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United States24514 Posts
For those who didn't read the introductory entry, head over to http://www.teamliquid.net/blogs/viewblog.php?topic_id=79275
Last time the question of the week was:
Question 5 Gravity Imagine two small droplets of water floating in empty space. Our conventional theory of gravity says that they will attract and accelerate towards each other. But what if instead you had water everywhere except for two small bubbles (empty spaces) nearby in the large mass of water?
The bubbles would:
a) Move Apart b) Not Move At All c) Move towards each other (attract)
The answer is c. You can think about the net gravity at a point due to the contributions from specific regions of water. Imagine point p equidistant from region R and region Q, both filled with water. The gravitational field from each region cancels, and results in 0 gravitational field at p.
Now imagine that region R is an air bubble, and region Q is still water. The gravity field at p is now towards Q because R does not provide any field. A rock floating at p would get pulled towards Q. But what about an air bubble originally at P? Bubbles tend to go the opposite direction that a rock would! I would rate this 3/5 difficulty because it takes a simple situation and complicates it.
Luddite provided an interesting explanation:
We can just think of the bubbles as particles of negative mass (holes, basically). However, since gravity goes as the SQUARE of the mass, the negative sign cancels out, so they still attract.
doghunter was the first user to get the question right, but I honestly don't know if his explanation works or not. Same with renezerg. MasterofChaos I don't think I understand your answer, however, I don't believe the bubbles need to be shielded necessarily.
Question 6 Fluids Three icebergs float in bathtubs brim full with ice-cold water. Iceberg A has a big air bubble in it. Iceberg W has some unfrozen water in it. Iceberg S has a railroad spike frozen in it. When they melt, what will happen?
a) Only the water in S will spill over b) The water in S will get lower and the water in A and W will stay exactly brim full c) The water in A will stay brim full, the water in W will spill over, and the water in S will spill over d) All will spill over e) All stay exactly brim full
As always, post your answers in spoilers and provide explanations.
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+ Show Spoiler +i'm guessing e, since the level of the water (brim full) already takes into account the "objects" in the icebergs
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United States24514 Posts
SCC-Faust provide an explanation you emo-blogging self-deprecating son of a broodling.
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On October 20 2008 02:03 kramus wrote:+ Show Spoiler +i'm guessing e, since the level of the water (brim full) already takes into account the "objects" in the icebergs + Show Spoiler +actually if something more dense than ice (or water?) were in the ice then it seems like the water level should decrease after the ice melts, because it would have been displacing more water than it "should have" since it had more mass in the same volume
edit: meant to change to B
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Doing this for a friend: + Show Spoiler +ANS: C explanation: the empty space in in ice A will compensate volume, for ice B, when water is frozen, vol drops unusually, assuming baths are full, the internal water will overcompensate..therefore it will spill..i dont know/never bloody see an railroad spike before..and A and B agrees with answer C.
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Germany2896 Posts
+ Show Spoiler +b) For floating objects only the total mass of it determines the amount of water in moves aside. So the phase transition of A and W doesn't make a difference. S is different, because before it melts the amount of water moved aside is proportional to the total mass. Afterwards the spike sinks to the ground where it will move only the volume of the spike itself aside which is less than the equivalent of the mass of the floating so the water sinks.
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+ Show Spoiler +e) The mass won't change in any of the tubs.
I just read masterofchaos' spoiler though and it seems pretty reasonable, don't have any knowledge of those kind of mechanics though, my instinctive answer would've been e).
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+ Show Spoiler +D. Ice is less dense than water. In the first two cases, the ice can displace no more than a volume of water equal to its weight, and when it melts, it expands to a volume that was larger than the initial volume of water displaced. In the third case, even if it were to be completely submerged in the water, the volume of the water after melting would exceed the volume of the ice before melting.
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+ Show Spoiler +My answer is b) The water in S will get lower and the water in A and W will stay exactly brim full
A floating object displaces its weight in water. Any part of the iceberg which was ice turns into water when it melts, displacing the same volume before and after. So we can disregard the ice part of the icebergs. The floating railroad spike displaces a volume of water equal to its weight, that is, a volume greater than the sinking railroad spike. The water in S will get lower as a result. Air displaces a little bit of water before, and none after, so the water in A will lower a little bit. For the third case, unfrozen water displaces the same amount of water before and afterwards.
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+ Show Spoiler + D In order for the ice to melt the temperature around the ice-cold water needs to increase. When the temperature of water increases the volume taken by the water increases, so if each one of the tubs is full to begin with, then melting the ice will overflow the tub.
I think my answer is more chemistry than physics >.>
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+ Show Spoiler + B According to Archimede's principle, the weight of the water displaced = however much your object weighs. This means with just ice, the water level will not change since weight of ice = weight of water from iceberg melting. Since air weight is practically negligible, this will mean that iceberg A's water level will not move. The same explanation holds for iceberg W, because the water inside the iceberg can be treated separately from the iceberg via archimede's principle. However, when we look at iceberg S, the water displaced is far more than it was in either situation A or B. However, there is the same amount of ice. The same amount of ice is going to create the exact same amount of water, and because the spike doesn't change, more water is displaced by iceberg S than the amount of water that iceberg S will generate when melted. Thus tub S will grow lower in its water level
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+ Show Spoiler + I will assume the volume of ice is negligible Iceberg A will release air which will go into the atmosphere and therefore not increase the volume => Brim full Iceberg W has water in it however the weight of it is equal to the volume it displaces and since water has a relative density of 1 => Brim full Iceberg S has something heavier than water so it should displace more water than it would if it was released. => Water level lowers Not sure if I remember the buoyancy (sp?) law correctly butok That makes my answer: => (b)
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United States24514 Posts
On October 20 2008 04:38 Gene wrote:
Perfect. Gene wins let's move on.
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On October 20 2008 05:10 Slayer91 wrote:+ Show Spoiler + I will assume the volume of ice is negligible Iceberg A will release air which will go into the atmosphere and therefore not increase the volume => Brim full Iceberg W has water in it however the weight of it is equal to the volume it displaces and since water has a relative density of 1 => Brim full Iceberg S has something heavier than water so it should displace more water than it would if it was released. => Water level lowers Not sure if I remember the buoyancy (sp?) law correctly butok That makes my answer: => (b)
+ Show Spoiler + get glass almost full of water and drop let say 10 coins and see what happens to water level
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What physics level/grade do you teach micronesia? If you do teach physics. I'm just curious... and do you like give these problems to your students as extra credit? lol
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On October 20 2008 05:57 micronesia wrote:Perfect. Gene wins let's move on. Dammit he beat me to it..
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United States24514 Posts
On October 20 2008 08:37 Insane Lane wrote: What physics level/grade do you teach micronesia? If you do teach physics. I'm just curious... and do you like give these problems to your students as extra credit? lol I teach high school physics (gr 11-12). I do not give these as extra credit.
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+ Show Spoiler +ANS: C explanation: the empty space in in ice A will compensate volume, for ice B, when water is frozen, vol drops unusually, assuming baths are full, the internal water will overcompensate..therefore it will spill..i dont know/never seen a railroad spike before..and A and B agrees with answer C.
hi i am a high school student, its my first time on tl..i am doing Quantum Mechanics..is there a forum somewhere here?
i really am not very sure of the answer..i din't do fluids in school +.+
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