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The physics most common in your life

  1. Aug 1, 2004 #1
    What physics do you encounter frequently yet habitually take for granted?
     
  2. jcsd
  3. Aug 2, 2004 #2
    My morning egg never boils if I stand by the stove , but the stove burns the pan the minute I'm not looking.
     
  4. Aug 2, 2004 #3

    Galileo

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    Murphy's Law ;)
     
  5. Aug 2, 2004 #4
    Newton's third law of motion. Not mostly but always.
     
  6. Aug 2, 2004 #5

    BobG

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    Why, is your head getting sore? :rofl:

    That reminds me. What does gravity smell like?

    To me, it usually smells like dirt. Tastes like it, too.

    But, to the question: Torque.

    Get a ruler, a piece of string, and a hammer. Put the string around both the ruler and the hammer, so the hammer kind of hangs from the string with handle of the hammer pushing up against one end of the ruler. Set the other end of the ruler on the edge of the table (the only thing that should be touching the table is the ruler). Place your bare foot beneath the whole contraption. Let go the ruler. (Okay, if you're not that confident, you don't have to do the place your bare foot beneath the hammer :tongue2: ).
     
  7. Aug 2, 2004 #6

    AKG

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    BobG

    Could you draw that? I don't get the bit about, "handle of the hammer pushing up agains one end of the ruler."
     
  8. Aug 2, 2004 #7

    BobG

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    I had some problems trying to attach a drawing, so I'll try a better explanation.

    The hammer should hang from the string, which is hanging from the ruler (the string needs to hang about 3 or 4 inches). The handle of the hammer should rest against the end of the ruler (in other words, the end of the hammer handle extends past the end of the ruler by about a 1/2 inch). The length of the hammer should lie directly below the ruler, with the ruler lying on the table, and the hammer hanging underneath the table at about a 30-45 degree angle.

    Then see how little of the ruler needs to be on top of the table to before the whole thing falls.
     
  9. Aug 2, 2004 #8

    AKG

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    BobG

    I'm attaching an image (hopefully it works). Is this what you mean?
     

    Attached Files:

  10. Aug 2, 2004 #9

    BobG

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    Yes. Except the string should hang from somewhere around 3 or 4 inches from the end of the ruler. i.e. - there should be lots of weight on the end of the hammer hanging below the table. The more torque you get on that end, the less of the ruler that has to actually be on the table. Play with it awhile and you can get down to an inch or less of the ruler on the table.
     
  11. Aug 2, 2004 #10
    Thanks for the cool visual, AKG. Seems like a center-of-mass problem with feedback. For some reason it also reminds me of the prime number problem involving a large number of playing cards stacked so the topmost juts out as far as possible from the bottom card.
     
  12. Aug 2, 2004 #11

    BobG

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    I did this for a bunch of fifth graders. They all knew it would fall off the table as soon as I let go. When I did let go, it kept them quiet for almost a whole second. Then they all immediately realized why the ruler stays on the table - torque. The force of gravity is stronger on the hammer head than the handle. Being connected, the hammer has the same torque as the hammer head about the pivot point (string). The force due to torque is greater than the force of gravity on the ruler (actually the torque resulting from the force of gravity, since the ruler has a pivot point, also).
     
  13. Aug 2, 2004 #12
    I try not to ignore physics in everyday life. I try to use it. For example: A friend of mind told me that his father in law just bought a miracle plate. It was suppossd to be something that you placed froze n food on and it would thaw rapidly compared by to placing in on the counter. Sounded fishy to me. So I did a bit of an eperin=ment and placed an ice cube ont the "Mirtacle plate" for which I used a metal pan, and I placed an ice cube in a wooden block. The ice cube on the wooden block took hours to melt and the cube on the metal plate too only minutes.

    SO I told my friend why his father in law wasted his money and that he should ask me about thermodynamics next time. :biggrin:

    Pete
     
  14. Aug 2, 2004 #13
    :confused:

    Wood=insulative to heat

    Metal=conductive to heat
     
  15. Aug 4, 2004 #14
    Hey BobG,
    I built your hammer set up...pretty cool! Got it to one inch. Maybe my hammer wasn't a good weight or something, but I found for the equipment I had it was easier to shift the hammer and string closer to the end of the ruler that's on the table. The hammer still hangs at about 45 degrees except instead of extending past the edge of the ruler, the ruler extends about 3 inches past the tip of the hammer. A little tape keeps the hammer from sliding out of the loop. Other than that, it's exactly how you described it. Very cool demonstration!
     
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