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How many kinds of force exist between Planets?

  1. Jan 30, 2012 #1
    As we know there is gravity force between Planets in order to hold them in orbits,
    Each planet has different speed in term of its rotation, will it create any force in any way?
    does anyone know how many kinds of force exists between planets as we know at this moment?
    Thanks in advance for any suggestions
  2. jcsd
  3. Jan 30, 2012 #2
    I suggest you look up tidal forces. You have to take these into account when considering planets as large bodies, not point like objects (for many purposes it is sufficient to consider them as point like objects).

    Consider the Earth and the Moon. Gravity falls off with 1/r^2 yeah? This means that the gravitational force from the Moon on the Earth will be greater on the side of the Earth closest to the Moon. Get it? This is the cause of tidal bulges. You may be aware that the Earth isn't spherical, it's actually bulging a bit in the middle. I should also mention that the rotation of the Earth also contributes to this bulge.
  4. Jan 30, 2012 #3


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  5. Jan 30, 2012 #4
    There are only four fundamental forces in nature (that we have observed at least):
    1) Gravity
    2) Electromagnetism
    3) 'Weak' Nuclear force
    4) 'strong' nuclear force

    The only fundamental interaction between planets is gravitational (the others are completely negligible).

    Because planets are complex objects, their gravitational interaction can give rise to addition effects which are sometimes referred to as independent forces---i.e. the 'tidal force', which is actually the difference in gravitational force between different parts of the planet.

    Additionally, especially in situations involving orbits and spin, the gravitational force can exert a 'torque' on/between planets which can make them spin-up, slow-down, etc.
  6. Feb 1, 2012 #5


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    Dear zhermes,

    I have a question regarding this statement. The two datapoints cited below indicate to me that significant electromagnetic effects take place between Jupiter and Io, and also at galaxy 3C303. Granted neither of these examples specifically relate to interactions between planets. However, since the effects do take place at the planet-to-moon scale, and also at the galactic scale, how is it possible to rule them out at the planet-to-planet scale? Perhaps they can and do take place in another solar system which we presently cannot observe?

    Thank you for your consideration.

    Respectfully submitted,

    1) As Io circles around Jupiter and through the plasma torus, an enormous electrical current flows between them. Approximately 2 trillion watts of power is generated. The current follows the magnetic field lines to Jupiter's surface where it creates lightning in the upper atmosphere.
    http://www.planetaryexploration.net/jupiter/io/io_plasma_torus.html [Broken]

    2) About two billion light-years from Earth, there's a huge jet of matter emanating from the core of galaxy 3C303. The jet carries a current of 10^18 amps, making it by far the highest electric current ever seen.

    Researchers at the University of Toronto made the find as they observed the interaction between the jet and radio waves found around the galaxy. The radio waves suddenly changed its alignment in response to the jet, which lead researcher Philipp Kronberg says is "an unambiguous signature of a current." The researchers say this incredibly high current - the equivalent of a billion billion amps - is being created by powerful magnetic fields found inside galaxy 3C303's central black hole. The jet itself extends out some 150,000 light-years, making it longer than the diameter of the entire Milky Way.

    Our analysis strongly supports a model where the jet energy flow is mainly electromagnetic.


    Evolution of Ohmically Heated Hot Jupiters

    Magnetoastrocoolness: How Cosmic Magnetic Fields Shape Planetary Systems
    Last edited by a moderator: May 5, 2017
  7. Feb 1, 2012 #6


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    No one is saying that there aren't ANY interactions other than gravity, just that the electromagnetic effects are negligible when compared to the interations through gravity. Given the right circumstances there can be extreme interactions involving the EM force, but these only happen under exceptional circumstances such as the black hole you listed.
  8. Feb 1, 2012 #7
    Even these two cases are negligible from a dynamics standpoint. Gravity only has a single type of charge---and mass attracts all other mass, and thus it compounds on large scales. Electromagnetism has opposite charges, and is so strong that it forces all macroscopic objects into virtual neutrality.
    Last edited by a moderator: May 5, 2017
  9. Feb 2, 2012 #8


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    While I do not doubt there are some exceptional cases where EM plays a role, gravity dominates in the cosmos.
  10. Feb 2, 2012 #9
    Does anyone have any suggestions on how to determine the amount of force from Jupiter's Gravity force affecting Earth?
    Thanks everyone very much for any suggestions
  11. Feb 2, 2012 #10
    @ oem7110

    Yeah, Sir Isaac Newton has a few.
  12. Feb 2, 2012 #11
    Could you please give me any suggestions on where to find the formula?
    Thanks everyone very much for any suggestions
  13. Feb 2, 2012 #12
  14. Feb 2, 2012 #13


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    Google "Gravitational Force Calculator" and you should find a few good ones. You can input the masses of both the Earth and Jupiter, and input whatever distance you want and see the force that each exerts on the other.
  15. Feb 2, 2012 #14
    If there is a huge amount of charges on Jupiter, then it must create some force between planets, I would like to know how much charges are on Earth surface?
    Does anyone have any suggestions on how to calculate this force too?

