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Does Gravity mean gravitational force?

  1. Jul 21, 2007 #1
    I feel sort of confused with some concepts. Does Gravity mean gravitational force? Is that every object has different gravitational force according to their mass? So what is 9.81m/s^2? This is not gravity as i thought before, isn't it? THis is only the acceleration due to gravity? SO gravity is simply a force? Hope you can clarify them for me.
  2. jcsd
  3. Jul 21, 2007 #2
    you can think of force as a push or pull on objects that causes them to accelerate, you can think of mass as an objects inherent resistance to being accelerated (hence, objects with more mass require more force to accelerate), acceleration is self evident I think
    this is a description of F=ma... true for all objects

    gravity is an attractive force between two objects which depends on both masses, a constant, and the distance between the objects...
    this is a description of the equation...
    Force of gravity=a gravitational constant x mass of object 1 x mass of object 2 / distance between objects squared
    ... true between any two objects with mass

    1) F=m1a
    2) F=Gm1m2/d^2
    ...substitute equation 1 into equation 2 (or vise versa)

    m1a=Gm1m2/d^2 as you can see the mass of object 1, the mass of the object falling to earth, cancels out (this is because of the mechanism of gravity described by Einstein)... and we are left with

    a=Gm2/d^2 where m2 is the mass of object 2

    now lets say object 2 is the earth, and object 1 something falling on the surface of earth, as you can see for yourself the mass of the thing falling doesn't affect its acceleration, the only thing that affects its acceleration is the mass of the earth and its distance from the earth, now if you plug in the mass of the earth, G the constant, and the distance (radius of the earth)... you get 9.81 m/s^2

    all objects on the surface of the earth accelerate towards the earth at the same rate (9.8m/s^2) regardless of the mass of the object... but if you change the mass of the earth (change planets) or move away from the surface considerably, the acceleration changes according to the above equation

    gravity is only 1 type of force, there are many different forces...
    Last edited: Jul 21, 2007
  4. Jul 21, 2007 #3


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    Gravity in the static sense is the force between masses
    as a result only of their mass.

    There are things like gravity waves that are dynamic
    aspects of gravity, but that's really very esoteric and
    unusual relating to relativity and very energetic

    Force = G * mass1 * mass2 / radius^2.

    Where the gravitational constant is:
    G = 6.67259 * 10^-11 in units of [m^3 kg^-1 s^-2]

    If we're talking about the earth's gravitational pull
    on objects near the surface of the earth, the force
    ends up being nearly 9.81 Newtons for each kilogram
    of the mass of the object due to the mass of the earth.
  5. Jul 21, 2007 #4
    he doesn't understand Newtonian gravity, and you bring up gravitational waves... might as well explain gravitons, black holes and string theory while your at it, hehe =)
  6. Jul 22, 2007 #5
    Is the gravitational force or gravity between objects come from the earth? Two objects will attract to each other because of the force that comes from earth? for example, a desk and chair in a room. Are the gravitational forces between objects are different? like the force between desk and chair and the force between car and truck? Since the unit for force is Newton, but the unit for acceleration is m/s^2, so they are two different things, right? 9.81 is the acceleration for all the things to fall on earth, but they have different forces?
  7. Jul 22, 2007 #6


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    The gravitational force is no way unique to the Earth. Anything with mass exerts a gravitational force on anything else with mass. You pull on the earth with the same force the earth pulls on you, it is just that that force is able to affect you much more than it affects the earth, since the earth is much more massive. The same goes for you and anything else. You pull on the keyboard in front of you with gravity and the keyboard pulls on you with and equal gravitational force. This force is just not noticeable, since it is VERY small.

    Yes, g=9.81m/s^2, is an acceleration. It is the acceleration CAUSED by the gravitational force between an object near the surface and the Earth. The FORCE of gravity between an object and the Earth is a different quantity measured in different units, and is NOT the same for all objects near the surface .
    Last edited: Jul 22, 2007
  8. Jul 22, 2007 #7
    so where does the gravitation force come from? for example, where does the gravitational force come from between a desk and a chair?
  9. Jul 23, 2007 #8


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    It's simply a function of mass. Any physical object warps the spacetime in its vicinity. If another object, which is also warping its immediate vicinity happens along, the two 'gravity wells' interact. The more massive object will have a greater distortion effect, and thus dominate the interaction between the two. And remember that it's measured from the centres of the two masses.
    Last edited: Jul 23, 2007
  10. Jul 23, 2007 #9
    you need to study the principles of general relativity a bit... it's not as easy to explain as two Newtonian equations, but like Danger said mass bends space-time, and objects travel through space time so are affected by the depressions in space-time... picture this, a piece of paper with giant depressions on it... now we take a pencil and try to draw a straight line on this paper. We do not see space-time, we see space... we make the paper flat (going from 4d to 3d, from space-time to just space), and the line that we thought was straight, now curves towards the places that had depressions (the places with mass). Fyi the faster you draw the line in 4d space-time, the less the line will curve towards the mass when view in 3d space, this is why faster objects deflect less, and why light barely deflects at all (unless the depression is enormous). This is all that Newtons forces are, objects curving or accelerating towards mass, there is no mysterious attraction, it only seemed this way because we were thinking in 3 dimensions, when the mechanism of gravity acts in 4 dimensions... When we start thinking in 4-dimensions the illusion of mysterious forces goes away and it all makes sense, but since you don't know 4 dimensional geometry and tensor calculus (and either do I), you can think of everything I have told you as the true origin of forces instead of some mysterious pull from the earth.... Stephen Hawking explained it like this... it's like flying in a plane, the terrain below gets hilly, and you see the shadow of the plane below meander and curve around the hills, even though the plane was going straight the whole time.... in this analogy you are going from 3d to 2d... but same principle
  11. Jul 23, 2007 #10


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    I've never heard that one about the shadow. I like it.
  12. Jul 23, 2007 #11
    This is my point of view:

    Every object has two properties:

    a) gravitational mass, which is the mass in the equation
    F= (G .m1.m2 )/d^2

    b) inertial mass, which is the mass in the equation
    F = m. a

    Note that we use the same "m" for both equations, but there is no reason for doing so.

