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Gravity question

  1. Feb 14, 2008 #1
    A question me and a buddy were pondering the other night.

    What causes gravity? Is it mass or weight? Would, say, a larger planet made up of gas have a stronger gravitational pull than, say, a smaller planet made up of dense, heavy metals such as lead? Does the mass or the weight determine gravity?

    Thanks for the help. Non physics guy here just curious about astronomy and such. If I screwed up the terminology, forgive me. Laymen's terms would be appreciated in your answer.
  2. jcsd
  3. Feb 14, 2008 #2
    Well, the term weight itself comes from gravity. Weight = mass * g (gravitational acceleration or gravity)
  4. Feb 14, 2008 #3
    Unfortunately, I don't know what that means. Would a larger, less dense (did I parse that correctly?) planet have more or stronger gravity than a smaller, denser one?

    Or if you understand what I'm trying to ask but screwing up the terms, can you give me an answer I can understand?

    I'm deducing from that equation that mass is the key factor.

    And what then puzzles me is how a black hole would have so much gravitational pull (right term?) without much mass? Isn't the matter compressed and compacted (crushed?) into a tiny mass? If so, why would it have such gravity? Or is it actually massive but since there's no light, the mass can't be seen?

    That was what we were pondering, really. Seemed that either gravity was caused more by density (if a black whole doesn't have much mass) or caused by mass but we can't see the mass of a black whole.

    Remember, this is the pondering of two people who are not physicists, astronomers, or scientists, so excuse any childish ignorance in my reasoning. If you can understand what I'm trying to say, even if I'm screwing up terms or not even phrasing it correctly, please forgive my ignorance and try to answer the question I'm trying to pose. And pretend I'm a six year so I can understand you.

    Thanks, guys.
    Last edited: Feb 14, 2008
  5. Feb 14, 2008 #4


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    No, it entirely depends on the mass - it is impossible to tell purely from gravity if the Earth is solid or is a hollow shell with a very dense object at the centre.
    There is an extra complication in the way you phrased it - if the Earth was twice the size it would have the same overall mass and the same gravity as far as a distant spaceship would measure it. But you would feel lighter on it's surface because you are further away from the centre.

    A black hole is a star crushed into a very small volume, it still has exactly the same mass as the original star

    There are three terms that have similair meaning to the public but very different meansing in physics.
    Mass - this is the amount of something, measured in kilograms. A big thing has more mass.
    Weight - this is the force you feel when you try and lift something on Earth. It is the gravity attracting the mass - if you go the moon the weight is less because there is less gravity, but it has the same mass.

    Density - this is how much space the mass is squashed into. So gold has a higher density than water. When somebody says gold is 'heavy' they mean it is dense. A small gold ring still weighs less and has less mass than a glass of water.
    Last edited: Feb 14, 2008
  6. Feb 14, 2008 #5
    mgb_phys, thanks for the tutorial. I'm guessing I'm at about a 6th grade level of physics here. I'm trying to comprehend the first part of what you wrote. Blowing my mind a little bit. Can't imagine the stuff that blows the minds of hardcore physicists.

    So let me try to pose my question differently.

    Two planets the same size, but one is made entirely of hydrogen and the other lead. Same mass, if I understand what you said, but the lead planet is much, much denser. Same gravitational pull for both or will the lead planet have a stronger one?

    So a black hole retains the same mass it had before it collapsed? I had the wrong idea about mass. I was thinking more like volume, size, area. Not sure how to picture it now.

    If you think you know what I'm asking even if I'm not asking correctly, just answer what you believe I'm trying to ask. It's possible I can't even ask the question properly. And thanks in advance for answering me. Your time and knowledge are greatly appreciated.
  7. Feb 14, 2008 #6


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    If the two planets had the same mass but were made of lead / hydrogen they would have the same overall gravity and so a moon would orbit them at the same distance.

    The strength of gravity increases with mass but decreases with distance - so you feel less gravity from an object a long way away.
    So if you stood on the surface of each, the gravitational field would be much stronger on the lead one simply because you are closer to the centre ( since lead is so much more dense the lead planet would have a much smaller radius) and so you would weigh more = feel heavier.

    But if you were at the same distance, say the lead planet had a hollow sphere around it to take the surface out to the same distance as the surface of the hydrogen planet then it would feel exactly the same.

    The reason a black hole is so weird isn't just that it has a lot of mass - by squashing the mass into such a small radius the strength of the field at the 'surface' of a black hole is incredibly strong.

