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!

Basic physics question

  1. Oct 11, 2011 #1
    Can you help me out and tell me if I am right or wrong when I say the following:

    Think of a free fall.

    As we see something in a free fall here on Earth we are really seeing/experiencing it in 2d not 3d. An object that moves in a straight line from one point to another is traveling/falling in 2d. Space/distance is increasing all the time between the object falling and its starting point.

    An object that falls in 3d must be falling “in”, or “out”. If something falls “in” it falls into one point from all directions. Space/distance is increasing from all directions. If something falls “out” it is falling outwards from one point in all directions, space/distance is increasing from one point and out.

    Please comment

    Best regards

    Halli
     
  2. jcsd
  3. Oct 11, 2011 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    What's your point? Is there a question in here somewhere? What is giving you trouble?
     
  4. Oct 11, 2011 #3
    Thanks for the reply phinds

    My trouble is that I have come to the conclusion that an object that falls straight down is falling two dimensionally, even though it is in a three dimensional world. It travels from point A straight to point B (no wind, resistance,.................)

    If that conclusion is right then an object that would fall three dimensionally must be falling from all angles/directions. Might compare it to an inflated balloon that is losing air. Its surface is falling into the centre.

    This is quite important.

    Does anyone agree or disagree with this conclusion??? Is there something that has already been written about this? Or am I just a very troubled person :)

    Please comment

    Best regards
    halli
     
  5. Oct 11, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    You can describe straight line motion as 1-dimensional, if you like.
    OK.
    I cannot make sense of these words.
     
  6. Oct 11, 2011 #5
    Thanks Doc Al

    See post 2 for ?better? explanation.
     
  7. Oct 11, 2011 #6

    Doc Al

    User Avatar

    Staff: Mentor

    I assume you mean post #3. Sorry, that's equally obscure. I don't see what you mean by "an object that would fall three dimensionally must be falling from all angles/directions". Why in the world would you think that?
     
  8. Oct 11, 2011 #7
    Thanks again Doc Al

    Why? Not easy to answer.

    The first question maybe should have been:

    Do you think it is possible for objects to "fall" three dimensionally (at least in theory)?

    But I needed to try to explain it somehow.

    So is it obscure or maybe a possibility in your opinion?
     
  9. Oct 11, 2011 #8

    Doc Al

    User Avatar

    Staff: Mentor

    I have no idea what you mean by 'falling three dimensionally'.
     
  10. Oct 11, 2011 #9
    Thanks Doc Al, I appreciate it

    I will try one more time with a statement:

    A balloon that is collapsing is “falling” 3 dimentionally.

    Could this be true in your opinion?

    Thanks again
    Best regards
    halli
     
  11. Oct 11, 2011 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    All I can make of any of this is that you are playing with words and confusing yourself. Collapsing is a perfectly valid description of what the balloon is doing and I guess if you like to think of that as "falling" in 3D you can but I don't get what that means.

    A steel ball that drops from the upper atmosphere to earth is falling in 3D due to wind shear and I seriously doubt that you could describe such an actual trajectory in less than 3D. An IDEAL situation could have something moving only in 1D as it falls.

    What is your basic problem here? What is it that you are trying to figure out? Playing with words isn't getting us anywhere.
     
  12. Oct 14, 2011 #11
    Hei guys

    You might say I am playing with words, that is right.

    But I am trying to understand some very basic forces. I am trying to understand them logically but not mathematically. And with that I have only words. The definition of things with words is sometimes confusing. Using one word instead of another can change everything because of our different understanding of different words.

    I am sorry if it seems like I was wasting your time, but the last answer really helped me and my weird head :)

    Thanks again
    Best regards
    Halli
     
  13. Oct 14, 2011 #12
    As has been said, you can think of simple falling as being in only one dimension, as the object is only experiencing the force of gravity, and it falls in a straight line (down). If we add another force that causes the object to veer left or right while falling (wind, or being shot out of a gun) well then we'll need to talk about it in two dimensions.

    If we add yet another force that's not in the same plane as gravity and the second force, then that object will fall down, left/ right, and in/out. Indeed, we'll need to discuss the object in all three dimensions in order to desrcibe its motion. A deflating balloon will spin around and forces will be imparted on it in all directions, so we'll need to use all 3 dimensions to descibe its trajectory.

    Realistically, to descibe the motion of ANY object, we need to know what it's doing in all three dimensions. However, the wording of a problem usually tells you about these other dimensions. For example, if someone drops a bowling ball from the 3rd floor, you know that it's side-side velocity is close to 0 due to the word "drops"....and hence we can ignore that dimension. You can also assume that there will be no other significant force due to the omission of words such as "explosion", "tornado", or "airplane" and hence using this information, we can simplify an otherwise 3d problem into 1d.
     
  14. Oct 14, 2011 #13
    Thank You L sos

    What I am trying to imagine is not exactly a balloon loosing air from one point and there for spinning around.

    It is more of a scenario of theoretical situation where I am trying to imagine this:

    A steel ball is positioned in vacuum. Space is "growing" or "increasing" all around it.
    Will that steel ball experience forces of collapse into its center of mass?

    Might be a stupid example to call it a "free fall in 3d" but that is how I have been trying to understand it.

    Best regards

    halli
     
  15. Oct 14, 2011 #14

    Doc Al

    User Avatar

    Staff: Mentor

    OK.
    What do you mean?
    Forces due to what?
     
  16. Oct 14, 2011 #15

    phinds

    User Avatar
    Gold Member
    2016 Award

    Seems like we've gone from object in free fall to objects undergoing gravitational collapse. You likely would have gotten a more coherent and helpful answer if you had asked the question you were trying to answer instead of asking a different question.

    Doc Al is gently trying to nudge you into thinking about what you are saying. You should read up a bit on the expansion of the universe and its effect (lack of effect, actually) on gravitationally bound systems.

    It's clear from your question that you don't understand that and I think you'd find it interesting.
     
  17. Oct 14, 2011 #16

    Drakkith

    User Avatar

    Staff: Mentor

    Halli, you could easily describe any objects motion as 1d. Where 3d comes into it is that it is free to change its vector of motion in 3 different directions. If I swing a bucket on a rope around my head you could say that at each infinitesimally point in time the bucket is only traveling in one direction, and that at the next infinitesimally point in time forward the direction it is traveling has changed but that it is still traveling in one direction.

    In your balloon example, each particle in the balloon is traveling in one direction that may change over time, yet overall that direction is towards the center of the balloon if it is collapsing.
     
  18. Oct 14, 2011 #17

    DaveC426913

    User Avatar
    Gold Member

    I think what he's grappling with is that, to describe an object falling in a straight line only requires one dimension of change. In the other two dimensions, coordinates do not necessarily change. (Though it is simply a question of what coordinate system you use. You could use an coordinate system that's oblique to the direction fall such that it tracks the object changing its coordinates across all three dimensions.)

    He's asking if it is possible to have an object fall in such a way that it requires coordinate changes in all three dimensions.

    Well, you can get two at least, by having the object fall in an arc. To track it requires a coordinate system adjustment to align at least one dimension, but you cannot describe the arc without tracking change in a minimum of two dimensions.

    Can anyone describe a circumstance in which a minimum of three coordinates are required to describe an object falling?
     
  19. Oct 14, 2011 #18

    Drakkith

    User Avatar

    Staff: Mentor

    Some sort of helix type motion in the fall, like a wiffle ball?
     
  20. Oct 14, 2011 #19

    DaveC426913

    User Avatar
    Gold Member

    Yeah. The trouble is, you end up invoking external forces other than free falling (i.e. gravitational attraction).
     
  21. Oct 15, 2011 #20
    I think you understand what I mean now. Thanks for the patience. I do not know all the right words to use.

    But I knew before I asked I could not mention GRA........ and I am not going to. I will be banned if I do.

    I read what I could find on the expansion of the universe and its effect (lack of effect, actually) on gravitationally bound systems. What I found did not help me. Could you give me some links to a good article or similar to this matter?

    But still I think about my ball. Made of whatever. All alone in space. And space is growing/increasing from all directions/all around. Do any forces „appear“ in the mass center of the ball?

    I have been bouncing from yes, no, yes, no, yes, no, yes, no. This damn ball has given me allot of headache. And it doesn’t even exist.

    But I really think (theoretically of course) it would get affected by the space increasing around it and seek for falling into its own mass center.

    But I am not 100% sure because if that would happen to the ball in my head allot of things could be answered.

    I am not surrounded by people I can discuss this hobby of mine with. So I searched for someone like you guys and girls to ask. And I want to repeat that I am not making any statements. I realize that I might be wrong. And I know that I know a lot less then you guys and girls about physics.

    Thanks and best regards

    halli
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Basic physics question
  1. Basic physics question (Replies: 4)

Loading...