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Hourglass's weight

  1. Dec 7, 2014 #1
    How does an hourglass's weight change while sand is dripping in it? (better be in a diagram)
  2. jcsd
  3. Dec 7, 2014 #2
    Start by saying what you think "weight" is and why it should be changing.
  4. Dec 7, 2014 #3
    W=mg that is what weight is. I think that when the sands are falling it is a kind of free fall; so some of the hourglasses's weight should diminish.
  5. Dec 7, 2014 #4

    There is nothing about falling, free or otherwise, or not falling at all, in the definition above. Why should the weight change then?
  6. Dec 7, 2014 #5
  7. Dec 7, 2014 #6
    That experiment does not measure the weight as you defined it in #3.
  8. Dec 7, 2014 #7
    OK then, whatever it is just explain it!
  9. Dec 7, 2014 #8
    The truth is that experiment measures the tilt of the scales (which they call "weight reading"), and nothing else.

    They explain why that "reading" is different from the static case. The details are complicated, and depend not only on the motion of the sand, but on the motion of the pans, too.
  10. Dec 7, 2014 #9
    Can you demonstrate it in a diagram?or give a link of it?
  11. Dec 7, 2014 #10


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    You're pretty demanding for someone with no bargaining leverage. :oldgrumpy:
  12. Dec 9, 2014 #11


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  13. Dec 9, 2014 #12


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    I've never actually heard of this particular question before, but it is really reminiscent of the one about jumping in an elevator or birds flying inside a truck.
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