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!

The Gravity of Sunnyside

  1. May 20, 2009 #1
    Can someone help me with these questions I am having?
    1. Does sunlight increase the the pressure on the sunny side of the planet?
    2. I'm told pressure has gravity. Is that correct?
    3. If it is, does this increase the gravity on the sunny side of the planet?
    4. If so any thoughts on how much increase would occur?
  2. jcsd
  3. May 20, 2009 #2
    Hi there,

    Indirectly, I would say so. Energy will be transferred to our plant's atmosphere, on the sunny side of the planet. This energy will heat up the air particles, making them move faster, therefore increasing the pressure.

    Then again, a meteorologist could give you a much more complete answer than this, but in the first idea, yes.

    Go and tell off these people of said that. Gravity (as understood normally) is a acceleration of particles on the surface of the Earth ([tex]g=9.81m/s^2[/tex]), where as pressure is a distribution of force over a surface area ([tex]\frac{N}{m^2}[/tex]). Pressure does not contain any type of matter, since it is the result of a force spread over an area, therefore NOT having any gravity.
  4. May 20, 2009 #3
    These people advocate learning GR.
  5. May 20, 2009 #4
    Thanks fatra & cesium. Fatra, I think that Cesium is saying that our science says that pressure does have gravity. I think that is the consensus but I'm happy to be corrected.

    May I add an addendum question?
    5. If a planet were one side gold and the other side fairy floss would the entire planet accelerate towards the gold side?

    I ask that because if you had a basket ball, one half semi-sphere solid gold and the other half air filled, and put if on a trampoline, with gold to one side and air to the other, the basketball would roll towards the gold side until the gold side was on the bottom. The curvature ("space-time") of the trampoline would start more towards the gold side so it tries to move that way.

    So that is why I am asking, would a planet accelerate towards its denser side?
    Or does that not matter and it wouldn't accelerate?
  6. May 20, 2009 #5


    User Avatar
    Science Advisor

    You are going to tremendous lengths trying to make sense of the trampoline model of space-time, which doesn't explain mass attraction. Check the explanation and links here:
  7. May 20, 2009 #6
    Hi there,

    In you basketball idea, gonegahgah, your are absolutely right. The gold side of the plante would be facing the sun. But this is only due to the fact that the center of mass of the planet would be shifted to the gold side.

    One very good example of what you are illustrating is the Moon. Why do you think the Moon is always showing the same side, and that there is a dark side to it. Precisely because the Moon's center of mass is shifted a tad towards the Earth. Over time, the Moon stopped "spinning" on itself, to always have the same side facing us.

  8. May 20, 2009 #7


    User Avatar

    Staff: Mentor

    Actually the Moon does spin on its own axis, once every 24 hours. The sun rises and sets on the Moon, just like it does on the Earth.
  9. May 20, 2009 #8
    Absolutely, that the Moon spins on its axis. This is not what I said before. It just does not spin for us on Earth, since we always see the same side of the Moon.
  10. May 20, 2009 #9

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  11. May 20, 2009 #10
    Thanks for your answers to my questions.

    Thank you for the answer about how it would face the Sun fatra. I was wondering, even if there were no Sun to consider, whether the dual density planet would move towards the denser side under its own gravity?

    AT, thank you for the info. I was just seeing if the analogy helped give me any answer. I'm just curious what happens when you have a denser side and a less dense side.

    Let's say you had two planets, one made of gold and one of fairy floss.
    The fairy floss planet would accelerate towards the gold planet more than the gold planet would accelerate towards the fairy floss planet, of course.
    But what happens if the gold planet and fairy floss planet are kissing (and somehow the planets don't collapse)?
    Both planets still try to move towards each other.

    Which of the following would be the result?
    a) Is it a stalemate and they both remain where they are in space?
    b) The fairy floss has greater acceleration and the combination moves them goldwards?
    c) The gold is stronger (or more inertia) and pushes the combination flosswards?
  12. May 20, 2009 #11
    Hi there,

    First of all, in your dual planetary system, the acceleration of both planets will depend on the reference you have. If you "stand still" compared to the two then, I agree with your statement. Otherwise, looking at the planets from the center of mass would result in an approach of equal velocity.

