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Why aren't forces N and G equal?

  1. Oct 11, 2009 #1
    I am struggling with this one. The question is to explain why the gravitational and normal forces are not equal. And I'll be darned if I know why they aren't. Logic tells me they should be equal. The small block has a mass of 2kg. And multiplying that mass times the normal (9.8N) gives me the gravitational (19.6N). Unless the normal is a constant 9.8N per kg for ALL objects?

    Forcesscreenshot.jpg
     
  2. jcsd
  3. Oct 11, 2009 #2

    Doc Al

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    Please describe the problem completely. What does that diagram show?
     
  4. Oct 11, 2009 #3

    D H

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    Did you make that diagram, or is it from other source?
     
  5. Oct 11, 2009 #4
    The diagram is a portion of a screenshot from an exercise in a program called Interactive Physics. The horizontal plane is a frictionless plane. The small block is just a 2kg mass. FN is Normal force and reads 9.807N. FG is Gravitational force and reads 19.613N. FT is the applied force from the green arrow of 8N horizontal.

    I'm supposed to explain why FN does not equal FG. But I thought they had to be balanced, hence equal?
     
  6. Oct 11, 2009 #5

    Doc Al

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    If the vertical acceleration of the block is zero, then the net vertical force must zero. Assuming that's the case here, that diagram looks incorrect.
     
  7. Oct 11, 2009 #6
    Doc, there is no vertical acceleration. And so I figured I must be screwing something up. According my admittedly limited understanding FN should be equal to FG. If things were as the program is showing me it seems to me the block should be sinking downward (due to FG) in addition to moving to the right from the applied horizontal push.

    There have been reports of this program screwing up in our online community. I just wanted to be sure the program was wrong and not me.
     
  8. Oct 11, 2009 #7

    Doc Al

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    I don't think it's you. :wink:
     
  9. Oct 11, 2009 #8
    OK, thanks.

    Love this new resource too. Glad you guys are here. That's the only thing I have found so far that I don't like about online college, nobody to discuss things with in real time.

    BTW, where's the link for the 20% discount on SciAm? I think I'd like to take advantage of that.
     
  10. Oct 11, 2009 #9

    Doc Al

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