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Conceptual Question: Stars in intergalactic space with speed

  1. Mar 4, 2013 #1
    1. The problem statement, all variables and given/known data

    http://postimage.org/image/7rgmo1527/
    How is the speed decreasing and acceleration decreasing in this problem?

    2. Relevant equations

    Kepler's Laws?


    3. The attempt at a solution

    I remember in lecture my professor was saying something along the lines of objects attracting each other. So, since there are two stars that are maybe attracting each other, as they get farther and farther apart by a distance of r, shouldn't the speed increase--because the force of attraction (already negligible) is even smaller as they go farther apart? Does this question pertain to Newton's laws or Kepler's laws?

    Thank you for your time!
     
  2. jcsd
  3. Mar 4, 2013 #2

    SammyS

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    Do you know the difference between acceleration and speed?
     
  4. Mar 4, 2013 #3
    mm speed is a scalar and acceleration is vector? Acceleration of the rate of change of the speed.
     
  5. Mar 4, 2013 #4

    SammyS

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    So the speed can be decreasing but at an ever slower rate of decrease, Right?
     
  6. Mar 4, 2013 #5
    Right that would explain the acceleration decreasing, sorry I understood that part already, (even thought I said "and acceleration decreasing") but am having a difficult time understanding how the speed is decreasing.
     
  7. Mar 4, 2013 #6

    phinds

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    What force is acting on each star? Is that force directed in the same direction as their motion or not?
     
  8. Mar 4, 2013 #7
    Okay, I think I understand it now, its like if a person was on the surface of the earth and as they got farther and farther away their speed would decrease because the force of gravity (which is the only force acting on the stars) would diminish and the person would go slower and slower as he got farther and farther away (decrease in acceleration)

    Thanks! SammyS and Phinds
     
  9. Mar 5, 2013 #8

    phinds

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    No, I don't thing you are thinking this through properly.
     
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