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Acceleration has begun to drive me crazy please help.

  1. Apr 1, 2013 #1
    From my understanding, we are not really moving "anywhere." By this, I mean direction is not absolute. Walking north, in terms of spacetime curvature, is the same as walking south. The only way in which we are moving is through spacetime. A guy going 70mph on a train in Paris is moving through spacetime the same as a guy in NYC on a train going 70mph, minor gravitational effects aside.

    Okay, here's where it starts to get confusing for me, and I will not attempt to be perfectly rigorous in my interpretation, as I foolishly did before.

    My pup is asleep (at rest). She is moving (relatively) through spacetime only at the rate at which the earth is rotating relative to other planets, stars, or whatever, but in the reference frame we're using here, she's at rest. Suddenly my pup awakens because she can smell the piece of bacon I put in her food bowl. She gets on all four paws and races toward the bacon. At this point she is accelerating, and this is where I'm getting confused.

    If she is accelerating relative to the earth beneath her paws (correct me if that is not correct), could we also say that it's the earth beneath her paws that is accelerating in the opposite "direction"?

    There is something occurring among space, motion, and acceleration that is very confusing to me.

    It seems that "space" (and its "contraction") is creating this idea of accelerated motion, when it's not really happening.
  2. jcsd
  3. Apr 1, 2013 #2


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    Yes, in the non-inertial rest frame of the pup the Earth undergoes coordinate acceleration. But that is different from the proper acceleration which the pup experiences.
  4. Apr 1, 2013 #3


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    There is no Inertial Reference Frame (IRF) in which the earth is accelerating and not your pup. In your pup's non-inertial rest frame, the earth is accelerating.
  5. Apr 1, 2013 #4

    There is something about proper acceleration that is not making any sense. The physics community is missing something here, in my opinion.

    Question: If my pup is levitating above the surface of the earth, does the bacon come to her (via earth's rotation, assuming she's in the right position), or does she go to the bacon?
  6. Apr 1, 2013 #5
    mostly gibberish.
    Do you know the distinction between 'speed' and velocity?? that might help you.


    Accelerated motion is real enough....you can FEEL it....as in an accelerating car. You can measure it by carrying an accelerometer with you as you move.

    Perhaps you have read of the similarities between accelerated motion and gravity? Both can cause spacetime to 'warp'..that is curve.

    Length 'contraction', however, is a term utilized in special relativity [SR] and refers to the relationship between space and time at high velocities...when one is altered so is the other.
  7. Apr 1, 2013 #6
  8. Apr 1, 2013 #7


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    Like what?

    In the rest frame of the pup the bacon come to her. In the rest frame of the bacon she comes to the bacon.
  9. Apr 1, 2013 #8


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    No, you're missing something. Since Newton wrote F = ma there have been no contradictions of this experimentally.

    Your pup is not levitating and before she starts moving is at rest in the rotating frame. Motion is relative, proper acceleration is not.
  10. Apr 1, 2013 #9
    The whole "feel" part, I believe, is not being interpreted correctly. I just do not see how being at rest and accelerating are not the exact same thing.

    Naty: yes, I read the first two paragraphs of the link. What I'm saying is that direction, which is an important part of classical velocity, is not making any sense. If we're not really going anywhere, how can direction be of any value? If you're telling me that going to Paris is the same as the earth dragging Paris to me (using dragging loosely), I cannot understand how proper acceleration can hold.

    Not trying to be difficult, but a lot of this is not making any sense.
    Last edited: Apr 1, 2013
  11. Apr 1, 2013 #10


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    Obviously they are NOT the exact same thing since they can be experimentally distinguished. Devices built specifically for the purpose of experimentally distinguishing rest from acceleration are called accelerometers.

    Try to simplify your examples. Confusing examples lead to confusion. Get rid of the earth, it is massive and has gravity and rotation, etc. Get rid of your dog (for purposes of examples only :cry:) dogs bounce around a lot. Express your example in terms of two rockets in deep space. Specify their initial velocities and positions in some inertial frame and any rocket thrusts performed in order to change that.
  12. Apr 1, 2013 #11


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    You can be at rest while undergoing proper acceleration. Rest is a frame dependent concept. Proper acceleration is frame independent.
  13. Apr 1, 2013 #12


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    At rest: the values of the position coordinates are not changing with respect to the time coordinate (the first derivative with respect to time is zero). Note that this definition requires that you specify a reference frame, because a system of assigning coordinates IS a reference frame - no frame, no coordinates! - but the frame need not be inertial. When we say things like "the sleeping pup is at rest" or "the running pup is not at rest" we are using a non-inertial frame in which the surface of the earth is at rest.

    Acceleration comes in two flavors.
    Coordinate acceleration: This is usually what people mean when they just say "acceleration". It is the rate of change of velocity with respect to the time coordinate, where velocity is defined as the rate of change of the position coordinates with respect to the time coordinate. Obviously this also requires that you specify a reference frame - no frame, no coordinates! - and you can generally choose a frame in which the coordinate acceleration is zero. For example, a freely falling object is experiencing coordinate acceleration in the (non-inertial) frame in which the surface of the earth is at rest, but it is not experiencing acceleration in the (inertial) frame in which it is at rest.

    Proper acceleration: I'll hold that definition until you're clear on the previous two definitions. They are pure classical physics, no relativity involved, and you have to be clear on them before you can start to make sense of relativity.
  14. Apr 1, 2013 #13
    I believe I pasted this link into one of your threads? To make a long story short, there is no physical theory which explains why some people "feel" acceleration why others "feel" being at rest. It is observed, but not understood.

    - http://www.calphysics.org/haisch/science.html
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