lecture # 3 cassandra paul physics summer session ii 2008
TRANSCRIPT
Heat and WorkLet’s Quickly review heat so we can differentiate it
from work…
00 C00 C
Ice-cube
Water
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 ice-cube C) Impossible to tell D) Neither of above
No Heat Transfer!
Temperature (K)
Energy added (J)
solid
liquid
gas
Temperature (K)
Energy added (J)
solid
liquid
gas
Ice Water
Heat Transfer
Heat Transfer can only happen if there is a Temperature difference.
Low temp High temp
HeatHeat 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.
Energy leaves hot objects in the form of heat Energy enters cold objects in the form of heat
WorkWork is done whenever a force is exerted.
KEKESpeedSpeed
Baseball
Work
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).
WorkWork is a transfer of energy (process) that takes place from a physical system to another physical system due to an
interaction that involves “Force”.
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!!!!
PEgravity PEspring
KEtrans
SpeedHeight Displacement from EquilibriumX
Work is done when there is ForceWork is done when there is ForceTo 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.
Block is already moving, you push in same direction:
direction of travel
direction of Force
KEKESpeedSpeed
Work
Consider a block being pushed by you on a level surface with no friction:
W=ΔKE=Fd
Block is already moving, you push in same direction:
direction of travel
direction of Force
KEKESpeedSpeed
Work
Consider a block being pushed by you on a level surface with no friction:
W=ΔKE=Fd
What does this d mean? A)The distance the block travelsB)The distance the force is exerted over
ForceForceProperties of forcesProperties of forces
Force is a vector quantityi.e. Forces have both magnitude and directionForce 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
ForceForceProperties of forcesProperties of forces
Force is a vector quantityi.e. Forces have both magnitude and directionForce 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 amount of work done by gravity on the ball as it falls from a height of 10 meters to the floor.
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.
KEtrans
Speed
Work
+ +
ΔKE = W
KEtrans
SpeedPEgravity
Height
ΔKE +ΔPE= 07A way convention… …but nothing wrong with this way too!
Work can enter or leave a systemExample: A book is initially at rest, you slide the
book across the table to your friend. It stops right in front of your friend.
KEtrans
SpeedKEtrans
Speed
System: BookInitial: Book is at rest (right before push)Final: Book is at highest speed (right after push)
System: BookInitial: Book is at highest speed (right after push)Final: Book is at rest (book has stopped)
Work Work
ΔKE = WΔKE = W
Work done by hand
Work done by friction
+ + - -
WorkWorkExample: 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?
KEKESpeedSpeed
System: Baseball
Initial: Ball at rest in pitcher’s hand
Final: Ball just leaves the pitcher’s hand
Work
∆KE = Work
KEfinal - KEinitial =1/2(m)(vf2) – 0 = W
(0.5)(0.3kg)(44m/s)2 = 290.4 J
Diving: Potential Energy
0m or 3m
2m or 5m
-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?
From board From floor
KEtrans
SpeedPEgravity
Height
ΔKE +ΔPE= 0
System: DiverInitial: Highest pointFinal: 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
B) 0m at board
C) 0m at water
D) It doesn’t matter
E) Need more information
How can it not Matter!?
KEtrans
SpeedPEgravity
Height
ΔKE +ΔPE= 0
System: DiverInitial: Highest pointFinal: Just before hitting water
½ m(vf2-vi
2) + mg(hf-hi)= 0
(0.5)(50kg)(vf2-0) + (50kg)(10m/s2)(hf-hi)= 0
0m or 3m
2m or 5 m
-2m or 1m
-3m or 0m
From board From floor
(0 - 5)
(-3 - 2)Δh is the same! Δh=-5 so vf = 10m/s
+ -
Instantaneous PE and KEΔKE +ΔPE= 0
(KEf – KEi) + (PEf- PEi) = 0
KEf + PEf - KEi - PEi = 0
KEf + PEf = KEi + PEi = Etot
KEanytime + PEanytime = Etot
The sum total of all of the energies at one point in time is equal to the total energy ofthe system. In a closed system that value is constant throughout the process.
Equations to memorize and more importantly know how to use this week
½ mΔ(v2)= ½ m (vf2-vi
2) = ΔKEtrans mgΔh = ΔPEgrav
½ k(Δxf2-Δxi
2) = ΔPEspring