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Homework Help: Meteor Entering Orbit of a Sun (cons. energy & momentum) [very hard algebra]

  1. Nov 16, 2008 #1
    1. The problem statement, all variables and given/known data
    Alright, so I've given this guy a few cracks, and I think I'm close to it, but apparently I'm not right, so here's how it goes. Simple in concept.

    A meteor moves toward the solar system with speed v0 in a direction such that it would miss the sun by a distance d if it were not attracted by the sun's gravitational force. Denote the mass of the sun by M. Find the distance b of the meteor from the sun at the point of closest approach in terms of v0, d, and M (and G, the gravitational constant).

    http://img515.imageshack.us/img515/8376/lg0704figgx8.jpg [Broken]
    http://g.imageshack.us/img515/lg0704figgx8.jpg/1/ [Broken]

    2. Relevant equations
    They effectively give me a few formulas in the helper questions provided:

    The momentum and energy are conserved
    Initial energy = 1/2mv0^2
    Final energy = 1/2mV^2 - GMm/b

    Initial Momentum = mv0d
    Final Momentum = mVb

    Where V=the final velocity of the object at point b

    3. The attempt at a solution

    I feel like the error here is most likely in my algebra, as I'm having a ton of troble easily isolating b. But here are the ways I've tried so far:

    I always start with the equations:

    1/2mv0^2 = 1/2mV^2 - GMm/b


    mv0d = mVb.

    I then cancel out all the m's and multiply out the 2 in the top one to get:

    v0^2 = V^2 - 2GM/b


    v0d = Vb

    From this point, I'll either solve for b in the top or solve for V in the bottom. If I solve for b in the top, I get:

    V^2 - v0^2 = 2GM/b, b = 2GM/(V^2 - v0^2)

    Then I would solve for V in the bottom one:

    V = v0d/b

    Subbing that in gets

    b = 2GM/((v0d/b)^2 - v0^2)

    And this is where I think I go wrong...trying to get b out of this. I've tried rearranging the equation to bring all the b's to one side, I've tried resubstituting stuff...most recently I tried quad formula:

    I factored out the v0's on the bottom:

    2GM/((v0^2)(d^2/b^2-1)) = b

    b(d^2/b^2 - 1) = 2GM/v0^2

    d^2/b - b = 2GM/v0^2

    d^2 - b^2 = 2GMb/v0^2

    b^2 + 2GMb/(v0^2) - d^2 = 0

    then, using 1 as (a), 2GM/v0^2 as (b) and -d^2 as (c), I did quad form and got:



    http://img395.imageshack.us/img395/482/e5a7daf972eacf000cabb54ld4.gif [Broken]
    http://g.imageshack.us/img395/e5a7daf972eacf000cabb54ld4.gif/1/ [Broken]

    But that was not correct

    I'm officially stumped by this point...I've taken 4 cracks at it, to no avail. Am I setting up the formulas incorrectly, or am I just screwing up on the algebra over and over?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 16, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You are really close.

    What is "-4ac", when c is -d2? (And a=1 of course)
  4. Nov 16, 2008 #3
    haha...wow. I see it...it just should be + 4d^2. Just put that in and it worked. Thanks a ton...Almost sure I wouldn't have picked that out without doing the whole thing over again.

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