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United States24644 Posts
As any seasoned tl blog reader would agree, physics is a topic seen most predominantly in 'help me with my hw' type threads. Physics is indeed a course that many people who lack the background or interest take due to a requirement for school/work. In fact, this effect has become so exaggerated that most people don't seem to realize just how interesting, relevant, and fundamental physics really can be (and is).
I'm not bringing this up due to some kind of attempt to be a responsible educator of physics. My motivation is more simple. I like physics, and I don't like to see it misrepresented until it drives many people to dislike it.
I have access to a nice collection of thought provoking physics questions of various levels. My plan is to post one question (multiple choice) minus the answer, and ask for people to provide in spoilers their prediction of what the answer is, and an explanation. I suppose I should pick the first response that adequately answered the question, and feature them as the previous week's winner.
I'm going to choose a question of varying difficulty from the following topics (probably will sequentially run through the topics):
Mechanics:
Kinematics
Newton's Laws of Motion
Momentum and Energy
Rotation
Gravity
Fluids
Heat
Vibrations
Light
Electricity and Magnetism
Relativity
Quanta
The goal is for every reader to encounter at least one question that is difficult enough to challenge them while being within their knowledge/ability to answer correctly. These problems are mostly simple at face value, but at times can fool even graduate students in physics programs, so don't sell them short! Upon the conclusion of the question, I will provide my own estimate of the difficulty of the question on a scale of 1-5 with 1 being easiest.
Question 1
Kinematics
Micronesia exercises his dog yubee on a 15-minute walk by throwing a stick that yubee chases and retrieves. To keep yubee running for the longest time as Micronesia walks, which way should he throw the stick?
a) In front of him
b) In back of him
c) Sideways
d) In any direction, as all are equivalent
+ Show Spoiler +Starting with an easy one that doesn't require much knowledge, but don't expect this to continue 8o
  
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+ Show Spoiler +b
throwing the stick backwards makes the dog have to cover the distance between you and the stick twice plus whataver distance you may have traveled by walking.
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+ Show Spoiler +
EDIT:
+ Show Spoiler +because you are walking forward away from the stick while yubee is fetching it so there is a longer distance for him to run to get back to you
but if its yubee you might want to throw it on a roof or something
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For question one + Show Spoiler + my first instinct is behind him because then the dog has to run in two directions, behind, then back to the owner giving the most exercise. This assumes the owner walks in the same direction the whole time.
That makes the most sense to me.
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United States24644 Posts
Feel free to post just the answer if you want, but an explanation of why you chose the answer would be nice, could illicit discussion (in spoilers I suppose), and also qualifies you for being the winnAr.
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So... you're using TL.net as a tool to measure the difficulty of problems that you intend to assign to your students? eh?
+ Show Spoiler +Ahh! I fell for the trick! It's d because it's about time. You take 15 minutes to walk. The total distance the dog runs is v*t. If you keep the dog running the entire time then nothing happens. I blame the guy who didn't spoilertag his answer
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United States24644 Posts
On September 26 2008 09:35 Hippopotamus wrote:
So... you're using TL.net as a tool to measure the difficulty of problems that you intend to assign to your students? eh?
Lol. I minimize how many problems I give to my students that require thought. Not because I don't want them to think, but because their ability to do it is generally limited (and I blame NCLB XD).
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On September 26 2008 09:35 Hippopotamus wrote: So... you're using TL.net as a tool to measure the difficulty of problems that you intend to assign to your students? eh? + Show Spoiler +Ahh! I fell for the trick! It's d because it's about time. You take 15 minutes to walk. The total distance the dog runs is v*t. If you keep the dog running the entire time then nothing happens. I blame the guy who didn't spoilertag his answer
+ Show Spoiler + yes but the dogs velocity would have to increase to catch up to you if you throw it backwards, so it's not like it's constant or anything.
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Haha I can actually think of a lot of complications with that first question. Like, can we assume that you can throw the stick equally far in any direction? Because I think I can throw it a lot farther in front of me then I can behind me. And, where's the dog when you throw it? is he right under your feet, or somewhere in front of you? And do you keep walking when you throw it?
Heh sometimes I feel like the only thing my education in physics has taught me is how to nitpick stuff like this.
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Also:
On September 26 2008 09:24 micronesia wrote:
probably will sequentially run through the topics):
Fluids
Heat
Vibrations
Who says physics isn't sexy 
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Based on the fact this is supposed to be simple, I believe a certain razor would easily explain the answer to all your questions
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Calgary25977 Posts
Nice! I spend most of my days working on fluid mechanics so I'm excited for that section! Good blog!
