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Homework Help: Velocity of an apple before strikes the surface of a white dwarf.

  1. Apr 1, 2012 #1
    1. The problem statement, all variables and given/known data

    An apple is dropped from a height of 12.8*10^6 m above the surface of the white dwarf. With what speed does the apple strike the surface of the white dwarf?

    M of white dwarf= 1.99*10^30
    gravity of white dwarf= 3.29*10^6

    2. Relevant equations


    3. The attempt at a solution

    I tried doing this v=srqt( 2(3.29*10^6)(12.8*10^6 ) and my result was far of the real answer i got 9.17*10^6 m/s and the answer sheet showed 5.28 *10^6 m/s.

    What did I do wrong?
  2. jcsd
  3. Apr 1, 2012 #2


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    Hi ScienceGeek24 :smile:

    (try using the X2 button just above the Reply box :wink:)
    that's only for constant acceleration

    a white dwarf is only about the size of the Earth, so 107 m is a long way up

    you'll need the radius …

    isn't there a relation between the radius and mass of a white dwarf? :smile:
  4. Apr 1, 2012 #3
    yes a=Gm/r^2 that's how i got the acceleration of the white dwarf

    The radius is equal of the earth's radius which is 6.37*10^2 and the mass was the same the sun which was 1.99*10^30. However, I still don't understand your question.

  5. Apr 1, 2012 #4


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    You'll need to find the gravitational acceleration as a function of distance, and integrate (or use potential energy). :wink:
  6. Apr 1, 2012 #5


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    tim's suggestion referred to super-scripts, not the whole post, so your post would look like:
    BTW: Earth's radius is significantly greater than 6.37*102m !
  7. Apr 1, 2012 #6
    The thing is that i don't think this problem should be base with integrals, it has to be an easier way, sqrt(3.27*106)(12.8*106+6.37*106) and still i don't get the right answer.

    I mean there has to be another factor that I am not taking into account. I know this problem does not need integrals.
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