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Thinking Physics Question 6

Blogs > micronesia
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1 2 Next All
micronesia
Profile Blog Joined July 2006
United States24701 Posts
October 19 2008 16:56 GMT
#1
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.

*****
ModeratorThere are animal crackers for people and there are people crackers for animals.
kramus
Profile Blog Joined November 2007
United States1259 Posts
October 19 2008 17:03 GMT
#2
+ Show Spoiler +
i'm guessing e, since the level of the water (brim full) already takes into account the "objects" in the icebergs
SCC-Faust
Profile Blog Joined November 2007
United States3736 Posts
October 19 2008 17:15 GMT
#3
+ Show Spoiler +
B
I want to fuck Soulkey with a Zelderan.
micronesia
Profile Blog Joined July 2006
United States24701 Posts
Last Edited: 2008-10-19 17:18:22
October 19 2008 17:18 GMT
#4
SCC-Faust provide an explanation you emo-blogging self-deprecating son of a broodling.
ModeratorThere are animal crackers for people and there are people crackers for animals.
kramus
Profile Blog Joined November 2007
United States1259 Posts
Last Edited: 2008-10-19 17:36:25
October 19 2008 17:25 GMT
#5
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
HypnoticPoo
Profile Blog Joined March 2008
Singapore291 Posts
Last Edited: 2008-10-19 17:26:41
October 19 2008 17:26 GMT
#6
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.
MEOW.
MasterOfChaos
Profile Blog Joined April 2007
Germany2896 Posts
October 19 2008 17:34 GMT
#7
+ 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.
LiquipediaOne eye to kill. Two eyes to live.
LaLuSh
Profile Blog Joined April 2003
Sweden2358 Posts
October 19 2008 17:47 GMT
#8
+ 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).
raiame
Profile Joined December 2007
United States421 Posts
October 19 2008 17:56 GMT
#9
+ 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.
datscilly
Profile Blog Joined November 2007
United States529 Posts
October 19 2008 18:10 GMT
#10
+ 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.


eborp
Profile Blog Joined January 2008
United States266 Posts
Last Edited: 2008-10-19 19:24:50
October 19 2008 19:17 GMT
#11
+ 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 >.>
UMCP CSL NEEDS YOU!
brian
Profile Blog Joined August 2004
United States9625 Posts
Last Edited: 2008-10-19 20:12:27
October 19 2008 19:38 GMT
#12
SerpentFlame
Profile Blog Joined July 2008
415 Posts
Last Edited: 2008-10-19 19:45:08
October 19 2008 19:43 GMT
#13
+ 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
I Wannabe[WHITE], the very BeSt[HyO], like Yo Hwan EVER Oz.......
Slayer91
Profile Joined February 2006
Ireland23335 Posts
October 19 2008 20:10 GMT
#14
+ 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)

micronesia
Profile Blog Joined July 2006
United States24701 Posts
October 19 2008 20:57 GMT
#15
On October 20 2008 04:38 Gene wrote:

Perfect. Gene wins let's move on.
ModeratorThere are animal crackers for people and there are people crackers for animals.
Too_MuchZerg
Profile Blog Joined February 2008
Finland2818 Posts
October 19 2008 21:09 GMT
#16
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
Insane Lane
Profile Blog Joined August 2008
United States397 Posts
October 19 2008 23:37 GMT
#17
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
Grobyc
Profile Blog Joined June 2008
Canada18410 Posts
October 20 2008 00:44 GMT
#18
On October 20 2008 05:57 micronesia wrote:
Show nested quote +
On October 20 2008 04:38 Gene wrote:

Perfect. Gene wins let's move on.

Dammit he beat me to it..
If you watch Godzilla backwards it's about a benevolent lizard who helps rebuild a city and then moonwalks into the ocean.
micronesia
Profile Blog Joined July 2006
United States24701 Posts
October 20 2008 01:15 GMT
#19
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.
ModeratorThere are animal crackers for people and there are people crackers for animals.
TimeShifter
Profile Joined October 2008
Singapore235 Posts
October 20 2008 03:01 GMT
#20
+ 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 +.+
strawberries~
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