Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I dont know how to solve this problem

  1. Apr 16, 2006 #1
    a) The comet released from rest at a distance rmax from the sun ( angular momentum =0 ). How long comet takes to reach the sun ? (take sun`s radius to be zero )
    b) Assuming the comet can somehow pass freely through the sun describe its overal motion and find its period ?

    I cant really think how to solve this problem , what I know it that since angular mom is zero, c=0 ( C= l^2/gama*reduced mass ,Gama=GMsMc)
    so so if c=o c=rmax*(1-e) e=1 therefore Energy E=0 ( accually E=0 doesnt make sence eather since it has a PE = -Gmsmc/rmax ) so that is not a bonded orbit but parabolic orbit, But How can I use these to find the time ? the hint is use the technique t= integral dx/xdot where we find xdot from the Energy ( KE=mxdot/2=E-U(x) ) but I still dont understand how to use this since my understanding ( e=0 so E=0 )

    please help, I would appreciate if anyone help. I still dont feel as if I completeley understood this subject but I have to turn the homework tomorrow :((
    Last edited: Apr 16, 2006
  2. jcsd
  3. Apr 16, 2006 #2


    User Avatar
    Homework Helper

    Well, since the angular momentum is zero, this is just a one-dimensional forced motion problem. You know the force as a function of distance, so set this equal to m d2r/dt2 and solve the resulting ODE.
  4. Apr 16, 2006 #3
    the answer is different than what I find

    thanks for your reply but when I set the equation, the 't' I find is
    t= r max^2/2GMs but the answer is t= (pi/2*root(2GMs))*(r max)^3/2 more like Kepler`s IIIth law. Do you have any suggestion for this one and the the b) ? ?

    thanks again
  5. Apr 17, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper

    How did you arrive at your expression for the time?

    Also, don't you expect the answer to resemble Kepler - since Kepler's Law relate to elliptical orbits and the particular orbit here is also an ellipse in the limit that the semiminor radius goes to zero (degenerate)?
  6. Apr 17, 2006 #5
    I actually expect that but I am confused a little. I understand that since angular mom=0 c=0 so c=rmax(1-e) e=1 a (semimajor)=rmax/2 and c=rmin(1+e) for c to be equal to zero rmin ( semiminor must go to zero. Right ? but But I still can not find the right answer :(( ( t= (pi/2*root(2GMs))*(r max)^3/2 )
  7. Apr 17, 2006 #6


    User Avatar
    Science Advisor
    Gold Member

    The formula [tex]1.1\sqrt{\frac{a^3}{GM}}[/tex] provides an impressively accurate estimation.

    b. The comet would be become trapped in the Sun's Schwartchild radius, as the Sun would be a black hole if it's radius were 0. But since it said "pass freely through", I'm just guessing, but wouldn't it distance itself from the Sun by the height it was dropped, except 180 degrees away, then fall back to the Sun, and continue this indefinately, like a pendulum?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook