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I was having a debate the other day with a work colleague where I explained that gravity is a weak force because it is easily broken. Then I remembered a lecture by someone, I forget who, that explained gravity is very weak because you can break its influence just by jumping or lifting a pencil, etc.

He countered that with something along the lines of 'that even though the pencil or your body is being moved away from the source of gravity it is still affected by gravity and thus it has weight'.

Is jumping a good example of gravity being a weak force?

P.S. You can probably tell, my colleague and I are not physicists but we enjoy our little debates, we just need to get our facts straight.

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Your colleague is correct that the pencil or your body (or any object) is still affected by gravity even though it may be moving away from the source of gravity. But that doesn't mean that gravity isn't a weak force. –  David Z Jun 11 '11 at 19:49
    
aye, but i was wondering if jumping, etc. is a good demonstration of its weakness? –  Gary Willoughby Jun 11 '11 at 23:35

4 Answers 4

I think what you heard in that lecture is this argument:

Gravitation is by far the weakest of the four interactions. Hence it is always ignored when doing particle physics. The weakness of gravity can easily be demonstrated by suspending a pin using a simple magnet (such as a refrigerator magnet). The magnet is able to hold the pin against the gravitational pull of the entire Earth.

Yet gravitation is very important for macroscopic objects and over macroscopic distances for the following reasons. Gravitation:

  • is the only interaction that acts on all particles having mass;
  • has an infinite range, like electromagnetism but unlike strong and weak interaction
  • cannot be absorbed, transformed, or shielded against;
  • always attracts and never repels.

Jumping, or lifting a pencil, is in your example "breaking" the influence of gravity because the electromagnetic interactions between your feet and the ground are able to counteract the gravitational force of the entire planet, thus demonstrating that gravity is a weak force, so I'd say yes, it's a good example.

Source: http://en.wikipedia.org/wiki/Fundamental_interaction#Gravitation

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Well, keep in mind gravity isn't being "broken" when you jump, it is still exerting a force of approximately F_g=G * m_1 * m_2 / r^2, where G is the gravitational constant (6.677 * 10^(-11)), m_1 and m_2 are the two masses, and r is the distance between them.

The idea of gravity being a weak force, relative the electromagnetic force is shown by jumping as the electromagnetic forces moving your muscles are able to overcome (a better word than "broken") the gravity of the entire earth for a period of time, so F_g < F_jump. Since you cannot jump again midair very effectively, it is temporary, as F_g is present still in the air. But assuming you had lots of energy and a space suit you could climb a very long ladder out to where gravities effects would be minimal.

Another good example that I like of electromagnetic force > gravity is the fact that when you jump out of a window (first story please!), land on the pavement. There the electromagnetic interactions between your electrons and the pavements electrons are stopping you from going all the way to the center of the earth.

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Gravity is a very "weak" force, but there's something special about it in how it is truly cumulative. All matter (as far as I'm concerned) has the same sign of gravity pull. By that I mean the force is "inward", or written as $-\vec{r}/|\vec{r^3}|$, so you could call it "negative".

user599884 gave a good reason the force can be called weak, which is true by all means. In fact, two balls containing exactly $1 C$ (Coulomb) of charge placed $1 m$ away will repel each other with $9 GN$ of force, which is something like a million tons of force. BUT, a ball with a $1 C$ charge on it has an imbalance of electrons versus protons of about $10^{18}/10^{23} Z\approx 0.00001$ times the total, or about 0.001%. That is some mighty force created by unthinkably small amount of matter.

The fact of life, however, is that we don't experience significant bulk forces from the electromagnetic or nuclear forces. I should clarify, however, that those forces give the form to everything around you on small scales. In fact, before humans started making magnets and electric machines, there was very little bulk E&M force in nature that could exert significant forces on macroscopic objects, even though those forces are so powerful! Why is this?

There is a very profound distinction between between gravity and other forces. Regarding electrostatic forces, there are 2 kinds of charges and matter will tend to try to balance those (and does a superb job at it actually). For magnetism, charges will ultimately move as a result of magnetic fields in a way that decreases the strength of the magnetic field. Not so for gravity. Gravity clumps matter together with doesn't degrade it's gravitational strength on large scales, making it truly cumulative. This is why over large time scales gravity "wins", the galaxies, planets, and stars are a result of gravitational clumping.

The gravity of $1 g$ you experience around you is the resultant force from every single atom in all of the planet. Just a tiny tiny tiny fraction of all those atoms exerting a different force like E&M or nuclear would be able to easily counter the $1 g$, but, the difference is that those other forces are balanced perfectly and don't give a resultant force that can operate on you.

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Balanced to first order , yes the argument holds, but fortunately not perfectly. The electrostatic is what is holding our bodies together by the molecular forces, and keeping us from falling through to the center of the earth, etc. The nuclear is more esoteric, but it is the higher order QCD forces that keep the nucleons together, and without nucleons atoms would not exist, and without atoms we would not exist. –  anna v Jun 11 '11 at 17:03
    
sorry that should be the electromagnetic force is what is holding..., of course –  anna v Jun 11 '11 at 17:39
    
@anna you know, I've always had an uncomfortableness with people saying that E&M explains atoms. Maxwell's equations don't predict atoms at all, Maxwell's equations applied to a quantum wave function does. However, I'm stumped there if asked for an alternative to QM. Say we have 2 particles that can't energetically interact to form a new particle but still attract. What should that lead to? In the case of gravity, absent balance from another force, they continue to attract until they form a singularity. Perhaps we are lucky QM doesn't allow this for electrostatic attraction. –  Alan Rominger Jun 11 '11 at 19:08
    
Your discomfort seems to come from the difference between necessary conditions and sufficient conditions. In order to have atomic and molecular forces it is necessary to have EM though it is not a sufficient description of the universe. Nevertheless, the necessity is a definite explanation to a statement of "a perfect balance and no forces". There exist the Wan der Waals molecular forces for which EM is a necessary ingredient. It takes many necessary ingredients, a main one is QM, to have a complete theory of the universe, but we are far away from that. –  anna v Jun 12 '11 at 4:08

More simply, I think you just need to understand what you were meant by "the effects of gravity broken by jumping". By jumping, that is, using your muscles (which work thanks to electric forces), you can compete with the gravitational effects of the entire Earth. But you are not going to escape from them ; gravity is still acting on you.

Gravity is actually the only force that non-scientific people think of; they are rarely conscious that electric forces are either existent and much more strong that gravity, and for this reason, they need to be explained why it can often be neglected.

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