    Furthermore, if the planet is far away from Sun, the planet's Rotation Period is faster when comparing with the planet closer from Sun, I think this Rotation speed may generate some kind of force too. Does anyone have any suggestions?

    Rotation Periods (Earth days):
    Jupiter : 0.41354
    Saturn : 0.445
    Neptune : 0.67125
    Uranus : 0.7196
    Earth : 1
    Mars : 1.026
    Mercury : 58.646
    Venus : 243
  16. Feb 2, 2012 #15


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    There are effectively none on the surface. The charges come from the top of the atmosphere from ionized particles. Ionization creates both types of charges, so there wouldn't be an imbalance. However if you throw in a magnetic field then you would get movement of the charges in different directions, leading to the generation of a "current" as the particles move through space.

    I'd bet that any force from an imbalance of charges is completely negated by the particles within the Solar System itself. The Solar Wind is a huge generator of charged particles that are continually streaming out.

    What kind of force are you referring to? There won't be one between it and the other planets, but on the planet itself there would be several fictitious forces such as centrifugal force. Also, the rotation would have a tidal locking effect on nearby small bodies, such as moons, but at the distance between planets it is completely negligible.
  17. Feb 3, 2012 #16
    Gravity is a very weak force between planets, but of course not negligible in many situations, as others have pointed out. I'd just like to stress, however, that <all caps> the entire universe! <\end all caps> is permeated, is filled with electromagnetic radiation. The entirety of the space between planets and galaxies is filled with oscillating through non-zero values of the electric and magnetic field. That is the source of our experience of light.

    Is this a "force between the planets"? It certainly is a force between stars and planets. When a chunk of matter like the Earth experiences a change in energy by absorbing radiation, is this not a force? We can think of force more generally than just "pushing" something. If there is a change in energy, that's how we know there has been a force.

    Then in the same, but a much smaller way, I myself experienced a force from the planet Jupiter when my eye absorbed a tiny amount of radiation emitted from the planet as I stared at it this very night.
  18. Feb 3, 2012 #17
    Let assume all raditiation to be a constant values and the amount of Jupiter's radiation from Earth to be 100, does anyone have any suggestions how to determine the Mar's raditation from Earth?
    Thanks everyone very much for any suggestions
  19. Feb 14, 2012 #18
    Say the Earth's distance from Jupiter is Rj(t) [note here Rj is a function of time, t, since in orbit, the Earth-Jupiter distance is variable] and say the Earth's distance from Mars is Rm(t).

    Say 100 Watts per square meter of radiation are received on Earth from Jupiter. Assuming a spherical spread to the radiation (the inverse square law) you could set up two fractions like X / 100 = Rj(t)^2 / Rm(t)^2 where X is the amount of radiation received from Mars. This would only be true if Mars was emitting the same amount of radiation as Jupiter, which it most certainly is not. To account for that you could multiply X by a fraction, like say we approximated that Mars emits one millionth (10^-6) the amount Jupiter does then we could rewrite the expression as X*10^6 / 100 = Rj(t)^2 / Rm(t)^2 where X is the amount of radiation received from Mars.

    But that is all kind of stupid, I recommend you use something like the Stephan-Boltzmann expression for the amount of radiation emitted by an object (in watts per square meter) as sigma*T^4 where sigma is the Stephan-Boltzmann constant and T is effective temperature. Use some approximate Mars temperature like 200 Kelvin and then let that fall off over the square of the Earth-Mars distance. You will find that it is a tiny tiny amount of radiant energy, but non-zero. There may be more energy in the sunlight reflected by Mars to Earth. To do that problem all you'd need is the Sun's output reduced by the square of the Sun-Mars distance, multiply by the fractional reflectivity of Mars (like it reflects 1/4 of it or something) and then reduce that by the Earth-Mars distance squared.

    Is this a homework problem of yours or are you just interested?
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