Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mg= A Pseudo Force?

  1. Jun 6, 2005 #1
    I was reading about the Principle Of Equivalence as proposed by Einstein which proved that Inertial Mass and Gravitational Mass is one and the same thing.
    Further it proved that if we are in a spaceship and flying in space with acceleration 'g' upwards, the person is bound to the surface of the rocket and there's no experiment one can do to distinguish if the person is on earth or in a spaceship.

    So maybe what appears to us as the earth's gravitational pull, maybe just a pseudo force, maybe we are accelerating upwards? and we are pulled towards the surface of earth who knows?...It can be possible that we are in the wrong coordinate system, we have chosen / or we are living in a coordinate system , which leads to emergence of a pseudo force 'mg' and therefore we need to consider whenever we consider vertical motion as per Galilean Transformation.

    And as all pseudo forces are directly proportional to mass , mg does the same.
  2. jcsd
  3. Jun 6, 2005 #2
    In Einstein's thought-experiment, the 'floor' of the spaceship is equivilent to the surface of the Earth, and the passenger is equivilent to an earth-bound object. So it would be the earth's surface that is accelerating, and only us by contact with it, and we are drawn to it by our inertia. As the earth's surface cannot be accelerating away from the centre of the earth in all directions, that's kind of where the equivilence ends. Also, the force applied to the passenger ma is not a pseudo-force, so nor would that applied to an earth-bound object mg, where g is just acceleration: 9.81 m/s^2.
  4. Jun 6, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    While this is certainly plausible, you will have trouble with it if (i) we discover the Higgs and (ii) if gravitons actually exist.

    If we discover the Higgs, then there is a chance that "inertial mass" may not be identical to "gravitational mass". So far, these two are one of the same. However, as in EM interactions and QED, there are higher-order corrections and self-energy interactions if we can formulate a QFT version of gravitational interactions. If this happens, then gravitational force will be like any other forces, and what we call as "mass" will be even more well-defined in terms of its inertial and gravitational properties separately.

  5. Jun 6, 2005 #4
    The problem with this idea is that the earth's surface doesn't accelerate upwards. Instead it spins and revolves about the sun. However the sensation on the feet (when one stands) is almost the same in both situations.
  6. Jun 7, 2005 #5
    That's not because of angular momentum though. The same would be true were the earth were not spinning or orbiting. Of course, there would be no life on the planet to notice this, but...

    Why does orbiting only have one t in it? Should it not be pronounced or-bite-ing?
  7. Jun 7, 2005 #6
    Could gravity be a nuclear reaction force?

    Because when the rocket engine accelerates the floor upwards in intergalactic space, the human standing on the floor will feel pressure on his feet. A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction.
    Last edited: Jun 7, 2005
  8. Jun 7, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Whoa! Let's not get out of orbit with our imagination here!

    Why would you need a "nuclear reaction" here? I could simply have compressed gas and use that as propulsion. There's no nuclear reaction of any kind there. It's just "evaporation". Yet, it will produce the identical effect as "gravity".

  9. Jun 7, 2005 #8
    To be exact they are said to be proportional to each other, i.e. inertial mass is proportional to passive gravitational mass.
    There's a twist to that. It the rocket is accelerating in the absense of gravitational tidal forces then no tidal forces will be detecable inside the rocketship cabin. If the rocket ship is at rest on the surface of the earth then tidal forces will be detectable inside the rocket ship cabin. Tidal forces are detectable if the device is sensitive enough and therefore you can tell the difference. Only when the gravitational field is uniform are you unable to tell the difference or when the device is not sensitive enough to detect the tidal forces.
    It was Einstein's view that the opposite is true. That what appears to us as what you call "pseudo force" (more properly known as "inertial force") is actually a gravitational force.

    That is inconsistent with the idea of general relativity where all coordinate systems valid.

    For details please see Einstein's Gravitational Field at

  10. Jun 7, 2005 #9
    Actually the rocket engine burns fuel (helium) which is a nuclear process. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The burning process accelerates the mass.
  11. Jun 7, 2005 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Then you've missed my counter example. I could produce the very same effect of "acceleration" simply by using compressed air! No "nuclear reaction" of any kind.