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On September 26 2008 09:47 Luddite wrote:Also: Show nested quote +On September 26 2008 09:24 micronesia wrote:
probably will sequentially run through the topics):
Fluids
Heat
Vibrations
Who says physics isn't sexy
I ROFL'd to that.
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+ Show Spoiler +d? assuming all else is equal, and micronesia will be tossing the stick as soon as he gets it back, then yubee will essentially be running the duration of the 15 minute walk. wait.....
i would say b, if yubee would wait (or walk with micronesia) a split second every time he brings the stick back for micronesia to wind up and throw it again. limiting this interruption would force doggy, er yubee, to spend more time running during micro's 15 minute walk.
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wow, i never knew the principle of relativity could trick so many people, assuming he's walking at constant speed of course.
+ Show Spoiler +
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+ Show Spoiler +Behind. The distance from Yubee's starting position to the place where the stick landed will always be the same and hence the time will be the same, so Micronesia wants to know how to be farthest from the stick when this time occurs, which occurs when he walks directly away from the stick.
Unless Micronesia continually throws the stick; then it doesn't really matter, although if he always takes a fixed time to throw the stick then behind still wins.
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+ Show Spoiler +Behind him, assuming that they're walking a straight path and the dog slows down when he picks up the stick and returns it. The dog will have a longer return trip than any of the other directions.
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O RLY?
Remember this timeless truth:
It's only physics if the deer are perfectly spherical and there is no air resistance!
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United States24644 Posts
On September 26 2008 11:29 Hippopotamus wrote: O RLY? Remember this timeless truth: Show nested quote +It's only physics if the deer are perfectly spherical and there is no air resistance!
The perfectly spherical one I'll give you, but I've had to do too much air resistance for you to say that!
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On September 26 2008 19:33 micronesia wrote:Show nested quote +On September 26 2008 11:29 Hippopotamus wrote: O RLY? Remember this timeless truth: It's only physics if the deer are perfectly spherical and there is no air resistance! The perfectly spherical one I'll give you, but I've had to do too much air resistance for you to say that!
haha, but back in grade school/etc, they always ignored air resistance and it made me so fucking mad.
They'd be like "John is riding on the back of a pickup truck. He throws a ball straight up in teh air while moving forward at 40 mph. When the ball comes back down, where will it land?"
And obviously the REAL answer is behind him because of air resistance, but those asshole teachers were always like "NO IT LANDS BACK IN HIS HANDS LOLOL U LOSE"
so I'm like fuck how about we go try it out to prove it and they're like "YOU GET AN F"
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United States24644 Posts
On September 26 2008 19:48 -orb- wrote:Show nested quote +On September 26 2008 19:33 micronesia wrote:On September 26 2008 11:29 Hippopotamus wrote: O RLY? Remember this timeless truth: It's only physics if the deer are perfectly spherical and there is no air resistance! The perfectly spherical one I'll give you, but I've had to do too much air resistance for you to say that! haha, but back in grade school/etc, they always ignored air resistance and it made me so fucking mad. They'd be like "John is riding on the back of a pickup truck. He throws a ball straight up in teh air while moving forward at 40 mph. When the ball comes back down, where will it land?" And obviously the REAL answer is behind him because of air resistance, but those asshole teachers were always like "NO IT LANDS BACK IN HIS HANDS LOLOL U LOSE" so I'm like fuck how about we go try it out to prove it and they're like "YOU GET AN F"
Are you saying they didn't understand it, or that they weren't recognizing publicly that it works differently? It's not that hard to play the game as a student if you understand it at that level.
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On September 26 2008 19:48 -orb- wrote:Show nested quote +On September 26 2008 19:33 micronesia wrote:On September 26 2008 11:29 Hippopotamus wrote: O RLY? Remember this timeless truth: It's only physics if the deer are perfectly spherical and there is no air resistance! The perfectly spherical one I'll give you, but I've had to do too much air resistance for you to say that! haha, but back in grade school/etc, they always ignored air resistance and it made me so fucking mad. They'd be like "John is riding on the back of a pickup truck. He throws a ball straight up in teh air while moving forward at 40 mph. When the ball comes back down, where will it land?" And obviously the REAL answer is behind him because of air resistance, but those asshole teachers were always like "NO IT LANDS BACK IN HIS HANDS LOLOL U LOSE" so I'm like fuck how about we go try it out to prove it and they're like "YOU GET AN F"
Since you do not know in which way the wind blows (it could blow 40 mph at the same direction as the truck) then you have to assume that any form of air resistance is negligible or the question is impossible to answer.
