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Weight, Acceleration, Gravitational Equivalence

  1. Jan 27, 2010 #1
    When you are in a box it is difficult to know the difference between if the box is accelerating in space or you are standing in the box on the surface of a planet. That is correct is it not?

    In both the accelerating box and on the surface of the planet you can weigh yourself and if the gravity and acceleration are equivalent you will weigh the same.

    Also if you have a standing vertical spring instead of yourself then it will compress the same in both circumstances.

    But, it occurred to me that if the spring were attached to the underside of the box that is accelerating then it will be stretched instead of compressing.

    Does this have any meaning when it comes to gravitational equivalence?
  2. jcsd
  3. Jan 27, 2010 #2
    im not sure i understand what you are getting at.

    gravity pulls towards the centre which is different than acceleration. but you know this so I wont continue ill just ask for you to specify your question
  4. Jan 27, 2010 #3

    George Jones

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    Turn the apparatus upside down in a gravitational field, and the spring expands.
  5. Jan 27, 2010 #4
    Hi dacruick

    The spring inside the accelerating box is squashing the same amount as the spring that is inside the box that is on the planet. So they can't tell the difference (except as you say the slight difference in the direction of the gravity and even micro-changes in the gravity at different heights). So these two can't tell if they are squashing because their box is accelerating or because they are on a planet surface. So they experience equivalence.

    But now, the spring that is hung from the underside of the accelerating box instead of squashing liking its counterpart inside the box instead it stretches due to the acceleration.

    The compressed spring inside the box is accelerating at the same rate as the stretched spring under the box. So, although the spring under the box is accelerating at the same rate it feels the opposite of weight (not quite weightlessness but perhaps negative weight; I don't know).

    So, they don't feel the same thing as each other even though they are both accelerating at the same rate. So, the spring underneath the accelerating box doesn't experience the equivalent weight that gravitational & accelerative equivalence normally give us.

    So what is the spring under the box experiencing? Is what it is experiencing equivalent to anything?
  6. Jan 27, 2010 #5
    That's true George. Thanks. That helps me understand better.

    If you stuck bathroom scales to the ceiling and stuck yourself to the scales then the scales would spin backwards.

    Can you be said to be experiencing negative weight?
  7. Jan 27, 2010 #6
    If I've understood this right, to see the equivalence, you just need to make sure you're comparing like with like.

    Case (1), spring on floor of box. The spring is compressed in the accelerating box, and compressed in the box standing on the planet's surface.

    Case (2), spring under box. The spring is stretched under the accelerating box (i.e. opposite to the direction of acceleration), and stretched under the box standing on the planet's surface. (To see this, you could dig a hole under the box for the spring to hang down into, or suspend the box at a fixed height, far enough above the surface for the spring to hang freely.)
  8. Jan 27, 2010 #7
    you can call whatever you want positive and negative, it plays no significance to anything other than what you've previously deemed positive and negative
  9. Jan 27, 2010 #8
    Thanks for all the answers here. It makes it much more clearer.
    I'm not sure I see it fully that way dacruick.
    Positive weight means that you are squashed; negative weight means that you are stretched; weightless means you are neither squashed nor stretched.
  10. Jan 27, 2010 #9
    you asked can negative weight be experienced. But negative weight is deceleration instead of acceleration. and acceleration and deceleration are the same thing just with a sign switched. So whether you want to call the weight you experience positive or negative is trivial as long as it is consistent with that you previously called it and what you will call it in the future.
  11. Jan 27, 2010 #10
    Are you sure? I agree acceleration and deceleration are the same. But our experience of weight depends on the side we are relative to it. If you are inside an object and it accelerates or decelerates you will experience compression when pressed against the accelerating or decelerating side. But if you 'hanging' from the object you will experience stretching when it accelerates away from you or decelerates before you.
    So even though acceleration and deceleration are the same thing; compression and stretching due to either are not.
  12. Jan 27, 2010 #11
    It is better to imagine the box "accelerating in space" is a box inside a large compartment in a rocket accelerating in space. This way a spring underneath the box in the rocket get compressed between the box and the floor of the rocket compartment just as a spring underneath a box on the surface of a planet gets compressed between the box and the planet surface. The same equivalence applies when the box is suspended above the planet or above the floor of the rocket.
    Last edited: Jan 27, 2010
  13. Jan 27, 2010 #12
    if you want to assign compression to acceleration and stretchign to deceleration thats fine. our experience of the weight depends on nothing but the direction of the acceleration. acceleration is the only vector quantity in the Force equation
  14. Jan 27, 2010 #13
    But I'm not assigning compression to acceleration nor stretching to deceleration?
    You can conversely get compression with deceleration and stretching with acceleration.
    It depends upon whether you are in front of or behind the accel/decelerator.

