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Canal Barge on Aqueduct
Name: Don
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
Will a canal coal barge crossing an aqueduct put an
additional load on the aqueduct?
Replies:
No. The condition that the barge floats means that it weighs precisely
the same as the water that it is displacing. So, as long as the water
level in the aqueduct is the same with and without the barge there,
the total weight is the same too.
Ha! It seems I have the setup wrong. I thought an aqueduct was like a
lock but in reading a little more, I guess it is a free-flowing
channel. Thanks, Internet!
OK, but even in this setup, I still think change is non-zero.
Water cannot flow instantaneously out of the channel. So the load
would not increase by the exact weight of the barge necessarily, but it
will increase by the amount of extra water the barge drags with it.
It depends on the barge (are these huge Mississippi River barges,
or little floating-barrel type things?).
So that begs the question if we could create a scenario where would
the supports see minimal change? I suppose we would have to have a
very large (wide, deep, and long) channel (in which we can neglect
wall effects and velocity gradients) and the barge is moving at the
same speed as the flowing water (in other words its velocity is zero
relative to the water). In this situation, the water is quiescent
relative to the boat at all points of contact, and it simply displaces
(moving) water.
I think the "weight does not change" answer sounds like
once of these cute yet misleading "physics mind benders" that in
reality selectively oversimplifies (it neglects momentum and
viscosity).
Wow!
OK, well, I think the load will change, but I would only be
speculating on how much. If we answer the literal question "does it
change *at all*, I think the answer must be yes for the reasons I
described. However, if we loosen the question to "does it change
materially, compared with load tolerances" then I would say the answer
is probably no (I mean, these things are not collapsing all the time,
so they must have broad tolerances the way bridges do). I do not see
any large wake or turbulence around the boat. So I
think the change would be small and not significant relative to
the structure.
Burr Zimmerman
Look at the attached picture and consider the weight of the people and
the weight of the barge. How is the load displaced? Why do things float?
How does a large warship float?
Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences
Consider the weight of the people and the
weight of the barge. How is the load displaced? Why do things float? How does
a large warship float?
Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences
http://www.youtube.com/watch?v=3Q6eetzzjMo
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Update: June 2012
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