By the way, this question:
You got a closed half sphere metal shell with a radius of 2A. (Thinks half a sphere were the opening is bridged)
You got a point charge q at a height A above the middle of the flat part.
In which direction do the charge move? (No gravity or anything like that, just electrostatics)
a: up
b: stands still
c: down
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+ Show Spoiler +
I would think that if the dog is walking with the owner, if you threw it backwards, it takes some time to turn around.
If the owner can throw it equally strong in every direction, the stick would go the furthest if he threw it forward, but the dog is already moving in that direction so s/he doesn't have to turn around and accelerate
I really have no clue so I'm gonna say forward, a)
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This blog rocks! Physics is the shit, I can't understand why basically no one thinks so. Will you include anything with theoretical physics?
+ Show Spoiler + If you take this question as it is given to you (without raising any questions of your own, I would say d. If micronesia throws the stick multiple times it really does'nt matter where as yubee(lol) will be running constantly. If anything else applies though, I would say b.
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+ Show Spoiler +
Just read all the others spoilers, and I find it strange that not more ppl came up with this solution...
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+ Show Spoiler +B. assuming that the throw in any direction is equally far. then, if you throw backwards, you will have walked an extra vel_walk * time_dog, where vel_walk is your velocity in the forward direction and time_dog is the time it takes for the dog to get back to you, and likewise the dog will have run that extra amount as well.
sweet, can't wait for the E&M / Quantum / Relativity sections
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United States24644 Posts
On September 27 2008 01:17 Makhno wrote:
This blog rocks! Physics is the shit, I can't understand why basically no one thinks so. Will you include anything with theoretical physics?
I'm starting with a set source of questions, which includes only the topics listed in the OP. This will last us for a while, and I'm open to suggestions for where to go from there, but I probably am not worrying about that for a few days.
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On September 27 2008 01:17 Makhno wrote:
This blog rocks! Physics is the shit, I can't understand why basically no one thinks so. Will you include anything with theoretical physics?
Geology rocks, physics is just plain awesome.
And the further you get from boring High School mechanics the more things you get to stop ignoring, which is when it gets fun. Especially when progress through E&M, you go from spheres to all kinds of weird charge and current configurations.
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Canada7170 Posts
+ Show Spoiler +b. And I bet that lots of others already mentioned why.
But I'd probably shoot the dog asap 
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To further clarify others' reasonings...
+ Show Spoiler +
If it takes a negligible amount of time to take the stick from the dog and throw it again, one can assume that the dog is running the whole time, so it doesn't matter which direction Micronesia throws it in.
However, if it does take a certain amount of time to do this, Micronesia would want to throw the stick in the opposite direction of his movement, assuming he always throws it some distance D, regardless of direction, as this would lengthen each iteration of throwing and retrieving, and thereby reduce the time that Yubee is waiting for Micronesia to throw.
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On September 27 2008 01:00 Purind wrote:+ Show Spoiler +
I would think that if the dog is walking with the owner, if you threw it backwards, it takes some time to turn around.
If the owner can throw it equally strong in every direction, the stick would go the furthest if he threw it forward, but the dog is already moving in that direction so s/he doesn't have to turn around and accelerate
I really have no clue so I'm gonna say forward, a)
+ Show Spoiler +
Oh wait, I think I understand now. My answer was assuming that micro was walking, threw the stick then stood still immediately after
If micro keeps walking, and assuming he can throw it with the same strength (I see this as initial speed with which the stick gets thrown) the stick will have a velocity (micro's walking speed + throwing velocity), so the stick moves with the same speed relative to micro regardless of direction
I'm not that good with imagining stuff, but now that I think about it, if you throw forward, the thing will move faster (thus farther) but micro walks toward the dog, but if you throw backwards, the thing will move slower so the dog doesn't take long to retrieve it, but takes longer for the dog to go back to micro. I'm having trouble working this out in my head, but it seems that the forward and backward times will be the same, so I'll go with doesn't matter
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+ Show Spoiler +well you have to make a few assumptions, such as the dog has constant velocity and that the owner throws the stick equally far in each direction, otherwise you'd be able to come up with countless scenarios and it would be impossible to answer the question, also the time it takes for micro to take the stick from the dog should be negligible. So, as someone mentioned, it will just be velocity*time = distance, and that's the only thing that counts. If you throw it backwards, the dog has to run more with each throw, but you will have to throw less times, they are just countering each other.
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Germany2896 Posts
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EPT
