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United States24753 Posts
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+ Show Spoiler +The can will be moving in the same direction as the side in which the hole was punctured. So it will be moving right (because it was punctured on the right side). This is because the vacuum must be filled, and air rushes in to fill it. However, the can also moves because it is not fixed, and moves in the direction that will aid in the air filling the vacuum. Wow that explanation was terrible.
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United States24753 Posts
+ Show Spoiler +On September 27 2008 10:24 ieatkids5 wrote:
The can will be moving in the same direction as the side in which the hole was punctured. So it will be moving right. This is because the vacuum must be filled, and air rushes in to fill it. However, the can also moves because it is not fixed, and moves in the direction that will aid in the air filling the vacuum. Wow that explanation was terrible.
Please spoiler it so people reading don't notice your choice.
edit: ty
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Belgium6774 Posts
+ Show Spoiler +C
it doesnt actually matter where the hole is punctured
a vacuum will just fill the void with the air it gets and perform force in every direction of the inside of the can, not just one side
...in my sleepy mind
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+ Show Spoiler +a) be moving to the left ya ya? It's like the can grew a mouth, and the air like a thin sausage. As the mouth eat the sausage it will move toward it. Or hmm let me see. Can we consider the air and the can as a system? The center of mass of that system does not change, while the mass of air is dispaced to the right into the can, the mass of can must dispace to the left.
edit: added picture
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On September 27 2008 10:27 Xeofreestyler wrote:+ Show Spoiler +C
it doesnt actually matter where the hole is punctured
a vacuum will just fill the void with the air it gets and perform force in every direction of the inside of the can, not just one side
...in my sleepy mind
I just want to say that I agree with this
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Calgary25991 Posts
+ Show Spoiler + The only part that bothers me is "After the vacuum is filled the can will". The can moves to the left, once the system is balanced, no motion will be taking place. We can assume the rate of air entry tapers off, so can's velocity must slow down as the can fills up. I assume at sometime before steady-state is achieved, friction is stronger than the air force, meaning the can is no longer moving.
C
Edit: The answers state "be moving", not "have moved", which is why I believe it's C.
I also disagree with your answer to question 1 due to the fact we can assume the dog is not running while picking up the stick, meaning we would want to throw it forward to maximize the amount of times the dog has to return. I would imagine you would give full credit to a student who explained their answer this way?
Edit: Whoops you wanted to maximize running! Throw them shits backwards son!
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United States24753 Posts
On September 27 2008 10:42 Chill wrote:
I also disagree with your answer to question 1 due to the fact we can assume the dog is not running while picking up the stick, meaning we would want to throw it forward to maximize the amount of times the dog has to return. I would imagine you would give full credit to a student who explained their answer this way?
Edit: Whoops you wanted to maximize running! Throw them shits backwards son!
Why are you assuming that the dog is not running while picking up the stick? There are 1001 ways you can impose your own minor restrictions that change the answer, but you are not using the most simple/obvious interpretation of the question. Also, I would not give a question like this to my class tbh. If I did, it wouldn't be graded for correctness so much as effort, and I suppose you would earn credit in that case.
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+ Show Spoiler +B? It seems to make sense as the air moves in the can moves in the opposite direction. Once it is filled up, it will keep moving in that direction by inertia.
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Closed system: air and can. Center of mass must remain in the same place and air moves "in the left" you say. If this means into the left side of the can, then the can must move to the left as the air is moving to the right. If this means towards the left (i.e. into the right side of the can) then the can moves to the right so that the center of mass is conserved.
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Calgary25991 Posts
On September 27 2008 11:06 micronesia wrote:Show nested quote +On September 27 2008 10:42 Chill wrote:
I also disagree with your answer to question 1 due to the fact we can assume the dog is not running while picking up the stick, meaning we would want to throw it forward to maximize the amount of times the dog has to return. I would imagine you would give full credit to a student who explained their answer this way?
Edit: Whoops you wanted to maximize running! Throw them shits backwards son! Why are you assuming that the dog is not running while picking up the stick? There are 1001 ways you can impose your own minor restrictions that change the answer, but you are not using the most simple/obvious interpretation of the question. Also, I would not give a question like this to my class tbh. If I did, it wouldn't be graded for correctness so much as effort, and I suppose you would earn credit in that case.
Because what kind of dog picks something up while running? Shouldn't common sense be the obvious restriction?
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+ Show Spoiler +b. the puncture of the can will reduce the air pressure on the left side. there will momentarily be a higher pressure on the right side of the can forcing the can left. As the air fills the can it will push on the inside right wall, but it will still be of a lower pressure than the outside, so the can will continue to accelerate left until the pressure equalises inside the can. at this moment, the can will still be traveling to the left.
I also think it is reasonable to assume that taking and throwing the stick does not take 0 time, and in that time, the dog will be waiting for the stick to be thrown. physics gets annoying when people rely on nitpicking - here is the above with some common-sense assumptions written out in full, so that they cannot be misinterpreted, see how it is a bit unnecessary.
+ Show Spoiler +b. the puncture of the can will reduce the air pressure on the left side. there will momentarily be a higher pressure on the right side of the can forcing the can left. (assuming outside the can is not of near vacuum, such that the amount of air involved is one the number of molcules scale) As the air fills the can, it will push on the inside right wall, (assuming the walls of the can do not react with the air in a chemical reaction, so as to consume the air) but it will still be of a lower pressure than the outside, so the can will continue to accelerate left until the pressure equalises inside the can. at this moment (assuming low enough air resistance so as not to slow the can appreciably, but of high enough resistance and viscosity such that the higher pressure of air gains purchase on the can as is assumed in the rocket can example), the can will still be travelling to the left. all this is assuming that the pressure gradient caused by the intake of air is greater than that which is created in the opposite direction by the can moving to the left (like a bow-wave). this is assuming the can is in an open space of isotropic pressure which maitains a global pressure value but not a local one to begin with and by "air fills the can" you mean that it is at the same pressure as this global air pressure. the moving can will eventually stop due to air resistance, and by the act that as it moves, the pressure in the can will get higher due to its movement forward with a hole at the front, and that the body of air in the can will be moving in the opposite direction. eventually, the can will stop due to this.
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United States24753 Posts
On September 27 2008 11:55 Chill wrote:Show nested quote +On September 27 2008 11:06 micronesia wrote:On September 27 2008 10:42 Chill wrote:
I also disagree with your answer to question 1 due to the fact we can assume the dog is not running while picking up the stick, meaning we would want to throw it forward to maximize the amount of times the dog has to return. I would imagine you would give full credit to a student who explained their answer this way?
Edit: Whoops you wanted to maximize running! Throw them shits backwards son! Why are you assuming that the dog is not running while picking up the stick? There are 1001 ways you can impose your own minor restrictions that change the answer, but you are not using the most simple/obvious interpretation of the question. Also, I would not give a question like this to my class tbh. If I did, it wouldn't be graded for correctness so much as effort, and I suppose you would earn credit in that case. Because what kind of dog picks something up while running? Shouldn't common sense be the obvious restriction?
At this level, it becomes impossible for there to be a 'correct' answer to any question like this :-/
The more physics problems you do, the more obvious it becomes which types of unstated restrictions are assumed by the question writer, and which are not even being considered. The example you are coming up with is a restriction where the question writer would specifically state that time was spent picking up the stick each time.
How do you prove who is right when arguing about what should or shouldn't be considered, giving the wording of a physics problem? Unfortunately, the best, or perhaps least bad way to do it is to interview a bunch of physicists and look for a consensus. Also I didn't make up the answer... taken straight from a published physicist.
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I just want to say that even though I am not answering the questions and providing reasons, I am following your blog and I appreciate you doing this. Keep up the questions micronesia, you are great! 
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The best things in life are free, and being punctured isn't the best thing that can happen to you, so the can owes someone money.
+ Show Spoiler +
I want to consider that most cans can't take that kind of pressure and would just implode.
Also, the problem's wording is poor. Air rushes in the left. Into the can from the left side? Into the can, moving in the direction "left"? Typically, such wording is correlated to the wording from the example given, but in this case the wording is "blows to the right" compared to "blows in the right." "To" and "in" could both be relative to the can or the observer, so that doesn't help much.
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On September 27 2008 13:27 BottleAbuser wrote:The best things in life are free, and being punctured isn't the best thing that can happen to you, so the can owes someone money. + Show Spoiler +
I want to consider that most cans can't take that kind of pressure and would just implode.
Also, the problem's wording is poor. Air rushes in the left. Into the can from the left side? Into the can, moving in the direction "left"? Typically, such wording is correlated to the wording from the example given, but in this case the wording is "blows to the right" compared to "blows in the right." "To" and "in" could both be relative to the can or the observer, so that doesn't help much.
read more physics >_> I dare say you're very not familiar with how physics is run conventionally so the confusion is understandible.
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My first interpretation is that the air is moving in the left direction, not entering the can's left side, and my intuition tells me it is what was meant. But if I was grading this, and someone was arguing with me about the wording, I'd fold. Dunno about Micronesia.
I'm just pointing out that there is real ambiguity here. I prefer unambiguous notation, and if you must use a natural language, be sure not to use unambiguous terms at all. In particular, the word "in" when used around containers is somewhat dangerous, if you do not actually refer to the inside of the container.
+ Show Spoiler +Since the wording is "after the can is filled," the can will no longer move. Consider that for it to be moving, some air somewhere must be moving also. The air inside the can would be moving in the same direction at the same speed, so that wouldn't help.
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EPT
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