Physics of astronomy week 4 winter 2004 astrophysics ch 2
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Physics of Astronomy, week 4, winter 2004 Astrophysics Ch.2. Star Date Ch.2.1: Ellipses (Matt #2.1, Zita #2.2) Ch.2.2: Shell Theorem Ch.2.3: Angular momentum (J+J, #2.7) #2.11: Halley’s comet Learning plan for week 5. Ch.2.1: Ellipses (Matt #2.1, Zita #2.2).

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Physics of astronomy week 4 winter 2004 astrophysics ch 2
Physics of Astronomy, week 4, winter 2004Astrophysics Ch.2

Star Date

Ch.2.1: Ellipses (Matt #2.1, Zita #2.2)

Ch.2.2: Shell Theorem

Ch.2.3: Angular momentum (J+J, #2.7)

#2.11: Halley’s comet

Learning plan for week 5


Ch 2 1 ellipses matt 2 1 zita 2 2
Ch.2.1: Ellipses (Matt #2.1, Zita #2.2)

Make an ellipse: length of string between two foci is always r’ + r = 2a.

Eccentricity e = fraction of a from center to focus.


2 1 derive the equation for an ellipse
#2.1: Derive the equation for an ellipse.

Distance from each focus to any point P on ellipse:

r2=y2+(x-ae)2 r’2=y2+(x+ae)2

Combine with r+r’=2a and b2 = a2(1-e2) to get


2 2 find the area of an ellipse
#2.2: Find the area of an ellipse.

so y goes between

and x goes from (-a to +a)

Area =


Ch 2 2 shell theorem p 36 38
Ch.2.2: Shell Theorem (p.36-38)

The force exerted by a spherically symmetric shell acts as if its mass were located entirely at its center.

The force exerted by the ring of mass dMring on the point mass m is

Where s cosf = r - R cos q and s2 = (r - R cos q )2 + (R sin q )2 and

dMring = r(R) dVring and

dVring = 2 p R sinq R dq dR


Substitute this into dF and integrate

Change the variable to u = s2 = r 2 + R 2 - 2rR cos q. Solve for

cos q =

sin q =

Substitute these in and integrate over du to get


Density = mass of shell / volume of shell

r(R) = dMshell / dVshell

So dMshell = r(R) dVshell = 4 p R2r(R) dR

Which is the integrand of

So the force on m due to a spherically symmetric mass shell of dMshell:

The shell acts gravitationally as if its mass were located entirely at its center.. Finally, integrating over the mass shells, we find that the force exerted on m by an extended, spherically symmetric mass distribution is

F = GmM/r2



Center of mass reference frame
Center of Mass reference frame

Total mass = M = m1+ m2

Reduced mass = m

Total angular momentum L=m r v = m rp vp


Virial theorem
Virial Theorem

<E> = <U>/2

where <f> = average value of f over one period

Example: For gravitationally bound systems in equilibrium, the total energy is always one-half of the potential energy.


Learning plan for week 5 hw due mon 9 feb
Learning Plan for week 5 (HW due Mon.9.Feb):

Mon.2.Feb: Introduction to Astrophysics Ch.3

Universe Ch.5.1-3, #6, 11 (Jared + Tristen)

Universe Ch.5.4-5, #25 (Brian + Jenni)

Universe Ch.5.6-8, #27, 29 (Erin + Joey)

Universe Ch.5.9, #34, 36 (Matt + Chelsea)

Universe Ch.19.1, #25 (Annie + Mary)

Tues.3.Feb: HW due on Physics Ch.6

Universe Ch.19.2-3, #34, 35 (Jared + Tristen)

Universe Ch.19.4-5, Spectra -> T,Z, #43 (Erin + Joey)

Universe Ch.19.6, L(R,T), #46, 50 (Annie + Mary)

Universe Ch.19.7,8, HR, #52 (Brian + Jenni)

Thus.5.Feb: HW due on Astrophysics (CO) Ch.2

CO 3.1, Parallax, #3.1 (Jared + Tristen)

CO 3.2, Magnitude, #3.8 (a-d) (Erin + Joey)

CO 3.3 Wave nature of light, #3.6 (Matt + Chelsea)

CO 3.4, Radiation, #3.8 (e-g) (Brian + Jenni)

CO 3.6, Color index, #3.13 (Annie + Mary)


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