Is that a caveat for the weight increases temporarily on the supports?
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- Thread starter noseall
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I suspect a typo here, 20k cube weighs 20,000 tons, which would have a different effect on the weight of the system than a 2,000 ton barge, but I agree with the spirit of what you say.
To float, said 2000 ton barge must have a volume greater than 2000 cubic metres. The amount of the barge below the waterline line will be 2000 cubic metres, displacing 2000 tons of water. When considering fluid level rise we do not count the part of the volume of the floating object that sits above the water line
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The weight increases permanently on the supports. The system as a whole weighs more and all parts of it must share the load out. If you drop that barge causing a huge splash and resulting wave, the weight borne by the bridge increases more as it carries the wave, then less than it as the wave passes (the height lowers to less than the sitting water line was before) then more than it was as the level refills and so on until equilibrium is restored.
It will restore at a new higher weight on the supports, but exactly what amount depends on the weight of the barge compared to the size of the system as a whole. If the bridge represents only 0.001% of the size of the entire section of the canal then it carries only 0.001% of the weight of the barge when things have settled
So the physics graduate was correct then.
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I think there has been some slight tweaking of the original exam question. I think most people in this thread agree, that a static barge anywhere in the canal already has its weight in the system. If it's repositioned over the aqueduct, nothing has changed. The position of the barge is irrelevant.
Some (one) are refusing to accept that a wave changes the volume of the water at a point where the wave exists and others are choosing to interpret your question as "a barge is placed in the canal".
Any additional barge placed anywhere in the canal will load the supports of the aqueduct, as long as no water escapes when it is added in (via lock or crane.)
Some (one) are refusing to accept that a wave changes the volume of the water at a point where the wave exists and others are choosing to interpret your question as "a barge is placed in the canal".
Any additional barge placed anywhere in the canal will load the supports of the aqueduct, as long as no water escapes when it is added in (via lock or crane.)
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Well, your question seemed to be more about a barge that is already in the system and contributing to the weight already, being sailed over the bridge - does the weight experienced by the bridge increase for the time the barge passes over? No, not in the sense i suspect you're asking. The system already weighs X tons, of barge plus water, and if you can move the barge around so you don't cause water to pile up somewhere then there is no local increase in weight.
As it's impossible to move a barge around without causing a temporary pile up of water, some small local increase is expected, but that's a few grams, not tonnes worth
Need to be really specific about the parameters of the question- if to you even a microgram of increase in weight on the bridge supports is "yes the bridge supports are carrying more" then it's yes, because it's practically impossible to move the boat without disturbing the water.
If the threshold is "do the bridge supports experience an increase of 20 tons?" then it's no (unless you can move that boat in such a way that you ramp 20 cubic metres of water up against it as it goes, no small feat)
If you were originally asking about adding a boat to the system via crane (anywhere on the system, above the bridge or not) then yes, the bridge supports will carry more. If you add that boat above the bridge centre, and the water around it immediately waves up, and the wave starts travelling towards the bridge ends but hasn't yet reached them then yes, the bridge is now experiencing the entire extra weight of the boat. That will change as the system levels out, and when things are perfectly level again the bridge will only be carrying the proportion of the weight of the whole system directly above it. If half of the system is bridge and then the other half is land, the bridge now carries half the weight of the boat
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Nah, more fun to just let folk run with it.
I suppose there will be modest increases with things like wind loading or some such.
No. They have moved on to a different question!
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Well, there is that "drop a smoke bomb into a crowded room" effect
Often questions of any kind are half baked and aren't really well specified enough to give a clear and definitive answer but people interpret it in their own way and then start arguing over the differences in two interpretations (interpretatiae?
) but if that's the aim then I don't suppose we'll get you to refine the question any. Surprised the physics grad didn't go through all that with you (or maybe they did)Indubitably, though in the interest of getting a quick answer hay's good enough we normally choose to omit stuff like that, with careful wording such as:
A bucket of negligible weight and holding 10L of water is sat on..
and other such that lets the answerer know they shouldn't factor the weight of the bucket, change in mass due to evaporation of the water during the time taken for the experiment to be conducted, and other such impossible-to-arrange factors
---
A truck weighing exactly ten tons drives onto a mile long bridge, that can only support exactly ten tons. Half way along the driver stops and gets out. A sparrow weighing 30g lands on the truck
Does the bridge collapse?
--
Once upon a time the answer was fairly clear but these days there is now an important factor omitted that changes the outcome completely. More questions are required before it can be answered
Well, there is that "drop a smoke bomb into a crowded room" effect
Often questions of any kind are half baked and aren't really well specified enough to give a clear and definitive answer but people interpret it in their own way and then start arguing over the differences in two interpretations (interpretatiae?
Indubitably, though in the interest of getting a quick answer hay's good enough we normally choose to omit stuff like that, with careful wording such as:
A bucket of negligible weight and holding 10L of water is sat on..
and other such that lets the answerer know they shouldn't factor the weight of the bucket, change in mass due to evaporation of the water during the time taken for the experiment to be conducted, and other such impossible-to-arrange factors
---
A truck weighing exactly ten tons drives onto a mile long bridge, that can only support exactly ten tons. Half way along the driver stops and gets out. A sparrow weighing 30g lands on the truck
Does the bridge collapse?
--
Once upon a time the answer was fairly clear but these days there is now an important factor omitted that changes the outcome completely. More questions are required before it can be answered
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The original question was does the weight of the barge increase the weight pressing down onto the aqueducts legs. No it doesn’t. You moved the goalposts and went into an entirely different question. I would say a typical noseall question wanting his usual drama.
I was responding to Highway man who suffers from changes in atmospheric pressure as he works from height.
Let me guess someone tied you to a helium balloon to help you get up.I was just out walking in the street and was pushed to the ground as the weight of the 400 ton Boeing 747 flew over me .
