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Opposing but equal forces

  1. Sep 6, 2009 #1
    First for the disclaimer I am not a student,I am just a layman so this is NOT a school homework question however I am at home and I am working on a thought problem.
    If two equal but opposing forces affect the same object, they cancel each other out, right? Do they cancel each other as they affect the object, or do both affect the object equally and therefore cancel themselves out? If they do affect the object and then cancel out, would that count as a force on the object even though it would be zero? Say for example if you could apply centripetal force to a vessel which had a molecule inside, the molecule which isnt attached to the vessel would move out until it came in contact with the vessel wall(i guess that would actually be losing its centripetal force until contacting the wall), but if you added an opposing centripetal force that was exactly opposite of the first, would the molecule return to the center(not dead center but somewhere on the center plane(xy))? If so, I guess if you could do this along the x,y,z axis' the molecule would be contained in a cube in the center, and if you could do this in an infinite number of axis' you would have a sphere in the center? Or am I just trying to overthink this? Thanks in advance for any help.
  2. jcsd
  3. Sep 6, 2009 #2


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    If the forces on the object are "inwards", the object experiences compression, if the forces are "outwards", the object experiences tension.
  4. Sep 6, 2009 #3
    Try this, ignore the fact that there are two forces. One thing about physics that's sometimes good to do, is to look at the effects of the things on each other in small LOCALITIES (all caps only because its a very important and rather metaphysical assumption, i.e. Einstein was ok with throwing away almost everything except locality which at times QM seemed to violate). That is, there's an assumption that it takes time for things to happen, for something felt "over there" to get "over here."

    So, if you have say, an object that's a cubic foot, and you apply two forces, equal but in opposite directions on opposite sides of the box...well, you have force A on side A and force B on side B and eventually they do "cancel out."

    But...for a little while...it should take some time for side A to even notice the force on side B right? Because there's a difference between them...

    so in the locality of side B, right when force B happens, it really can't feel force A or side A at all. It takes time before it can feel that thing's effect. So, its like those aren't there at all yet, and its just force B.

    So the particles on the very surface are pushed back by force B, to a certain extent. The reason why the box doesn't really move is because eventually those that are being pushed back by A are going to hit up against those being pushed back by B. That is, eventually force A's effects will find their way over to B and "cancel out."

    Another good tool in physics is to take any given quantity in physics and see what happens if you make it ridiculously big.

    So imagine a normal cardboard box. Now imagine I bought two of those beautiful new camaro's and positioned them on either side of the box, and had them run at it so that their "forces" would be exactly the same during collision. Does it just cancel out? Do the camaro's both hit the box, just stop, and do nothing? NO.

    The box is epically crushed. So it seems pretty evident that even equal forces in opposite directions affect the structure of the box, if only because there's a time gap before everything is equalized (before force A reaches force B) and in that time your box might have just turned into a pancake. So yes, the forces both affect the object, locally, and eventually they "cancel out" for some proportions, though that's usually only because we don't see the object compressing a bit, we just see it standing relatively still. But add a few muscle cars...

    I guess the moral is, before considering esoteric molecule in space box scenarios, consider the box on your driveway :biggrin:
    Last edited: Sep 6, 2009
  5. Sep 7, 2009 #4
    That does make it a lot easier to think about, atleast mechanically(since I have experience with crashing camaros), but would that also work with fields? Electrical or magnetic or I guess electromagnetic fields? I have a hard time imagining fields. The reason I ask is I read somewhere that if you have a large magnet( big enough to pull the nails out of your walls), you can just put an equally sized magnet next to it and it will keep your nails from coming out of the wall, so as one force pulls the nail the opposite force pushes the nail back in? Sorry I think I just answered my own question, it does also apply to fields, but I posted to double check. Thanks duke of duke for your explanation and patience with a newbe.
  6. Sep 7, 2009 #5
    don't worry Jasongreat, I too am still pretty much a noob and I don't really know for sure exactly how would work in a field because fields are weird things. My understanding of them is still rather Maxwellian/Classical, I don't know much about the high energy particles that create them. But in the Maxwellian model, the field permeates every point in space (that's how the fields work in pure mathematics too btw)

    So, your nail...its located somewhere in space. Unlike the camaro, which hits only one side of your box, the field actually pulls at every particle in the nail at once, because every particle is at a point in space and the field is at every point in space. So instead of one large camaro hitting the side of the box, its a lot more like...well I have no idea what a good metaphor for this would be but I'm pretty sure you get the picture. Instead of just hitting one edge of the box and then letting the molecules transmit the shock waves, you're first having the field pull on every particle in the nail and THEN letting those particles hit each other and transmit shock waves (though they're much much smaller).

    So the reason why it works much better with fields is because you can kind of skip the whole idea of preserving the locality, in a way, because you've already said the field is everywhere at once and affects all. Unlike your unfortunate box (and camaros) your nail is much more evenly squeezed, so long as the mag/electric fields are equal everywhere (if they're not equal, you're back to the differences in forces represented by the two camaros versus the force the molecules in the box were applying against each other while at rest...a very large difference that decided to equal out by forcibly changing shape).

    Of course I'm pretty sure high level physics would make a mockery of the classical models we're using, but they're good enough for me right now!

    But always keep thinking and asking!
    Last edited: Sep 7, 2009
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