  13. May 20, 2009 #12


    User Avatar
    Science Advisor

    You are confusing two things here: acceleration (which is absolute) and movement (which is relative). If the planets are in contact, they don't accelerate anymore. Their movement depends on the frame of reference. In the frame where they where initially both at rest (before starting to accelerate to each other) they don't move after contact (conservation of momentum).
  14. May 20, 2009 #13
    Sorry for confusing things.

    Velocity is distance / time and acceleration is change in velocity / time so I just assumed acceleration implied some movement? Thanks for the other clarification, I did mean that they start kissing at rest; not colliding.

    So what you are saying AT is that if I was a long distance away from these two, and therefore not moving with them, then I would observe them remaining stationary relative to each other while they kiss; no movement would result from the density differential.

    So, even though the space time dimple resides towards the denser one; they would not try together to cozy into this dimple which would keep moving with them?
    Or, even though the denser one spits out more gravitons giving the less dense one more impetus to move; and the less dense one spits out less gravitons giving the denser one less impetus to move; no overall movement/acceleration will result?

    I just want to clarify because if you were standing on them they obviously would not appear to move so I just want to clarify the at a distance thing?
    Last edited: May 20, 2009
  15. May 20, 2009 #14


    User Avatar
    Science Advisor

    The distance is completely irrelevant here. It depends on your frame of reference if you see them move. If you want to now if they still accelerate while kissing - no, they don't.
  16. May 20, 2009 #15


    User Avatar

    Staff: Mentor

    Velocity has a direction and acceleration has a direction, so since acceleration is a change in velocity, if the acceleration is perpendicular to the direction of motion, there is no change in speed. Ie, acceleration can change the speed or it can change the direction of motion, the two components of velocity.
  17. May 20, 2009 #16
    Thanks. That was what I was asking.

    I still wonder though; sorry to do so.

    It occurred to me just before to wonder what would happen if you had a mouse in space with a much stronger rocket (nuclear) on its back; and an elephant with a less powerful rocket (ion); that still left the mouse combination less heavier than the elephant combination.

    Obviously if you have two identical rockets kissing each other in space and one is allowed to produce greater thrust than the other; than the combined system will have a resultant acceleration in the direction the greater thrust is pushing.

    So if our mouse and elephant were kissing in space, although the mouse system weighs less than the elephant system, its greater thrust towards the elephant would result in a resultant acceleration in the direction from mouse to elephant.

    So how does this differ for gravity?
    The gold planet exerts a stronger "pull" on the floss planet than vice versa.
    So the floss planet has a greater "force" acting on it than does the gold planet.

    When the planets are separated, and the observer is far away, this is seen from a distance as the floss planet accelerating faster than the gold planet towards each other; colliding closer to where the gold planet started.

    The same would be the case for the mouse and elephant. The mouse would accelerate faster towards the elephant than vice-versa.

    But if the planets are kissing (at rest) would not the greater force acting on floss planet (towards the gold planet) than the force acting on the gold planet (towards the floss planet) cause the total system to accelerate towards the gold planet as it does towards the elephant?

    Are the analogies not equivalent?

    Its hard to demonstrate with a rope analogy. Let's say you had two people in space.

    If the two people were actually separated in space and one was pulling in harder than the other, the pulling less harder one would move faster than the pulling more one. Or perhaps that is incorrect?

    If they were standing upside down to each other feet to feet and they had a rope between them which they both pulled then if they pull the same amount they will stay in balance. If one pulls harder then either the other has to resist equally harder or they collapse. So essentially the pull remains equal both ways.

    So essentially there is no way for this system to exert an uneven pull. But that isn't the case for the two planets - they continue to exert an uneven pull? Or are we saying that they start to exert an equal pull on each other? Or something else?

    Does the equivalence not apply in this case between the mouse/elephant and gold/floss planets? Or is it that even though the less dense floss planet has more impetus to move, once they come into contact, is their ability to realise this lost?
  18. May 20, 2009 #17


    User Avatar

    Staff: Mentor

    Oops, make that once a month (more or less). I must have been thinking of Mars or something. :blushing:
  19. May 23, 2009 #18
    AT. I'm betting that you are right. I just need help to understand it better.

    Here's something else I thought of that maybe someone can help me with please.

    Originally I wondered what would happen if you put a pole between the gold planet and the candy floss planet (which remember is somehow not crumbling).
    I wondered if the pole would make it simpler to understand them pushing each other.

