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Yeah I know that you're pointing out there is a real ambiguity. What I'm saying is if you deal with physics on a regular basis there should be very little ambiguity in this question. I find this question perfectly worded and I'm sure my fellow physics friends would agree too.
For comparison, imagine now I tell you PvZ you can go 10/12 gate. Now this is perfectly clear what I mean right? But somebody who's not playing starcraft very often might be confused as to "Oh you build 10 gateways over 12 gateways ? what's going on?". That does not mean my wording is ambiguous, that just mean that person, (in this case, you), is not familiar with the conventions of the subject
But lemme reword it so maybe it'll help, because convention shouldn't held people back from thinking recreatively:
Can is vacuumed out.
You poke a hole on the left of the can
Obviously, since the can is vacuum, it'll start sucking in air from the left side cuz there's a hole on the left side.
Which way does the can move as a result?
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Much clearer.
+ Show Spoiler +However, I'm not confident that it's the correct interpretation. Either way, the answer does not change, as we're interested in velocity of the can when it has been filled, ie no longer moving.
By the way Chill, you don't need friction. The momentum of air is enough.
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So if the hole is in the left side, my answer would be reversed 
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On September 27 2008 12:38 micronesia wrote:Show nested quote +On September 27 2008 11:55 Chill wrote: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 :-/
Nah, Chill's right. Maybe he should be the physics teacher.
Stick to bowling and silly ads! :p
Btw the real answer is that you could just pretend to throw the stick and yubee would just keep running forever.
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don't u tell Micronesia what to do ;(
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+ Show Spoiler +
My answer is C, even if gas is ideal and there is no friction. The only moving parts of this system are can and gas that blows inside the can and eventually they are forced to move in one direction. So the only mean to fulfill momentum conservation law is to assume that they will not move at all. So the can will be just shifted in the process.
In this solution i assumed that gas will blow only inside the can and not outside which based just on common sense because actual description of this process is quite difficult.
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Q2: The compressed air can moved because there was a lot force trapped into the can in the form of compression. Because a can is a vacuum assuming it doesn't just suck into itself the air shouldn't move it at all since the only force is suction into the can, which shouldn't move the can. If it does, it will move to the left because the air comes in the left the can should occupy that space. But I don't think thats the right answer.
Q1: I disagree with you on this micro. If you throw the ball behind you every time when you walk, at the end of the 15 minutes the dog will have to catch up to you since he would have been running the opposite direction. So his t will be t+15
MN t=14:30min UB
<------- --------->
... t=15min
Home+MN <---------UB
If you throw it in front UB will actually be home faster than MN and be running less, but i guess he still has to return it but he will have to run less distance.
EDIT: throw sideways would work but not as well as shown in the diagram: MN has moved after throwing stick and UB has just reached it.
....................^
................ MN
.................|3m
1UB>=====|
........4m
He will only run 5 m because of Pythagoras' Theorem. If you through it backwards the same diagram will be thus:
MN
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3m
|
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|4m
UB
Path will be 3+4 = 7m.
and if you through in front: it will be 4-3 = 1m. By the time you get home the dog will have run less because your walking will help him. Not to mention all the amount of extra time you were picking up a stick because the dog got to you much faster.
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+ Show Spoiler +
As ppl have pointed out, the correct answer must be "not moving"
The system starts in rest, nothing is moving. Then air will rush in from the left, moving the can the opposite direction. But AFTER the can is filled, the air inside the can cannot move in any direction other than the direction the can is moving (macroscopically speaking).
Since the system started out in rest, the added velocities of the air and can will be 0. Since they both has to move in the same direction, the answer must be that both velocities are 0.
System starts in rest: v_air + v_can = 0
Restriction after can is filled: v_air = v_can
Put the two together: 2*v_can = 0
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+ Show Spoiler +c
The vaccum is "filled" the instant that any air enters that can, so immediately after the puncture. At that time, the can won't be moving yet. Shortly thereafter, the can will move towards the left.
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+ Show Spoiler +"The air blows in the left as it enters the can."
- The air blows in [through] the left [hole] as it enters the can?"
- The air blows to the left [direction] as it enters the can?
I think the second is the most intuitive, which means that the hole is on the right side.
I guess to the right. Before the hole opens, the atmospheric pressure on the can is constant everywhere; summing up the forces gives you zero. Once the hole opens, though, there's a surface on the right side of the can that no longer has leftward pressure exerted on it, since the interior of the can exerts no pressure on the outside. The net force on the metal of the can is then rightward.
Obviously if its blowing in through the left direction hole reverse the answer.
I think.
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+ Show Spoiler +it wont be moving, it will just be filled with air. Because for it to be able to get moving there has to be acceleration, and for acceleration there has to be a force, but there is no force applied to the can.
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On September 27 2008 17:00 Slayer91 wrote:
Q1: I disagree with you on this micro. If you throw the ball behind you every time when you walk, at the end of the 15 minutes the dog will have to catch up to you since he would have been running the opposite direction. So his t will be t+15
MN t=14:30min UB
<------- --------->
... t=15min
Home+MN <---------UB
If you throw it in front UB will actually be home faster than MN and be running less, but i guess he still has to return it but he will have to run less distance.
EDIT: throw sideways would work but not as well as shown in the diagram: MN has moved after throwing stick and UB has just reached it.
....................^
................ MN
.................|3m
1UB>=====|
........4m
He will only run 5 m because of Pythagoras' Theorem. If you through it backwards the same diagram will be thus:
MN
|
3m
|
|
|4m
UB
Path will be 3+4 = 7m.
and if you through in front: it will be 4-3 = 1m. By the time you get home the dog will have run less because your walking will help him. Not to mention all the amount of extra time you were picking up a stick because the dog got to you much faster.
but you are miss interpreting the question. It says that yubee walks for 15 minutes, and you have to assume that the dog also runs for 15 minutes. As I also said in the other blog, you have to do some assumptions, such as the dog runs with constant velocity, and that the time it takes to pick up the stick is negligible. Otherwise you would be able to come up with countless scenarios, and there wouldn't be a "correct" answer.
As someone mentioned in the last blog, it's all about time*velocity = distance, and the question was, did it matter in which direction the stick was thrown? no, because the time is still constant, and so is the velocity with the assumptions we already made.
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+ Show Spoiler +On September 27 2008 10:17 micronesia wrote:
...After the vacuum is filled... C - Since the can has been filled there will be no pressure and the can will be motionless.
On September 27 2008 10:17 micronesia wrote:
Makhno was the second person who seemed to give a nice explanation! :D
This seriously made my day ^^, I love this blog.
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+ Show Spoiler +I think options C that it wont move at all since we can assume the pressure and temperature outside the can will remain constant even after the it has filled the can with air.
Then we can assume quasistatic and adiabatic (this hasnt to be assumed but makes the calulations much simpler) then:
TdS = dW + dQ = -P_o*dV (this is then entropy of the system where the can is included) =>
-P_o*dV = |irreverseble process, not really but there are ways around this, as we are only looking for the states of equilibrium) then TdS = 0 =>-P_o*dV = 0. This means the net force acting on the can is zero, thus no movement.
I might be wrong, if so please correct me 
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+ Show Spoiler +C. I hate your tricky questions. After the can completely fills up, the system will once again have found an equilibrium with respect to its center of mass, so the can will not be moving after it is completely filled.
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+ Show Spoiler +after it's filled the forces are balanced so the air resistance would very quickly stop any motion. C
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yes, some things need to be clarified:
is this an open or closed system? is it ST&P so that the speed of sound slow enough to set up pressure gradients?
what do you mean by 'filled'?
filled=of the same pressure as the outside? (this is what I think you mean)
filled=with any air inside? (not really relevant: true vacuums are not really possible, because the inside of the can will evaporate in a true vacuum to create some independent molecules)
filled=a steady state with no acceleration? - the answer will always be C because the total momentum at the start is 0, and must be the same at the end steady state, without any consideration of vacuums or air pressure. So I'm guessing this is not what you mean.
you should specify whether friction is negligible, or if the air is viscous, because if air resistance exists, the answer will always be C after time. So I'm guessing you intend there to be no air resistance.
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On September 27 2008 10:41 fusionsdf wrote:Show nested quote +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
This one wins
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United States24701 Posts
On September 27 2008 23:48 betaben wrote:
yes, some things need to be clarified:
is this an open or closed system? is it ST&P so that the speed of sound slow enough to set up pressure gradients?
what do you mean by 'filled'?
filled=of the same pressure as the outside? (this is what I think you mean)
filled=with any air inside? (not really relevant: true vacuums are not really possible, because the inside of the can will evaporate in a true vacuum to create some independent molecules)
filled=a steady state with no acceleration? - the answer will always be C because the total momentum at the start is 0, and must be the same at the end steady state, without any consideration of vacuums or air pressure. So I'm guessing this is not what you mean.
you should specify whether friction is negligible, or if the air is viscous, because if air resistance exists, the answer will always be C after time. So I'm guessing you intend there to be no air resistance.
I took this question from someone who's better at this than me, so I'm trying not to change which clarifications are/aren't made. Usually the most obvious assumptions are the ones that are meant to be made.
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EPT
