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Physics of AstronomyTuesday, winter week 7 (21 Feb.06)
Math-A: Giancoli Ch.7&8 – Work and Energy
Discuss rotational dynamics lab
Math-B: Giancoli 11, Raff Ch.11.2.1-3: Intro to QM
HW setup
Dr. Neal Hurlburt visits Wednesday – lunch discussion 1:00 here
Looking ahead: quiz this Thus.
Relationship between force and work/energy
Examples of energy …
Work done = force . displacement in the same direction
Dot product: product of PARALLEL components of vectors
Do Ch.7 (p.171) Forces & Work #4, 6, 7, Dot products # 21, 25
xx x x
dv dxW m dx m dv mv dv
dt dt F dx
7-3: Work done by a varying force
Example: Spring obeys Hooke’s law: F = -kx
Do Ch.7 (p.172) F(x) #31-34, 39
( )W kx dx F dx
7-4: Work & Energy
Loss of potential energy→work done→increase in Kinetic energy
- U → W → + K
Ex: falling mass in last week’s experiment
Ch.7 (p.173) # 41, 46, 49, 52, (p.175) #65,
Energies vs time
-1.00E-020.00E+001.00E-022.00E-023.00E-024.00E-025.00E-026.00E-027.00E-028.00E-02
0 5 10 15 20 25 30 35
Time (s)
En
erg
y(J)
U=mgh
Kt=1/2 mv̂ 2
Kdisk
Ktop
Kpulley
Ktot
Etot
Ch.8: Energy conservation
Conservative force:
• Work done doesn’t depend on path taken (curl x F = 0)
• Net work done around a closed path = 0
• potential energy U depends only on x, and Fx= -dU/dx
• Etot = K + U = constant (conservation of mechanical energy)
• Gravity and Felec are conservative; Friction and Fmag are not
Ch.8 (p.200) #7,9, 10, 16, 17, (p.202) 32, 36(a), 41, 42-47, 73, 82, 89, 93
Gravitational potential energy and force
Near earth far from Earth
Force F
Potential energy U
Ch.8-8,9: Energy diagrams and Power
Power = rate of change of Energy
P = dE/dt
Minimum energy = stable state (F=0)
Ch.8 (Power, 203) #57, 59, 62, 65, 67, (Diag) 68-71, 94-97
Phys.B: Raff 11.2.1-3: Introduction to Quantum Mechanics – Blackbody radiation
• Blackbodies were carefully studied in labs in late 1800s• Rayleigh-Jeans theory explained long-wavelength tail:
~ 1/T (Wien’s law)• Ultraviolet catastrophe at short !
Planck’s quantization of photon energy
• Planck’s phenomenological relation fit, but why?
• Three weeks later, Planck’s revolutionary explanation
Probs. 11.7 and 11.8: Long and short- limits of blackbody radiation.
Blackbody radiation: notation
Q = energy (joules) energy density u = Q/volume
Q = dQ/d (J/m) energy density /wavelength
u(T, ) = Q /volume
Power = dQ/dt (J/s) emissive power = dP/d / area
Luminosity = Power/area
Prob. 11.9: Find energy maximum where du/d = 0
Photons as particles: Photoelectric effect
Photons can knock electrons out of metal, if they can overcome the binding energy to the metal, or work function W.
Ephoton = KEelectron + binding energy: h = KEmax + W
Brighter light yields more electrons.Shorter wavelength light yields more energetic electrons.Even “weak” light beam of single photons can release e.
Atomic models