    Then I wondered what would happen if you put a spring scale at both ends of the pole to semi-weigh each of the planets (each spring would be affected by the other spring I would guess?)

    Then I wondered what would happen if you put a spring scale on the ground, a pole on top of this, a spring scale upside down on top of this and lastly ourself on top of the upside down spring scale.

    Effectively we would be measuring together how much we weigh on the Earth and how much the Earth weighs on us but as a total combination.

    I imagine the measure would be the same so saying that the Earth is applying as much weight on us as we are applying on it? Is that correct?

    A simple way of course is to just turn a single set of scales upside down and we will see that the Earth weighs as much on planet 'us' as we do on planet Earth.

    So this would tend to suggest that the push is reciprocal and that the bodies would remain stationary with respect to each other.

    So, I assume that if you put two spring scales on top of each other that they would both show your weight as exactly half?

    But, the only thing that gives me some pause is that I was wondering if, even if the total of the two were the same as for a single spring, if somehow the bottom spring would compress slightly more due to the greater pull on the midpoint by the massier object.

    I'm not sure about any of that so does someone have the info to clarify for me that this isn't so?

    I guess the basic question can be simplified further as, if you have a long spring attached to the ground and have a platform on the top which you stand on then: does the spring compress closer to the ground due to the higher gravity there; or does the spring have constant compression throughout its height due to counteracting factors?

    Can someone help me?
  20. May 23, 2009 #19
    I was hoping you'd catch your error, excellent.

    Since the same surface of the moon remains pointed at the earth at all times, the moon only completes one full rotation about its axis per each full orbit around the earth therefore, every 27.3 days.
  21. May 25, 2009 #20
    Maybe I can ask another question that might make easier sense to help me with?

    If you again had a large mass and a small mass, but instead of a spring, put a bag of gas between the two so that the bag can only compress vertically.
    Would the gas be denser at the bottom of the bag towards the bigger mass?

    I know our atmosphere and oceans are denser (have greater pressure) towards the bottoms and I guess you could count the top layers as the smaller mass so I'm guessing the answer is yes? Is that correct?
  22. May 26, 2009 #21
    Hi there,

    In the sense that the mass of gas would be more attracted to the large mass, yes it would also be denser. But to see a noticeable effect, you would need a very large and a very small mass. Otherwise, the effect would be negligeable.

    Don't forget that our atmosphere, in this sense, is denser near the Earth's surface. The analogy to your example is the Earth's mass ([tex]10^{24}kg[/tex]) compared to the emptyness of space ([tex]\sim 0[/tex]).

  23. May 26, 2009 #22
    Thanks fatra

    Can we also think of the above atmosphere as the less denser mass and the lower atmosphere as the 'spring' in between?
    Would the upper layer of atmosphere then be pressing on the rest of the atmosphere below?
    Would this then be like: MASS (Earth) - spring (atmosphere less upper layer) - mass (upper layer)?
    Would this show how a 'spring' would compress less towards the smaller mass and more towards the the larger MASS?

    I'm leaning towards the idea that this wouldn't matter anyway.
    But it is just interesting to know things in their actuality.

    I would have assumed at first that a spring would compress evenly; but now I am leaning towards the spring compressing more at the denser end.
    Does that sound okay?

    Even if that were so, the greater MASS would have greater inertia than the lesser mass.
    So the greater spring compression near the bigger MASS may make no overall difference?
    It may sit in perfect balance anyway.
    Can someone help me with what the science says?
  24. May 27, 2009 #23
    Hi there,

    Nothing stops you from interpreting the atmosphere as different layer completely independant. As a matter of fact, for some problems, you might have to do just so. For others, you have to consider the atmospher as an entity, where the particles can move from the lower part to the upper part.

    Your question, and your thoughts are much more related to atmospheric physics, than to gravity at this point.

  25. May 27, 2009 #24
    Thanks Fatra

    But gravity is one of the principle drivers of atmospheric pressure by level though isn't it; acting on the particles which in turn act on each other?
    So wouldn't a resistance spring still vertically follow this pressure principle with greater compression/pressure at the bottom and less compression/pressure at the top?
    The gravity related behaviour would be the same overall wouldn't it; whether solid or gas or spring or atmosphere?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook