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Done mult. times need the correct answer w/in an hr! help!

  1. Oct 12, 2007 #1
    A woman of mass 51 kg is standing in an elevator. If the elevator floor pushes up on her feet with a force of 405 N, what is the acceleration of the elevator?
    ______m/s2 down

    So far I've used the F=ma equations and I've gotten 7.94 m/s which was wrong and then I figured maybe it was negative since its down and submitted -7.94. Then some how I redid it and got 27.. not sure where that came from but I'm at a loss and I need an answer within an hour. Can someone get to the bottom of this for me? I don't understand why the first solution I got was wrong. Help! Thanks!
     
  2. jcsd
  3. Oct 12, 2007 #2

    Hurkyl

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    It sounds like you're just blindly plugging things into formulae. Have you learned how to actually analyze problems involving forces?
     
  4. Oct 12, 2007 #3
    yes, i've been taught briefly how to do this, that's why i'm asking for help. My prof is horrible and I've checked my work multiple times and it only makes sense that the first answer I got is correct. I know the website we have to use has been marking correct answers incorrectly, but I just wanted someone to check my work and hopefully get to the bottom of this. I've spent too much time on this one problem and I can't find anyone in my class who actually understands this question, other than the way I do. So I'd like help please, that's what this is for.
     
  5. Oct 12, 2007 #4

    Hurkyl

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    So then share your analysis! That way, we can confirm if you're doing it correctly, or point out where you made a mistake.
     
  6. Oct 12, 2007 #5
    I did... I said I used... F=ma which would be A=f/m for acceleration which I used. So i plugged A= 405N/51kg and got 7.94 which should be the correct way as I've been told. Why am I not getting the right answer with this?
     
  7. Oct 12, 2007 #6

    Hurkyl

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    But why did you use that equation? In what way did you analyze the problem that led you to conclude that you use F=ma like that?

    Incidentally, what precisely is being represented in your calculation by the letters F, m, and a?
     
  8. Oct 12, 2007 #7
    Its an equation used in physics often and that's what's been covered in my class. So it only makes sense to use an equation that uses F=force, m=mass, and a= acceleration. Did you not know what that meant? I figured if someone were to help me with this question they would know what the equation stood for in order to help me, wouldn't you need to know that since I obviously don't understand what I'm doing wrong. Now that I stated what each part is, any ideas as to what I'm doing wrong and how to solve this problem?
     
  9. Oct 12, 2007 #8
    force diagram!
     
  10. Oct 12, 2007 #9
    Never mind, I figured it out. Thanks though!
     
  11. Oct 12, 2007 #10

    Hurkyl

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    Because F=ma involves a force, a mass, and an acceleration, it does stand to reason that it might be a useful equation to use. But you have done nothing to justify its use in any manner whatsoever -- what quality does this problem have that allows you to make the calculation you are making?

    Actually, you haven't. There are lots of forces, masses, and accelerations in the world. You have to be more specific.

    I do have a pretty good idea -- you haven't yet grasped the physics of the problem. A good way to start correcting that is by thinking more carefully about the problem -- think about what physical quantities are in the problem, think about what meaning you are assigning to letters, and think about the physical principles that allow you to use equations to relate physical quantities.
     
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