    Our personal experience of weight can be one of three things it can be compression, nothing or stretching.

    When we are being compressed (due to gravity or accel/deceleration) we are said to experience weight.
    If we are not being compressed (or stretched) we are said to experience weightlessness.
    So mathematically it would seem that we could conversely say that if we are being stretched (due to gravity or accel/deceleration) that we could be said to be experiencing negative weight.
    Certainly a set of bathroom scales stuck to our feet and to the ceiling would reflect this.

    The experience of being stretched can not be said to be the equivalent experience as that of being compressed. Our very atoms know the difference.
  15. Jan 27, 2010 #14


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    If you stand on the floor, you feel compression in your legs through a force on your feet. If you hang off the ceiling, you feel your arms being stretched through a force in your hands. If you felt your legs being stretched through a force on your feet you wouldn't conclude you had negative weight, you'd conclude you were upside down.

    Everything I've said applies equally to a "stationary" room on Earth or a room in an accelerating rocket -- you wouldn't be able to tell the difference (unless you moved around the room and made very, very accurate measurements of how your weight varied).
  16. Jan 27, 2010 #15
    If you have 1 & 0 & then the opposite of 1 what do you have? -1.
    The bathroom scales even reflect the situation.
    What more proof do I need?

    And I don't even understand why there is reluctance to look at it in this light.
    It's not as though it even effects some deep held belief of science.
    I know science doesn't like to look at the basic things in new ways.
    This forces people into a pattern or reiteration of basic facts rather than looking at things from different perspectives or with their own mind.
    Why for something so basic?

    As far as I can see we measure weight (not mass) with a scale.
    And weight shows a relationship to compression.
    And stretching due to gravity/speed change don't cause scales to reflect +ve weight.
    Instead they reflect -ve weight.

    There is nothing mathematically wrong with this.
    It is just a different way of looking at it and it is logically consistent.

    I would have regarded it as an interesting new perspective!
  17. Jan 27, 2010 #16
    More accurately you feel your arms being stretched through the force of gravity.
    The force upon your hands just prevents you from moving away from the ceiling so that the gravity can stretch you.

    Though under equal & opposite the hands are applying a balancing up force as the gravity is providing down force.
    But, the hands would be applying no force without the gravity being present.
  18. Jan 30, 2010 #17
    I don't believe antigravity exists. Just as an aside I would be curious what science thinks on the subject if anyone would be kind enough to give me an insight.

    I think that if it existed then anything that exhibited it would not cluster so it would not form into anti-gravity planets. So you wouldn't get very big bodies of the stuff anywhere?

    I was just thinking of it because if you had an antigravity planet (which I don't think is possible) then it would push you away. So if you stuck bathroom scales to the surface and your feet to the scales you would see the results I speak of too.

    Again the scales would show you have negative weight and your body would stretch.

    You can see these results without the need of an antigravity planet but it adds another perspective I hope.
  19. Feb 22, 2010 #18
    i dont understand?
  20. Feb 22, 2010 #19
    What are you confused about jfy?
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