1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Skydiver question

  1. Mar 12, 2009 #1
    When a skydiver falls from the sky, the person feels as though he or she is completely weightless, and when on the ground, the person does not.

    We can assume weight to be equal to the force of acceleration of the ground according to the equation F=ma. Yet we don't typically say that the ground is accelerating us upwards, we say the force of gravity is pulling things downward. What's wrong with the notion that the thing we experience as gravity is the result of the ground accelerating the person upwards?
    Last edited: Mar 12, 2009
  2. jcsd
  3. Mar 12, 2009 #2


    User Avatar
    Science Advisor

    You mean "isn't" or "is"? In General Relativity it is the result of the ground accelerating the person upwards. In Newtonian Gravity it is the result of the person being pulled down by a force. There is nothing wrong with either view.
  4. Mar 12, 2009 #3

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    nothing ....free country ..all notions pretty much ok..but i can tell you..when i jump...it is a definite ' off Earth " experience.....
  5. Mar 12, 2009 #4
    Well that was kind of a loaded question, and both of you FAILED, so I will tell you what you should have found wrong with the idea.

    the notion that the ground is an accelerating platform when applied to the whole earth would mean that the earth is growing, and at an exponential rate, so wouldn't things of mass grow into each other? Wouldn't the earth and moon expand into each other? Obviously this doesn't seem to happen, the moon looks like it's just about as far away as it was 10 minutes ago.

    So now my question is, how do we explain this phenomenon, how do we manipulate the universe so that the perspective of the ground really is accelerating upwards? what do we have to modify to make it so that f=ma, the earth is expanding, without having things expand into each other?
    Last edited: Mar 12, 2009
  6. Mar 12, 2009 #5
    is I'll fix that post
  7. Mar 12, 2009 #6


    User Avatar

    Staff: Mentor

    That's superfluous. The whole point of such exercises (usually) is that if you are in a closed room, it is impossible to conduct an experiment which can differentiate between your room accelerating and gravity acting on you. How the room comes to be accelerating or have a gravitational force act on it is not typically part of the question and is completely irrelevant to it.

    So it seems like your question wasn't loaded the way you thought it was...
  8. Mar 14, 2009 #7
    Read this before commenting!!!


    I was trying to debate this idea in a Sophocles style, but
    I can see that people are more annoyed with this than anything. So I'll stop it with these loaded questions. I just want to give a bit of insight, that's all.

    The point I want to make is this; It is conceptually intensive and requires a little bit of imagination so read it all the way through and pay attention. Hopefully if I set it up right it will sound ridiculous and then start to make some kind of sense.

    Take the example of two circles, these two circles are of equal size, and they are separated from one another by a distance of 3 of their diameters. When you return at time t later you see that they are separated by a distance of four diameters. The standard convention of science is to express this in terms of a velocity.

    However, there are at least two other ways you can describe this phenomenon, you might express it as the distance in between the circles "expanded" from a distance of three to a distance of four over time t, or that the two circles shrunk in such a way that the proportions between their size and the distance which separated them increased from three to four diameters. Or any combination of the two, because the expansion or contraction between the two circles and the distance that separates them is relative.

    All of these succeed in describing the phenomenon of motion, and you can apply them for any form of motion.

    Take the example where the circles are static, or unmoving to something else. In general science, the standard convention is to say that the distance does not change, but with this perspective, we cannot make that assertion, all we can say is that the rate of expansion of the distance is equal to the rate of expansion of the two circles. (or the rate of contraction is in equilibrium - depending on how you see the half ___ glass of water)

    In science the convention is to describe it in terms of a velocity, whereas if you disregard the notion of a distance as something that is absolute and static, then you have an infinite number of ways of describing the situation, and as far as gravity is concerned, there might be some benefit in describing the phenomenon of motion in such a way.

    I don't know what that benefit might be, probably I'm dumb and crazy and this is all loaded in the way you think it is. But I just think the thought is interesting - that's all, maybe worth considering.
    Last edited: Mar 14, 2009
  9. Mar 14, 2009 #8


    User Avatar
    Homework Helper

    Only during the initial seconds from a base jump. Once at terminal velocity, the skydiver would feel an upwards blast of air supporting the skydiver, and other than turbulence, the skydiver feels the pull of gravity internally.
  10. Mar 14, 2009 #9
    It was more for conceptual purposes. Lets just say there is no wind resistance.
  11. Mar 14, 2009 #10


    User Avatar
    Science Advisor

    Wrong. You are confusing acceleration and velocity. Just because something is accelerated away from (or towards) a certain point (here center of earth), does not mean that it is also moving away from (or towards) that point.

