MadSci Network: Physics Query:

### Re: How long would it take an hourglass to expire on the moon?

Date: Mon Jun 12 19:20:34 2000
Posted By: Graeme Bushell, Faculty, Chemical Engineering, The University of New South Wales
Area of science: Physics
ID: 960738218.Ph
Message:
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Sorry, but I think you're going to be disappointed on this one.

Particulate systems such as hourglasses are very hard to predict. Although
the motion of each individual particle is simple, and governed by Newton's
laws of motion, when you add a whole lot of them together the motion
becomes unpredictable. It's a great example of a complex system built from
the behaviour of very simple parts.
In engineering applications of particle technology we rely extensively on
experience, computer intensive simulations and some engineering common
sense.
About the only thing you could say about your system is that since the
forces encouraging the sand to move downward are less, the hourglass will
take longer to drain than it would on earth. However, it might not even
drain at all, the sand may remain stuck in the top chamber if the gravity
forces are not enough to overcome friction. It also depends on the kind of
sand used and the design of the hourglass.

So, the only way to answer your question, in my opinion, is to actually do
the experiment on the moon, or, more cheaply, to run a discrete particle
simulation on a computer. An example of this kind of simulation work
(actually these guys simulated something very like an hourglass) is
"Simulation of Descending Particles in Water by the Distinct Element
Method": K.Asakura, S.Harada, T.Funayama and I.Nakajima, Powder Technology
94 (1997) pp195-200

Cheers,
Graeme

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