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Author Topic: Does This Hourglass weigh less when the sand is falling ?  (Read 7884 times)

Offline neilep

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Dear Hourglassologists,

See this Hourglass ?




It's a thing of beauty eh ?

When the sand is free falling..............once it has passed through the gap.....does this make the hourglass lighter ?...after all..it's not supporting it's weight is it ?

whajafink


Take your time !


Thank Ewe


Neil
Hourglass Weight Investigator

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Offline turnipsock

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Does This Hourglass weigh less when the sand is falling ?
« Reply #1 on: 19/05/2008 23:04:41 »
Could you not just put it on a set of scales (or some of these fancy modern electronic scales)and see?
 

Offline neilep

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Does This Hourglass weigh less when the sand is falling ?
« Reply #2 on: 19/05/2008 23:30:12 »
I suppose you could...but I imagine the sand must be very light so you'd need a well sensitive piece of weighing scale eqpt.

Great idea though.

Thanks Turnipsock
 

lyner

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Does This Hourglass weigh less when the sand is falling ?
« Reply #3 on: 19/05/2008 23:52:12 »
The sand that is actually falling will not be pressing down on the glass so the weight will be a bit less. However, there will be a bit of drag as the sand falls through the air and that will cause a tiny (even tinier) force downward to counteract this a bit.

It reminds me of the story of the man in his car following a large van down a narrow lane. The van kept stopping and the driver would get out and hammer on the sides  then drive on.
Curious, our friend overtook and stopped the van, asking the driver what he was up to.
"Well, mate, this is a 5cwt van and I've got 6cwt of budgerigars on board. I don't want to be overloaded so I have to keep some of them flying."
 

Offline Madidus_Scientia

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Does This Hourglass weigh less when the sand is falling ?
« Reply #4 on: 20/05/2008 01:48:58 »
lol @ story

But would the force of the sand hitting the bottom counteract the weight lost as its falling? Would it only weigh less while the sand starts to pour but before it starts hitting the bottom?
 

lyner

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Does This Hourglass weigh less when the sand is falling ?
« Reply #5 on: 20/05/2008 09:21:42 »
That's a point.
Before any sand has landed, my first post is true.
Although there will be less static weight, the falling sand grains will be gathering momentum, which is lost at the bottom as it hits the glass and a high force will act briefly on the bottom, as the particle slows down. The 'impulse' (force times time) on the way down will be the same as the impulse at the bottom. It must total zero for each grain because it starts and ends with zero speed. Whilst there are grains falling, the average force would be zero.
There are some other points of interest;
The energy situation has changed by the end of the process; the sand has lost Gravitational Potential Energy.
The characteristics of the actual weighing machine are relevant. They all work with a spring of some sort. The GPE of the falling load must go somewhere and must make the spring oscillate. It will depress a bit and will spring back up when the falling sand stops.
If you look at the situation in its simplest form; a box and a single ball falling inside it. That shows MS's point very well.
1. Initial conditions - weight of ball + weight of box
2. Ball falling- just weight of box
3. Ball hits bottom - weight of box + one big thump
4. Settled down - weight of ball + box
The details of 3. depend on the characteristics of the spring in the weighing machine. The machine could keep bouncing for ever if there is no friction; it would even bounce upwards when the ball first starts to fall.
 

Offline graham.d

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Does This Hourglass weigh less when the sand is falling ?
« Reply #6 on: 21/05/2008 13:47:11 »
Yes, I think this is right. If no sand is falling it has a certain weight W(say). When the sand starts to fall the weight will reduce by the increasing mass of sand in the flow until the sand-stream hits the bottom of the glass when it rapidly rises to become constant at W again. During this period the loss of weight due to the sand in transit is balanced by the integrated impulses as the falling grains' momentum is transferred to the glass. When the sand runs out the transfer of momentum continues but there is a decreasing amount of sand in transit so the weight now increases until the last grains hit the bottom. When the final grains have landed the weight returns to a constant value again. The weight varies thus:

                               /|
__________   _________________/ |__________
          \ |
           \|


Sorry I don't know how to import graphics and my drawing here turns out rubbish when it moves from Times New Roman to proportional text. It is supposed to be a constant value, then a slope down and rapid rise, constant value then a slope up and rapid fall, then contant value. Perhaps someone can draw this and import it.

Of course this is approximate as reality would have lots of imperfections compared with this, but I think this is the basic physics.
« Last Edit: 21/05/2008 13:53:56 by graham.d »
 

Offline neilep

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Does This Hourglass weigh less when the sand is falling ?
« Reply #7 on: 21/05/2008 15:55:04 »
I am constantly amazed at the level of responses. I ask what I think is a simple question and the answers forthcoming are most generous and educational.

THANK YOU sophiecentaur,Madidus_Scientia and graham.d

 

Offline rosalind dna

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Does This Hourglass weigh less when the sand is falling ?
« Reply #8 on: 21/05/2008 17:24:30 »
It is a simple thing called Gravity that makes the sand go through the hourglass or as some used it as a timer.

 

lyner

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Does This Hourglass weigh less when the sand is falling ?
« Reply #9 on: 22/05/2008 16:11:04 »
GD  Despite your graph being 'not quite perfect' I believe you have got the right idea apart from the horizontal line would be lower during the time that sand is flowing.
My diag was done on Powerpoint and saved as jpeg.


Quote
It is a simple thing called Gravity

All sorted, then, Rosalind!?
 

Offline graham.d

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Does This Hourglass weigh less when the sand is falling ?
« Reply #10 on: 23/05/2008 09:21:50 »
Yes, you are right Sophie. The sand that is in transit is in free fall and would not be contributing the the total weight.
 

Offline rosalind dna

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Does This Hourglass weigh less when the sand is falling ?
« Reply #11 on: 23/05/2008 11:13:39 »
GD  Despite your graph being 'not quite perfect' I believe you have got the right idea apart from the horizontal line would be lower during the time that sand is flowing.
My diag was done on Powerpoint and saved as jpeg.


Quote
It is a simple thing called Gravity

All sorted, then, Rosalind!?


This is what I meant about Gravity because the sand in the Picture of Neil's hourglass, The sand is falling down so that is what
the pull of gravity does.

 

lyner

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Does This Hourglass weigh less when the sand is falling ?
« Reply #12 on: 23/05/2008 21:01:20 »
Fair enough Rosie. But gravity just ain't simple.
 

Offline rosalind dna

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Does This Hourglass weigh less when the sand is falling ?
« Reply #13 on: 23/05/2008 21:11:15 »
Fair enough Rosie. But gravity just ain't simple.

What is simple?








Being called Rosalind is what I like.
 

lyner

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Does This Hourglass weigh less when the sand is falling ?
« Reply #14 on: 23/05/2008 23:31:04 »
Sorry Rosalind.
Simple means you can predict what will happen easily with no significant Maths and under all circs. It means that it can be appreciated intuitively. Clearly, gravity is not simple because it took us millennia to get as far as Newton, even.
Most people 'get' gravity wrong. For a start, they label the downward force on an object on Earth as "gravity" when they should label it "weight".
Gravity is a Field (Force per Unit Mass) and not a simple Force.
Then, there's Mass and Weight etc. etc.
 

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Does This Hourglass weigh less when the sand is falling ?
« Reply #14 on: 23/05/2008 23:31:04 »

 

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