    Simple example: A mass on a string rotating in a circular path is accelerated towards the center of the circle, but it doesn't move towards it.
  12. Mar 14, 2009 #11


    User Avatar
    Homework Helper

    From an inertial (non-accelerating) frame of reference, someone standing at one of the poles of the earth (so no centripetal related factors) would show no signs of acceleration. Clearly different than observing an object in free fall in space.

    If you're referring to general relativity, then there are no inertial frame of references (every frame of reference is relative).
  13. Mar 14, 2009 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    But then the parachute won't do any good. :biggrin:

    (Sorry, couldn't resist that one.
    We now return you to your regularly scheduled program ...)
  14. Mar 14, 2009 #13
    I don't see how the example applies, it's not like you can assume a rotational acceleration for a sphere with the acceleration in the direction perpendicular to the surface. The only assumed motion is an accelerated expansion. I am just extrapolating an equally valid perspective of gravity, please read my other post to understand the conceptual argument, this has very little relevance to the point I am making or was trying to make. Please read the thing about the two circles.
  15. Mar 14, 2009 #14
    That's a good piece to this puzzle. I'll have to think about that some more.
  16. Mar 15, 2009 #15


    User Avatar
    Science Advisor

    I does not apply to gravity. It just disproves your assumption that acceleration implies movement in the same direction. Your assumption is wrong even in classical Newtonian mechanics and flat spacetime. It is even more so in the curved spacetime of GR.
    No, there is no expansion. The Earth's radius is constant, and in the curved spacetime of GR objects have to be accelerated to keep a constant distance to the Earth's center.
    Maybe you should draw a picture of this, that shows what distances you mean. It is confusing:
    How would this equilibrium look like, for three circles in a line - keeping all apparent separation distances constant? Picture please.
  17. Mar 15, 2009 #16

    It's not a picture - it's more like a motion, with two circles, and a distance which separates them growing larger uniformly or shrinking uniformly. In the case of the two static circles, circles which are unmoving relative to one another, the perspective takes into account ALL the ways that the appearance of a static state can happen. It's up to you how you want it to appear, with a static state, you can say everything is shrinking uniformly with time or growing, so if there is nothing to compare this expansion or contraction then nothing changes.

    Of course it would seem superfluous for this reason, and maybe it is - but I have a feeling that there might be something to this.
  18. Mar 15, 2009 #17


    User Avatar
    Science Advisor

    Then draw a picture of two states of that motion at two different time points for three circles arranged in a line, with static configuration (all separation distances are constant).
    If you cannot tell the difference to the standard model, it cannot be disproven, but is just as usefull as the https://www.physicsforums.com/showthread.php?t=104076&p=869356"
    Last edited by a moderator: Apr 24, 2017
  19. Mar 15, 2009 #18
    Please, be patient. This is just an analogy, and it's not a complete idea, I still have some problems that I haven't thought out, like the one you brought up about centrifuges.

    I don't think it's like the hollow earth theory - or any other crackpot theory, The way I hope people understand it is that it's just a different way of looking at the same thing. I just think it is a more precise model, to take into account all the ways the appearance of a motion can be interpreted.

    But here is why it is useful; if I brake down the universe into 2 parts, and for simplicities sake lets just for now, disregard any rotations or centrifugal forces.

    you have one quantity which is a mass, which again for simplicities' sake will say to only have one property, and this property is expansion
    The other quantity will be 'empty' volumetric space, which also has an expansionary property.

    For mass, we will say that it has a much greater expansion rate, than that of 'empty' volumetric space. and for every volume of mass, it expands at 1.5 times the rate of an equal volume of volumetric space.

    for now, we will make the assumption that you may superimpose mass and empty volumetric space, that volumetric space cannot superimpose itself and that also mass cannot superimpose itself either.

    Now imagine having three of these pieces of mass side by side to one another, since they cannot superimpose themselves they grow outward, so the accelerated expansion rate from the center adds, the radius would expand in this case 2.25x faster than empty space (maybe a little less). The more mass you have side by side, the more the difference in expansion between the body of mass, and the empty space around it.

    Now if you can imagine it, with these rules you can create a pseudo-gravitational effect in some ways it mimicks gravity quite well. It even kind of succeeds to explain dark energy - to a certain extent however it fails to explain certain phenomenon, like shearing, and rotation and stuff like that, I have an idea to fix this problem but it gets pretty complicated.
    Last edited by a moderator: Apr 24, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook