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Velocity Problem of neutron star

  1. Apr 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A neutron star has a mass of 2.0 x 1030 and a radius of 5.0 x 103. Suppose an object falls from rest near the surface of the star. How fast would it be moving after it had fallen a distance of 0.010 m? (Assume that the gravitational force is constant over the distance of the fall, and that the star is not rotating.)

    2. Relevant equations

    (a) F=(G*m1*m2)/r2
    (b) F=ma
    (c) V2=Vo2 + 2ax

    3. The attempt at a solution

    Using equation (a), I found the force to be about 1.068 x 1043 N

    Using equation (b), I found the acceleration to be 5.338 x 1012

    Using equation (c), I found the velocity to be 231042.68 m/s after the object had fallen a distance of 0.01 m.
  2. jcsd
  3. Apr 13, 2008 #2


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    What's the question? b) looks fine. I don't see how you got a) since you didn't give a mass for the object. c) doesn't look correct numerically.
  4. Apr 13, 2008 #3
    Yeah, (c) didn't look right.

    For (a), I squared the mass because I only had one mass given, and that was 2 x 1030. Then I divided it by the distance squared, which I used 5 x 103 for, and multiplied it by G.

    F = (G*M2)/r2
  5. Apr 13, 2008 #4


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    If you did that then the force in a) is the force between two neutron stars with centers 5000m apart. For c), I don't mean that it's REALLY wrong, it just looks like somebody forgot a 2.
  6. Apr 13, 2008 #5


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    Yeah you don't want to be doing any mass squaring on this one. What happens is you set F=ma in the Gm1m2/r^2 equation and the m of the falling object cancels out, so it's not needed to solve the problem.
  7. Apr 13, 2008 #6
    Now I'm confused. :s

    So what equation(s) would I use to go about solving this?
  8. Apr 13, 2008 #7


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    If M is the neutron star mass and m is the mass of the object, F=G*M*m/r^2. (If they don't give you a value for m, that's all you can say). Since we also have F=m*a, a=G*M/r^2.
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