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Homework Help: Velocity in a Vacuum

  1. Sep 30, 2006 #1
    Here is a question on a quiz I took the other day:

    An object is in free fall in a vacuum. Which of the following is true?

    A. Velocity is increasing
    B. Acceleration is increasing
    C. A and B
    D. None are correct

    I might be thinking about this wrong, but I assumed it was a vacuum in space, which means there would be no gravity, which would mean the answer is D. However, if it was a man-made vacuum on Earth, gravity would increase it's velocity, which would mean the answer is A.

    Can someone clear this up for me, please?
  2. jcsd
  3. Sep 30, 2006 #2


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    "...a vacuum in space, which means there would be no gravity..."


    Go over that logic again?
    Last edited: Sep 30, 2006
  4. Sep 30, 2006 #3


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    Vacuum and zero gravity are unrelated. One doesn't mean the other.

    What do you think keeps the Moon and all the satellites from flying off into space? What do you think the astronauts are experiencing on the Moon?
  5. Oct 1, 2006 #4
    What would the answer be then? I'd also have assumed D but you seem to be suggesting otherwise?
  6. Oct 1, 2006 #5


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    I think they simply want you to ignore the effects of air resistance. Imagine the object falling down a long evacuated chamber at or near the earth's surface.
  7. Oct 1, 2006 #6
    If that is the case then wouldn't it be A) velocity is increasing and the acceleration would be 9.8 m/s/s
  8. Oct 1, 2006 #7
    Yea i thought that it was trying to tell you to ignore air resistance when reading this.so i think the ans might be A?so it is correct to say that the object will not reach terminal velocity when falling thru the vacuum.....rite?
  9. Oct 1, 2006 #8
    Yeah that's correct. A quick definition for terminal velocity tells you that. "The terminal velocity of an object falling towards the ground, in NON-VACUUM, is the speed at which the gravitational force pulling it downwards is equal and opposite to the atmospheric drag (also called air resistance) pushing it upwards."
  10. Oct 1, 2006 #9


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    So, we have an object that is falling freely. Since it is falling, we know it is under the influence of gravity. Since it is in a vacuum, we can discount terminal velocity. The acceleration will be g.

    You know its velocity is increasing.
    Is its acceleration increasing, or is it constant?
  11. Oct 1, 2006 #10
    It's acceleration is constant. The answer is A. I made a foolish assumption.

    Thank you for the help!

    If I knew the question meant that, I would have said A without a doubt (re-read my first post).
    Last edited: Oct 1, 2006
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