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Vector force& and Optic question

  1. Apr 15, 2012 #1
    6ru4iu.jpg

    Log question

    The diagram shows a log being pulled up an earth slope from X to y.

    How much work is done against friction, the answer I get is not among the 4 choices..

    also the part messed up by the scanner says 1000N force due to friction


    2s1rifb.jpg

    The diagram shows a boy doing homework. He has a difficulty reading his book under the small light bulb.

    Draw a ray diagram to show the shadow produced by his head on the table. What difference would it make to the shadow if the light bulb were replaced by a long fluroescent bulb? Draw a diagram to illustrate this case.

    my attempt...
    rgy1ix.png
    the darker part is the umbra

    and the strokes are the penumbra does that drawing make any sense?


    Please respond.
     
  2. jcsd
  3. Apr 15, 2012 #2

    PeterO

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    "Log question

    The diagram shows a log being pulled up an earth slope from X to y.

    How much work is done against friction, the answer I get is not among the 4 choices..

    also the part messed up by the scanner says 1000N force due to friction
    "

    The question also says the log was dragged 50m

    If you oppose a 1000N force over a distance of 50m you certainly get one of the options.
    Perhaps you were working out a total energy change brought about by dragging up the hill as well.
     
  4. Apr 15, 2012 #3
    is it 50,000 J?
     
  5. Apr 15, 2012 #4

    PeterO

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    In short, the drawing does not make any sense.

    Better look up umbra and Penumbra

    http://en.wikipedia.org/wiki/Umbra

    is a start, though most Physics texts have great diagrams.
     
  6. Apr 15, 2012 #5

    PeterO

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    That sounds logical to me.
     
  7. Apr 15, 2012 #6
    Okay thanks sir I'll try to redraw the diagram, I drew the diagram in a rush.
     
  8. Apr 15, 2012 #7

    PeterO

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    Don't forget they said "replaced by a long fluroescent bulb".

    That means it behaves like an array of globes perhaps 1m long.
     
  9. Apr 15, 2012 #8
    I'm sorry I don't follow, what would be 1m long the shadow?
     
  10. Apr 15, 2012 #9

    PeterO

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    The light source.

    Have you seen the long flourescent tubes they use in many businesses, schools and many home garages / basements - instead of a normal Globe light fitting.

    http://en.wikipedia.org/wiki/Fluorescent_lamp
     
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