    But, it seems, that the two masses are the same: if an object A has double the gravitational mass than an objetc B, objetc A will have double the inertial mass than objetc B.

    Thats why all objects are acelerated at g when they fall. A is attracted by a force twice as B ( more gravitational mass ), but A is twice more difficult to accelerate than B ( more inertial mass ).

    Its not obvious than the two masses are equal. As long as I know experiments have been made in order to veryfy this equality.

    Ive read something about that relativity tell us why the two masses must be equal but I dont understand it.
    Last edited: Jul 23, 2007
  13. Jul 23, 2007 #12
    As I tell my own students, the term *gravity* is devoid of any concrete meaning. The most frequent evidence of this is that many, many students think of *gravity* as a substance that is possessed by an object, which is nonsense. I do not permit my students to use the term *gravity* and this problem completely goes away. When most people say *gravity*, what they really mean is *gravitational interaction*, which is a valid term for the behavior that two objects experience when attracted to each other without any electrical influences. Sometimes, people mean *mass* when they say *gravity*. The more mass an object has, the stronger will be its gravitational interaction with another object, but there is no substance or property called *gravity* that is involved. Eliminate *gravity* from the vocabulary and speak in terms of *gravitational interaction* and all will be well.
  14. Jul 23, 2007 #13
    why would two objects attract to each other? Does earth make them do it or other reason?
  15. Jul 23, 2007 #14


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    The "why" is that gravity exists and is
    the effect of any massive objects having
    force effects on other massive objects.

    Gravity exists and is a property of the
    physics of the universe. It doesn't
    depend on the earth specifically.

    Any object with mass or energy
    (i.e. anything) causes a gravitational
    force field.

    If the earth didn't exist, the
    other planets would still go around the
    sun due to gravity, and the rocks on
    Mars still wouldn't fall up into the
    Martian sky.

    If our solar system didn't exist, the
    rest of the stars and planets in the
    galaxy would still have their own
    gravitational fields and would still
    orbit the center of the galaxy.

    If our galaxy didn't exist, other galaxies
    would still have gravity..

    If nothing at all existed in the universe
    besides one blueberry, the blueberry
    would still have a gravity field. :)

    The reason the earth's gravity seems
    important and dominant to us is
    because we live on the earth and its
    gravity field produces the most
    easily perceived force in our lives,
    our weight and attraction 'downward'.

    If we were on the moon we'd feel
    the moon's gravity as being the most
    significant force effect on us.
  16. Jul 23, 2007 #15


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    Earth has absolutely nothing to do with the gravitational interactions between two other objects, other than the fact that it adds a larger component to the situation. For instance, a book sitting on a table has a minor mutual attraction to the table. It wouldn't be enough to even keep them together against the forces of their temperatures bouncing their molecules around. The fact that Earth is acting in a large way upon both of them ensures that the one on top will push against the one on the bottom. (And it has nothing to do with mass in the way that you might be thinking. It's entirely mutual, as can be proven by the fact that you can prop up an unbalanced table with a book under one leg just as easily as you can support the book with the table.)
  17. Jul 23, 2007 #16
    Asking what causes gravitational attraction is a completely different question from asking how to *describe* gravitational attraction. Gravitational attraction can be described in at least two ways. One uses the concept of *force* and the other uses the concept of *spacetime curvature*. The former was invented by Newton and the latter was invented by Einstein. No one knows what *causes* gravitational attraction and we may never know.
  18. Jul 23, 2007 #17
    when you say, "what causes gravitational attraction"... you essentially mean why mass affects space-time, right?

    while we're on the subject... could someone please explain why pressure bends space time? Is it because pressure is a type of energy? if it's too technical or requires mathematics don't bother. Thank you.

    How do you get negative energy (of the sort described in inflationary cosmology)? I read somewhere that anti-particles are particles traveling back in time... is this true?

    now im getting carried away =) but I would appreciate any links, answers, books to read to get these kinds of answers
    Last edited: Jul 23, 2007
  19. Jul 23, 2007 #18


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    No, the earth does not cause gravity. Gravity is an inherent property of everything with mass. You have gravity. I have gravity. Everything with mass has a gravitational field which causes everything else with mass to be attracted to it to some extent. The earth happens to have a much stronger gravitational field since it has much more mass.

    Now, are you asking something like, "Why does gravity exist in the first place?"

    I don't think anyone knows the answer to this question.
    Last edited: Jul 23, 2007
  20. Jul 24, 2007 #19


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    Dr. Robert Forward is the only guy that I've ever met who understands negative energy. On the one fortunate occassion that I got to speak with him, he certainly couldn't manage to educate me about it. In fact, we gave up after I couldn't understand negative matter.
    As for antimatter being matter travelling backwards in time, I believe that it's just a simplification used by particle physicists. If you examine a particle interaction, then play it backwards on video, it looks exactly like an anti-particle interaction going forward in time. This is definitely a ZZ or Arildno question, though.
  21. Jul 24, 2007 #20
    ZZ or Arildno question?
    "If you examine a particle interaction, then play it backwards on video, it looks exactly like an anti-particle interaction going forward in time"

    I read a brief idea based on this concept, instead of a particle and an antiparticle annihilating to form photons, a photon hits a particle with enough momentum, to send it moving backward through time in the form of an antiparticle... the idea was that perhaps there are very few particles instead of the billions that we detect, we just keep bumping into duplicates.
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