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Free-fall acceleration

  1. Jul 12, 2004 #1
    We have already discussed the definition of g to death, but I have another question regarding the use of "free fall." This is also a term that is misleading, since a body doesn't have to be falling to be truly in free-fall.

    Can anyone come up with a better term?
     
  2. jcsd
  3. Jul 12, 2004 #2
    please explain what could be in freefall without falling
     
  4. Jul 12, 2004 #3
    An object rising.
     
  5. Jul 12, 2004 #4
    ??? then it wont be falling.....
     
  6. Jul 12, 2004 #5
    and if something is rising, then ur going against gravity, which is something completely different
     
  7. Jul 12, 2004 #6
    When it is rising the object, lets say a rock, is still considered to be in the "free fall" scenario. This is because even though it is has been given enough force to counteract its weight, gravity is still doing work on the rock. So you could say it is falling in the negative direction. This is what JohnDubYa is trying to say i think. The term free fall is not a very good term because it gives us the impression that the object must be falling.
     
  8. Jul 12, 2004 #7

    chroot

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    To be in "free fall" means to have no forces other than gravity acting upon you. In the parlance of general relativity, it means you're following a geodesic (a straightest-possible line) through curved spacetime.

    Perhaps you should just call it "geodesic motion."

    - Warren
     
  9. Jul 12, 2004 #8
    hum...good point, it accelerates upwards but negatively, -9.81m/s^2.
     
  10. Jul 12, 2004 #9
    RE: "then it wont be falling....."

    Precisely the problem.
     
  11. Jul 12, 2004 #10

    chroot

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    Consider also just calling it "inertial motion."

    - Warren
     
  12. Jul 12, 2004 #11
    I would think inertial motion would apply where NO forces act on the object.
     
  13. Jul 12, 2004 #12

    chroot

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    No, an inertial frame is one in which Newton's laws hold. In the parlance of general relativity, gravity is not a force. The only situations that forces are involved are those situations in which a body is not allowed to follow its natural trajectory. The chair you're sitting on is preventing you from following the trajectory you'd otherwise follow, onto the ground. When you're freely falling, you don't feel your own weight, which means no forces are acting upon you.

    Einstein's principle of equivalence states quite simply that the physics in an inertial frame is indistinguishable from that in a freely falling frame -- so calling free fall "inertial motion" is entirely valid.

    - Warren
     
  14. Jul 12, 2004 #13

    robphy

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    In the context of Newtonian mechanics, a "free-falling projectile" is falling with respect to an object starting at the launch point and moving with constant velocity equal to the projectile's launch velocity. Pictorially, draw the projectile's parabolic trajectory and the tangent line to that parabola at the launch point.
     
    Last edited: Jul 12, 2004
  15. Jul 12, 2004 #14
    I am not saying that "free fall acceleration" is inaccurate, but simply misleading.
     
  16. Jul 12, 2004 #15

    robphy

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    I guess the problem is that "falling" suggests decreasing.

    Thinking about it more, it seems to me that what is falling (decreasing) is the y-velocity. As the y-velocity decreases, one can picture the velocity vector turning downwards.

    In terms of the trajectory itself, it seems that one may need to capture the notion of "concave down".
     
    Last edited: Jul 12, 2004
  17. Jul 12, 2004 #16
    Well, that assumes the object isn't thrown perfectly vertical. :)
     
  18. Jul 12, 2004 #17
    how about ...gravity acceleration :D
     
  19. Jul 13, 2004 #18
    The definition of freefall is "no other forces are acting on it apart from gravity".
    so therefore it could not be rising, going sideways or anything else but falling
     
  20. Jul 13, 2004 #19

    LURCH

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    Sure it could; A sattelite going to a higher orbit burns its engine for a relatively short period of time, then the engine is shut off. But the sattelite is still gaining altitude (relative to the Earth's surface). It is, quite litterally, "falling up". Once the transfer orbit burn is finished, no force (other than the pseudoforce of gravity) is acting on the sattelite, but it continues to climb.
     
  21. Jul 13, 2004 #20

    chroot

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    Or, even simpler, consider the first half of a baseball's trajectory, just after you've thrown it. It's going away from the earth, but is acted upon by no other forces besides gravity. It is therefore in free-fall.

    - Warren
     
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