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

How is gravity considered as the weakest force?

  1. Oct 7, 2012 #1
    Between the four main forces of nature how can gravity be the weakest when gravity seems to govern the entire movement and attraction of mass within the universe.
    I know that more than 80% of the universes matter and more than 90% of its mass/energy is undetectable dark forces (dark matter and energy), but for anyone of the other three main forces to be dominant over gravity it would seem like one or each of them would have to be a governing or binding force of dark fluid as well (the combination of dark matter and energy).
    If someone could be as detailed as possible when explaining how and/or why gravity is the weakest force of nature I would greatly appreciate it.
     
  2. jcsd
  3. Oct 7, 2012 #2
    Well I know the Electromagnetic force is way way stronger than gravity because when you jump off a building, you wont plummet to the core of the earth, the ground will stop you. That is essentially the EM force of the outer electrons in the atoms of your body repelling the electrons in the atoms of the ground. Also the covalent and ionic bonds that hold all the individual atoms together(which is also EM).
     
  4. Oct 7, 2012 #3
    Ok. A magnet weighs what... 50 grams?

    Yet I take 2 magnets and the magnetic force of the magnets, each weighing 50 grams, beats the entire freaking 10^30 something tons of the entire Earth's gravity.
     
  5. Oct 7, 2012 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You don't see the other forces directly in everyday life as most objects do not have a (significant) net charge. Magnets, and the repulsion when you hit the floor give some hint how strong the electromagnetic force can be.

    However, you can compare the forces between elementary particles: Calculate the gravitational attraction between two protons, and compare it with the electromagnetic interaction. You will see that those differ by a factor of 10^36 or something similar.
    The strong force has a tiny range, but within that range it is even stronger.
    The weak interaction is a bit tricky, but in setups where it is relevant it is much more important than gravity, too.
     
  6. Oct 7, 2012 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The difference between Gravity and 'the others' is that is only has one sign; it is always attractive. So, unlike the attraction and repulsion due to the equal numbers of protons and electrons in a lump of stuff, which mostly cancel each other out, every particle in a lump of stuff is attracting every other particle in any other lump of stuff. That means gravity, despite being relatively 'weak' has an effect that goes on 'for ever' and is never cancelled out.
    Anyway, it's not a beauty contest. Each one will beat the other under the appropriate circumstances.
     
  7. Oct 7, 2012 #6
    I think it helps to stop thinking that gravity is a force at all, as Einstein explained it is the distortion of space. Just imaging things constantly falling faster and faster, satellites are always trying to fall to Earth as is the Earth constantly trying to fall into the Sun. Space is like water, if you get into water then the water has to move out of the way but is always trying to fill that space back up. Maybe I am wrong but this is how I see it.
     
  8. Oct 7, 2012 #7
    Mfb stated it well!
    Gravity seems to be effective over massive distances, such as the span of a galaxy, but on atomic levels it is nearly UN-detectable. I like to think of the gravitational force between two nucleons vs. the strong nuclear force between them....
     
  9. Oct 7, 2012 #8
    Yes but only across a short distance.Take one of your 50 gram magnets and drop it from any height anywhere on Earth.Now seperate your 2 magnets by a couple of feet and measure the magnetic force.Don't know who coined the phrase weak force but it certainly seems a misnomer if you take into consideration it's universal pulling power.
     
  10. Oct 7, 2012 #9
    Because it takes a lot of mass to exert the same amount of force as the others.
     
  11. Oct 7, 2012 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Place both magnets on the ground, on top of each other. The lower magnet now has:
    - magnetic force upwards (from a tiny 50g-object)
    - gravity downwards (from everything below it, including mass at close distance)

    Gravity is proportional to M/r^2, and the mass of earth is related to its radius to the third power, so the gravitational pull on the object actually increases for a larger earth radius (at constant density), even if some parts of earth are far away.

    Repeat the same experiment in space (without that extremely big mass nearby), and you won't notice gravity between the magnets at all.
    As alternative, hypothetical experiment, remove all electrons from earth, leaving only the protons and neutrons - this removes just ~1/4000 of the mass, so gravitational attraction remains nearly the same. However, the result will be a BIG explosion, as the electromagnetic repulsion is so extreme that gravity is completely negligible.
     
  12. Oct 7, 2012 #11
    You might not notice gravity acting on the magnets but eventualy they will with or without a big mass nearby, you can't get away from mass, distance is no object to gravity.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook