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Stupid question about Newton's third law

  1. Oct 24, 2009 #1
    OK, first of all, I wasn't really sure where to post this; it's not really homework because we covered the material weeks ago and moved on, leaving me totally in the dark. My physics teacher has this annoying habit of saying something is "ALWAYS true" or "NEVER true" and then beginning the next class with an exception, so I need to know if this is true here or not.

    My question is this: Is Newton's third law valid only in inertial frames of reference? That is, can the reaction force ever be unequal to the action force, and under what conditions? We did the classic "elevator experiment", wherein we weighed ourselves at different points of the elevator's motion and found that our apparent weight change is caused by a differing strength of the normal force of the floor on us. I realize that this is an experiment designed to verify Newton's second law, and not the third, but it seems like should be more consistent - the reaction force in these cases is very clearly not equal to the action force. I can't help but feel like I'm missing some element in this that's keeping it all from "clicking". I asked my teacher, and he went on some tirade about pseudoforces (???) and circular motion ... thus cementing my decision to become an English major when I enter college.

    Any and all help would be appreciated, and try to be gentle - physics is not my forte.
     
  2. jcsd
  3. Oct 24, 2009 #2

    Doc Al

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    Your apparent weight is the normal force of the floor on you. (True weight is the gravitational force on you from the earth.)
    Where do you think the "reaction" force does not equal the "action" force? Point out the forces you think are "action/reaction" pairs.
     
  4. Oct 24, 2009 #3
    Er ... your force on the floor equal to your actual weight, and the force of the floor pushing back on you, equal to your apparent weight?
     
  5. Oct 24, 2009 #4

    Doc Al

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    Nope, those are not 3rd law pairs. The floor pushing up on you and you pushing down on the floor--that's the correct 3rd law pairing. Your actual weight is the earth exerting a gravitational pull down on you, so the 3rd law pair to that is you exerting a gravitational pull up on the earth.

    It happens that your apparent weight equals your actual weight under some conditions (zero acceleration). But that's got nothing to do with Newton's 3rd law, which is safe and sound.
     
  6. Oct 24, 2009 #5

    cepheid

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    Newton's Third Law says that the force with which the elevator pushes up on you is equal to the force with which you push back down upon the elevator. These are both the same and are equal in magnitude to what we are calling your apparent weight. There is no violation of Newton's Third Law.

    I deleted my earlier post because it had some conceptual errors.
     
  7. Oct 24, 2009 #6
    You know what's messed up?

    That same thought occured to me, and I asked my teacher about it. He waved me off and told me I was wrong. So you're saying that you push down on the floor with the same strength that the floor pushes back on you - the normal force?
     
  8. Oct 24, 2009 #7

    Doc Al

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    Absolutely. That's Newton's 3rd law.
     
  9. Oct 24, 2009 #8

    Cleonis

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    I concur. A third law force pair is by definition a pair that is exerted by two objects upon each other.

    Imagine an elevator located on an asteroid. Or rather, a lifting machine that can lift tons of weight. Let the asteroid be several kilometers across. When an object is put on the platform of the lifting device there is a third law force pair between the object and the platform.

    Next imagine that the object that is lifted is another asteroid, that was hugging the first asteroid. The lifting machine goes to work, lifting the second asteroid. Well, assuming the second asteroid is comparable in mass to the first asteroid, what is happening is that the two asteroids are being pushed away from each other.

    On Earth, the cargo of an elevator has way, way less mass than the Earth, but ultimately when a lifting device causes upward acceleration of a load that upward acceleration is at the expense of the entire Earth being accelerated downward.

    Going back to the example of two hugging asteroids: the lifting machine is causing them to accelerate away from each other. How much will each asteroid accelerate? Clearly the asteroid that has more mass will accelerate less. And the second law tells you what the ratio will be. That is where the Third law and the Second law are effectively acting as a single principle.

    Don't worry if you find yourself doubting whether a particular case is an example of Second law or Third law. In some cases, such as this one, it's probably not worthwile to even want a distinction.

    Cleonis

    btw: Californians, I'm told, like to say to each other that when their governor, Arnold Schwarzenegger, does his push-ups, it's the Earth that moves away from him.
     
    Last edited: Oct 24, 2009
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