    Thus, your assertion that "gravity could be a nuclear force" is false. No nuclear reaction of any kind is involved in my example. Yet, I could duplicate the same dynamics.

    Furthermore, "rocket engines" that we have currently doesn't use any "nuclear reaction". They are all chemical involving atomic/molecular reactions. The shuttle booster and external rockets do not use "nuclear reactions".

  12. Jun 7, 2005 #11
    But combustion is defined as the burning, or oxidation, of matter to produce energy. Two substances are necessary for combustion to occur - a fuel and an oxidizer. The same principle is also valid in liquid-propellant rockets.

    My point was that discrete particle interactions are required to generate motion.
    Last edited: Jun 7, 2005
  13. Jun 7, 2005 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    But how does what you have said here implies "nuclear reaction"? A "nuclear reaction", by definition in physics, involves interaction at the NUCLEAR level. This include fission, fusion, transmutation, radioactivity, etc. What you have said is CHEMICAL reaction, at the atomic/molecular level.

    Remember, you're asking if gravity and nuclear reaction/force are the same. I have produce what is nothing more than a "conservation of momentum" example of the expulsion of gasses to generate an acceleration. It required no reaction of any kind, be it chemical or nuclear. Somehow, you didn't understand this. I've just generated motion and acceleration (those two are not identical) without any "nuclear interaction". Thus, "gravity" is not equal to "nuclear force".

    BTW, do you even know what "nuclear forces" are? The strong interaction mediated by gluons have VERY different properties than gravity.

  14. Jun 8, 2005 #13
    What do you mean by 'equivalence' ends? The inertial mass and Gravitational mass equivalence still remains but your arguement that 'earth cant be accelerating outwards is true, if that is the only possible way for 'mg' to emerge as a pseudo force.

    If we assume that 'mg' is a pseudo force, as I told before , the only possible reason can be that we are living in a coordinate system that leads to emergence of this 'mg' force..(?) .

    On the other way , if someone outside solar system was observing earth, and if he applied a force on earth in some direction, would he account for the force 'mg'?....If no ... then his coordinate system is correct .. and if he takes in account 'mg' then it is a kind of universal force that should be accounted for whenever we take into account the forces on earth.So probably that argument is valid.

    I havent studied Einstein's Gravitation yet but I am quite sure about the fact that choosing the wrong coordinate system will always lead to some kind of pseudo force.As per Galilean Transformation, a person in rest or in uniform motion will see all physics laws as they are but the one accelerating coordinate system will have to take into account a pseudo force 'mg'


    I was wondering , suppose that our whole solar system was in some kind of matter 'A' and that matter is accelerating upwards/downwards with 'g' ...then we ppl on earth will definitely feel that force..wont we?...and that 'mg' is what I call could be a pseudo force..?
  15. Jun 8, 2005 #14
    Analogy, rather than equivilence, would have been a better word. The OP has taken the man-in-the-box scenario and run with it, to the point of supposing gravity has the same causes as a space-ship's acceleration (propulsion). I'm simply pointing out when to stop trying to relate gravity to a rocket. It has nothing to do with the ways in which mg may emerge.
  16. Jun 8, 2005 #15
    But what generates spin?
  17. Jun 8, 2005 #16
    On a cosmological scale? Gravity. But gravity does not necessitate spin and spin does not cause gravity. For example, a black hole or neutron star may not necessarily be spinning, but it still has a gravitational field.
  18. Jun 8, 2005 #17


    User Avatar
    Science Advisor

    Absolutely right! English used to have these spelling rules, but it is totally degenerating. Other examples: limitting has totally become limiting; focussing is now focusing; travelling is now traveling; accommodate is slowly becoming accomodate. Sigh...
  19. Jun 8, 2005 #18
    And may I know whats going on here?...english classes?
  20. Jun 8, 2005 #19
    No. English clases!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook