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Agenda. Friday&Monday – Problems Ch. 21-23 Tuesday – lab 5 & “Curve” Quiz Can Improve score by 5-20 pts Or replace quiz 2 (not a popular quiz) Today Potential & Potential Energy Chapters 6&7. Potential Energy. Measure of Energy “Stored” in a system Akin to work.

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Agenda
Agenda

  • Friday&Monday – Problems Ch. 21-23

  • Tuesday – lab 5 & “Curve” Quiz

    • Can Improve score by 5-20 pts

    • Or replace quiz 2 (not a popular quiz)

  • Today

    • Potential & Potential Energy

    • Chapters 6&7


Potential energy
Potential Energy

  • Measure of Energy “Stored” in a system

  • Akin to work


Gravitational pe i know u
Gravitational PE [I know, U…]

  • How much work does it take to raise a mass M to a height H in a gravitational field g?

Negative work done by gravity

Implies gravitational energy stored

Work done by something else (Outside)


Conservative forces pe
Conservative Forces & PE

  • Energy from Conservative forces can be described in terms of PE

  • Spring PE (E stored by spring)

  • Gravitational PE (E stored by gravity)

  • Electrical PE (E stored in Electric Fields)

  • Conservative = Path Independent

  • Conservative = No energy lost

  • Conservative N.E. to friction


Mechanical energy conservation
“Mechanical” Energy Conservation

  • Have

    • Q = DU + W

    • Heat, internal energy, Mechanical

  • Most large systems, Heat irrelevant

    • Thermal energy of a golf ball? Small!

  • Need to look closer at macroscopic here

  • WNC = DE = EF – E0


Mechanical energy conservation e f e 0 w nc
“Mechanical” Energy ConservationEF = E0 + WNC

  • WNC = DE = EF – E0

  • WNC Work done by Non-Conserved

    • “Outside” or Friction, etc….

  • E = Energy = PE + KE

    • See how thermal might come in?

  • Wonder of Energy

    • No Directions

    • If no WNC, then no cares about path!

    • Can often ignore everything but initial & final


Relativity
Relativity

  • No – not the extra cool one

  • Energy is relative

  • Can you tell what floor I’m on when I drop something?

  • Gravitational PE comes into play as relative height change, not absolute height.


Potential vs fields
Potential vs. Fields

  • Energy ~ Integral of Force

  • Field from a point

C = constant (k, G)

S = stuff (Q, M0

r = distance from object emanating field


Potential vs fields1
Potential vs. Fields

  • Energy ~ Integral of Force

  • Field from a point

C = constant (k, G)

S = stuff (Q, M0

r = distance from object emanating field

P  Potential

Could be gravitational Potential

Could be electrical potential (Volts)


Examine gravity ch 8
Examine GravityCh. 8?

  • How fast must something be traveling to escape the pull of the Earth’s gravitational field?

  • Needed

    • Gravitational Potential Field

    • Energy Relationship

    • Beginning “height”

    • Final height”


Examine gravity ch 81
Examine GravityCh. 8?

  • How fast must something be traveling to escape the pull of the Earth’s gravitational field?

  • Needed

    • Gravitational PE = PEG = -GmME/r

    • Energy Relationship  EF = E0 + WNC

      • Given an initial velocity, no other “NC”  WNC=0

    • Beginning “height” RE (~ Surface of Earth)

    • Final height”  Far Away (infinity)


Find escape velocity e f e 0 w nc
Find Escape VelocityEF = E0 + WNC

  • Initial Energy

    • KE = 0.5mv2

    • PE = -GmME/RE

    • Negative implies object attracted to earth

    • As r increases, PE becomes less negative

    • As r increases, h increases, PE increases (mgh)

  • WNC = 0

    • Only force is gravity

  • Final Energy

    • PE = ?

    • PE = 0 [no earth pull]


Find escape velocity e f e 0 w nc1
Find Escape VelocityEF = E0 + WNC

  • Initial Energy

    • KE = 0.5mv2

    • PE = -GmME/RE

  • WNC = 0

    • Only force is gravity

  • Final Energy

    • PE = 0 [no earth pull]

    • KE=?

    • KE = 0 [minimum initial energy to escape earth]


Find escape velocity e f e 0 w nc2
Find Escape VelocityEF = E0 + WNC

  • EF = 0, WNC =0

  • E0 = 0

  • E0 = PE0 + KE0

  • E0 = -GmME/RE + 0.5mv2 = 0

  • v2 = -2GME/RE

  • What does escape velocity depend on?

  • How does this relate to electricity?

  • V = kQ/r & PEE = kQ1Q2/r

  • Same method, gravity easier as no + or -


Reference for pe
Reference for PE

  • When dealing with “points,” what is a good reference for energy?

  • Hint: Earth (from outside) looks like point source (G Law)



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