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Negative Normal Force (Conceptual Problem)

  1. Oct 3, 2008 #1
    1. The problem statement, all variables and given/known data

    There is an object glued on a plate that rapidly accerlates upwards (then stops) and then accelerates downwards. Graph the y-component of the normal force on a force vs. time graph. There is an interval of time on the graph inwhich the normal force is negative. How can this be? What does it mean physically for normal force to be negative.

    3. The attempt at a solution

    Since normal force is caused by electrostatic interactions between molecules, it can only occur perpendicular and AWAY from a surface. It would be impossible for normal force to be negative in this instance but yet my TA says that the question is correct...??
    How could normal force be in the same direction as gravity?? Am I missing something important or did I make a mistake somewhere is my understanding?

    Please help, thanks in advance
  2. jcsd
  3. Oct 3, 2008 #2
    It means that you've broken physics. In that you've eliminated the equal and opposite force and turned it into an equal force.

    Maybe possible theoretically, but as far my knowledge of Newton's third law goes.. This is impossible.

    Edit: Hmm if the object is adhered to the plate such that it can accelerate down and still remain intact. Then you will have a tough time determining which forces are acting where between the object and the plate as they will act as a single object.
  4. Oct 3, 2008 #3

    Gib Z

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    Homework Helper

    The negative sign merely indicates the opposite direction of the Force. In this case, signifying the plate is bringing you down instead of up. It seems both of you thought a bit too deep into this one lol.
  5. Oct 3, 2008 #4
    so would it be correct to say that the negative normal force is really the force applied by the plate (pulling the object down)? So would that mean that my FBD would show normal force pointing in the same direction as gravity?
  6. Oct 3, 2008 #5

    Doc Al

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    Staff: Mentor

    I'd say that the use of the term "normal force" in this problem is quite sloppy. In general, what's usually called the normal force does act the way you describe--normal forces PUSH, they don't PULL.

    Yes. They used the term "normal force" when they should have said "the force exerted by the plate on the object".
    When the plate pushes up on the object, the "normal force" is positive; when it pulls down, it is negative. Gravity is always negative.
  7. Oct 3, 2008 #6


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    Hi zmike! :smile:

    "Normal" just means "perpendicular to the surface, measured in the direction from the other surface".

    If the normal force on the graph is negative, that means that if the object wasn't glued on, it would have lost contact with the plate as soon as the force was zero. :smile:

    That's why glue was invented! :biggrin:
  8. Oct 3, 2008 #7

    Doc Al

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    Staff: Mentor

    tiny-tim makes a good point: You can take the component of any contact force between two surfaces perpendicular ("normal") to the surface and that can be properly called the "normal force". Nonetheless, your TA should have explained the difference between the "usual" examples of normal force and this one.
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