Initial: Book is at rest (right before push)
Final: Book is at highest speed
(right after push)
System: Book
Initial: Book is at highest speed (right after
push)
Final: Book is at rest (book has stopped)
Work
Work
done by
hand
KEtrans
Speed
+
+
ΔKE = W
Work
Work
done by
friction
KEtrans
Speed
-
-
ΔKE = W
Work
Example: A pitcher throws a 0.3kg baseball 44m/s
(100mph) how much energy is transferred from the
pitcher’s hand in the form of work?
System: Baseball
Initial: Ball at rest in pitcher’s hand
Final: Ball just leaves the pitcher’s hand
KE
Work
Speed
∆KE = Work
KEfinal - KEinitial =1/2(m)(vf2) – 0 = W
(0.5)(0.3kg)(44m/s)2 = 290.4 J
Intro to Graphing PE and KE
Tuesday you will be doing some graphing, let’s
practice.
Diving: Potential Energy
From board
2m
From floor
or
5m
0m
or
3m
-2m
or
1m
-3m
or
0m
At highest point, Tricia Woo is 2 meters above the board and 5 meters above the
water, how should we calculate her PE? Where should we measure the height from?
System: Diver
Initial: Highest point
Final: Just before
hitting water
We want to make sure to calculate the
correct final velocity for the diver, where
should we set the height equal to zero?
A) 0m at top
KEtrans
Speed
PEgravity
Height
B) 0m at board
ΔKE +ΔPE= 0
C) 0m at water
D) It doesn’t matter
E) Need more information
How can it not Matter!?
System: Diver
Initial: Highest point
Final: Just before
hitting water
KEtrans
Speed
+
From board
2m
PEgravity
From floor
or
5m
0m
or
3m
-2m
or
1m
Height
-
ΔKE +ΔPE= 0
-3m
or
½ m(vf2-vi2) + mg(hf-hi)= 0
(0 - 5)
(0.5)(50kg)(vf2-0) + (50kg)(10m/s2)(hf-hi)= 0
Δh is the same! Δh=-5 so vf = 10m/s(-3 - 2)
0m
Instantaneous PE and KE
ΔKE +ΔPE= 0
(KEf – KEi) + (PEf- PEi) = 0
KEf + PEf - KEi - PEi = 0
KEf + PEf = KEi + PEi = Etot
The sum total of all of the energies at one
point in time is equal to the total energy of
the system. In a closed system that value
is constant throughout the process.
KEanytime + PEanytime = Etot
Equations to memorize and more
importantly know how to use this week
mgΔh = ΔPEgrav
½ mΔ(v2)= ½ m (vf2-vi2) = ΔKEtrans
½ k(Δxf2-Δxi2) = ΔPEspring
DL sections
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Swapno:
11:00AM Everson Section 1
Amandeep: 11:00AM Roesller Section 2
Yi:
1:40PM Everson Section 3
Chun-Yen: 1:40PM Roesller Section 4