    The importnt thing to understand is that mass is a property of an object and doesn't depend on where it is. Weight is the force pulling an object toward something else - it depends on the mass of the object, but also on the mass of the thing doing the pulling and the distance between them.

    This is especially confusing in the US system which uses 'pounds' for mass in everyday language when they are really units of weight. It is a little clearer in metric.
  8. Feb 14, 2008 #7
    The picture's getting clearer but need a little clarification.

    These two planets, let's say they are the same size. If you measured them, they'd be identical. But one is made up of light material (hydrogen or helium or whatever would be construed as light) and the other is made of heavy material. [like if I had two marbles the same size, though one is made of light plastic and the other is steel]

    The one made of heavy materials has more gravity then? If I understood your reply below, it would be the same on both planets because their size is equal. Did I get it?
  9. Feb 14, 2008 #8


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    If they had the same size AND the same mass they would be identical. Otherwise the one with more mass would have more gravity.
    It's like you had a wooden marble and a plastic one with the same size but a steel core so they weighed the same and were the same size - the gravity on the surface would be the same.

    There are two differences here:

    The 'amount' of gravity depends on the mass - so the sun has quite a lot of gravity

    The force due to gravity depends on the gravitation field strength at that point - which depends on the mass and the distance. So your weight is due to the gravity from Earth because you are close to Earth this is much stronger than the pull on you from the Sun because the sun is further away.

    So a denser planet doesn't have more total gravity but it does have a stronger gravity field because you can get closer to the centre than a less dense planet.
  10. Feb 14, 2008 #9
    By George, I think I may have it. Didn't help that my concept of mass was incorrect.

    Tell me I have this right.

    Let's say God reached out from the heavens, grabbed Jupiter in his hand and squeezed. And when he let go, Jupiter was much smaller but the same mass (hope I got this right). Then he grabbed me and put me down on the surface. I would feel a stronger gravitational pull toward the center than I would if I had landed on Jupiter before it was given the squeeze, right? Because I'm closer to the center (i.e, same mass as before but denser).

    Did I get my mind about this correctly?
  11. Feb 14, 2008 #10


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    Absolutely correct - well done !!!

    But we, and Jupiter's moons wouldn't feel any difference.
  12. Feb 14, 2008 #11
    You had to add to it, didn't you?

    So let me see if I can reason this out. We wouldn't feel a difference on earth because the earth's pull on us is much greater than Jupiter's pull on us. If that's what you meant, that's easy to conceptualize.

    But Jupiter's moons not feeling any difference in gravitation pull is a puzzler. Here is my best guess at an explanation, and I hope that it's right because it is so simple and elegant, it deserves to be correct.

    Jupiter's moons aren't affected because they are still equidistant to the center of Jupiter, regardless of the planet's density? As long as the mass is the same, doesn't matter if it's larger (less dense) or more dense, because the center remains the same distance from the moon.

    I hope this is right because any other explanation will probably be beyond my comprehension.
  13. Feb 14, 2008 #12


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    Exactly !
    We feel the same effect because we are the same distance from Jupiter we were before, and it's mass hasn't changed. Only the effect on the surface has changed, and that has only changed because the surface has moved.
  14. Feb 14, 2008 #13


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    You probably picked-up on this by now, but just to make sure...
    Density=mass/volume, so if two planets have the same mass and volume, they have the same density. Realistically, you cannot have a metallic planet and a gaseous planet with both the same mass and volume.
  15. Feb 22, 2008 #14
    mass = how much matter an object has

    weight = the gravitational force exerted on you by the earth (F = ma), where F is the weight, m is your mass, and a is the acceleration. Basically, wieght is your mass * the acceleration at which things fall on Earth near ground level, which is 9.8 (g), so...
    F = mg on Earth. Weight should actually be measured in Newtons on scales rather than pounds, kilograms, etc.

    Density = mass/volume. Why is a brick of concrete heavier than a brick of styrofoam? They have the same volume... but the brick of concrete has more mass, so... it has more density.
  16. Feb 22, 2008 #15
    If the mass of planet A = mass of planet B. Then if Planet A had a higher volume but lower density and planet B has a lower Volume but higher Density. Then the gravitation pull of either A or B are the same.
  17. Feb 23, 2008 #16


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    But since You understand it now, I'll try to confuse you: The surface gravity of the two planets will be different. Why?
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