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Dynamics and action-reaction pairs

  1. Oct 3, 2007 #1
    I know for an action-reaction pair, two forces should act on different objects and equal in magnitude, opposite in direction. Check out this: A brick resting on a flat surface. The normal force acting up is just equal to the weight acting down. Is this an action-reaction pair? My teacher said no. But I think it was an action-reaction pair because brick exerts its weight on the surface,it's a common sense, and because of the weight, the surface exerts normal force on the brick, and they must be equal in magnitude, then how come they are not. My teacher's explanation was like because there were three objects, the other one was earth. The weight came from the earth, it meant the earth exerts brick's weight on the surface? IT doesn't make sense to me. Hope u can help. thanks.
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  3. Oct 3, 2007 #2
    [EDIT: on rereading, I can't actually tell exactly which position belongs to mia vs the teacher.*shrug* :biggrin:]

    There are a couple different balanced forces on the brick: its weight (always equal and opposite to the lift its gravity exerts on the earth) and the normal force (always equal and opposite to the compression exerted on the surface).
    Last edited: Oct 3, 2007
  4. Oct 3, 2007 #3

    Doc Al

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    Your teacher is correct. The action-reaction pair is this: (1) The upward normal force of the surface on the brick, and (2) The downward normal force of the brick on the surface. If the brick is not accelerating, then those normal forces will happen to equal the weight of the brick, but that doesn't mean those forces are gravitational forces.

    What if the brick was accelerating? Imagine that it was lying on the floor of an elevator that was accelerating upwards. Certainly the weight of the brick hasn't changed, but the normal force has: it will be greater than the weight.

    Since weight is the force of earth pulling the brick, the reaction to that is the equal and opposite force of the brick pulling up the earth.
  5. Oct 3, 2007 #4

    D H

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    Just to elaborate a bit here, suppose the brick is in your hand. If the action-reaction pair was as described in the OP, you would not feel the brick. You feel the weight of the brick because of the action-reaction pair between the brick and your hand.
  6. Oct 4, 2007 #5


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    My most common exposure to the action/reaction pair is:
    Feel something nice/feel slap across face.:grumpy:
  7. Oct 4, 2007 #6
    My teacher said brick and the surface was not an action-reaction pair. But I thought it was. She said about because earth exerted gravitational force on brick, and another normal force on brick exerted by surface. so there is only one object here, so it's not. THIS WAS NOT WHAT I SAID. because i think how come there was a normal force exerted by surface is because brick exerts its weight on surface. So therefore these are two objects. And should be an action-reaction pair.
    Last edited: Oct 4, 2007
  8. Oct 4, 2007 #7
    Same here, I agreed it to be an action-reaction pair. But my teacher said NO! Because the forces exerted on the same object--brick? I don't understand my teacher's reason.
  9. Oct 4, 2007 #8


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    Which that post disproves.

    There are TWO distinct action-reaction couples that the brick is involved in here:

    1. with the table. Here, the table's normal force on the brick and the brick's reaction force to that on the table constitutes the pair.

    2. With the Earth. Here, gravity is the force by which the Earth pulls on the brick; the brick exerts an equal and opposite gravitational force on the Earth.
  10. Oct 4, 2007 #9
    I totally agree.But my teacher said because this problem involved three objects, brick, surface and earth, that's why it's not an action-reaction pair. But still, as you have said, 3 objects can have two action-reaction pairs. I think my teacher was wrong. Btw, you said the table's normal force on the brick, Is this force happening because of the weight/force that the brick gives it? I know the forces happen at the same time, but I want to know the cause-effect. In other words, which is which's reaction?
    Last edited: Oct 4, 2007
  11. Oct 4, 2007 #10

    Doc Al

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    In the first post you were questioning whether the brick's weight and the normal force of the surface were an action-reaction pair. That sounds like what your teacher thought you said, since that is not an action-reaction pair, as has been explained here. Even though there are two objects: brick and surface. (Just because there are two objects, does not mean that you've correctly identified the action-reaction pairs.)

    The point your teacher was making (I presume) was that the brick's weight is the force of the earth on the brick, and thus is not part of the brick-surface interaction. Thus there are really three objects interacting here.

    It something of a semantic issue, since common language would say: The brick's weight presses down on the surface. Nothing really wrong with that as long as you understand that you really mean: Due to the brick's weight, the brick presses down on the surface with a force equal to its weight. But that force between brick and surface is a contact force (the normal force), not really the force of gravity.

    Three objects can potentially have 3 action-reaction pairs: A-B, B-C, A-C.

    FYI: "Action-reaction" is somewhat of an old-fashioned term, which implies cause and effect even though there's none. A better term would be "3rd-law pair".
  12. Oct 4, 2007 #11
    Thank you, Doc Al. I get it now. The force that presses down on the surface is equal to the weight of the brick, but this force is not the weight but a normal force/contact force. Because I was confused with this downward normal force and the downward weight. Though they are same in magnitude, they're not the same thing. Weight was exerted by the earth actually.
  13. Oct 4, 2007 #12

    Doc Al

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