Lecture # 3

Cassandra PaulPhysics

Summer Session II 2008

Today

• Work• Mechanical Energy Systems• Force• Graphing

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”.

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!!!!

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

More on this in 7B!

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

Intro to Graphing PE and KE

Tuesday you will be doing some graphing, let’s practice.

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

DL sections

• 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