Hourglass on a balance scale

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  • Thread starter DrDavy
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  • #1
DrDavy
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Here's my question:

Two spent hourglasses that weigh exactly the same on a balance scale. You flip one over so that it's running. What happens to the balance?
 

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  • #2
bob012345
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First, welcome to PF!

As to the question, what do you think will happen and why?
 
  • #3
erobz
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I think the scale tilts to the flipped slide after first very briefly rising while the sand begins to descend. While the sand is falling it’s weightless( causing it to slightly rise), but upon impacting the base the falling sand would impart its weight plus an additional impact force from the momentum change and start tilting in the opposite direction. That’s my guess.
 
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  • #5
DrDavy
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Before I look at the spoiler, I want to say that I think the one that is running would be lighter, because at any given moment, there would be grains of sand in the air. I guess that's the point of the first part of Erobz's answer. I'm not smart enough to grasp the second part of that answer.
 
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  • #6
DrDavy
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Thank you very much, bob012345, for the definitive answer!
 
  • #7
bob012345
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Thank you very much, bob012345, for the definitive answer!
You're welcome. You might enjoy setting up a 'kitchen' experiment to play with this situation.
 

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