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Homework Help: Dynamical friction due to gravity

  1. Dec 30, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi there. Can anyone help me with this question, or maybe give me some approach to this question? I really dont know how to start solving it.

    In this question we will show that gravity can act as an effective friction
    force. Massive objects moving in a uniform medium will be slowed down due
    to gravity. When the massive object moves it will create a wake behind it
    of over dense gas that will slow down the object due to gravity interaction
    between the object and the over dense region behind it. We will assume that
    we have a massive object with mass M moving at a velocity vM in an infinite
    gas cloud with a density ρ.

    1. Assume each small gas particle feels only the gravity of the big object
    and moves at a constant acceleration that is given by its initial position
    relative to the object. In a short time span Δt, gas particles up to which
    radius will reach the center object (neglect the center objects motion)?
    How much mass do they have?

    2. During the time the excess gas mass has fallen the massive object has
    moved by what distance (assume constant velocity)? Assume that all
    of the fallen mass has reached the original position of the object, what
    force will the object feel from the extra mass behind it?

    3. If two massive objects are going through a uniform medium, who would
    be slowed down more, the heavier one or the lighter one? How does
    this compare to aerodynamical drag whose force is proportional to the

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 30, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Not even an attempt at the first part? It's very easy. Just use the usual s/t/u/v/a equations for constant acceleration. (Radius here refers to the distance to the massive object, not the radius of the particle.)
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