[0450] Under A Killer Balloon: Episode 1, Page 8
└ posted on Monday, 28 January 2013, by Novil
Update on 7 February 2013: I updated Sandra’s dialog in the first panel since the original description of the puzzle was probably not precise enough. It’s been a long time since I had physics in school…
- Sandy South: Two similar looking metal rods lie on a table. One of them is magnetic, the other one is not. I have no tools and mustn’t take the bars off the table. How can I find out which one is magnetic?
- Sandy South: As strange as it may sound, if I don’t solve this tricky puzzle soon, I may never find out who stole Cloud’s action figure.
- Qoo: Shouldn’t we have a second look at the scene of the crime?
- Sandy South: Excuse me, but I can’t do physics homework and solve crimes at the same time!
![[0450] Under A Killer Balloon: Episode 1, Page 8](/comics/2013-01-28-0450-under-a-killer-balloon-episode-1-page-8.png "[0450] Under A Killer Balloon: Episode 1, Page 8")
Holy crud, she’s taking physics in the seventh grade?
*looks at panel 2*
Careful how often you make those faces, girl. It might stay that way.
Metal table and blow, the moving one is the non-magnetic, if its not a trick question and they’re Both magnets really and can therefore attract/repel.
I’d prefer her physics work to my own…
It’s called multitasking, Woo. Look it up.
Spin the rods repeatedly, the magnetic one will always point north…
Would this work? I just thought it up.
JDad wrote:
I don’t know why you’re so surprised with that. I already took physics at 7th grade too.
It was really bothering me so i found the answer online. I won’t spoil it for you by writing it here. I posted the link to the answer instead and you can choose to look yourself.
http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_NextTime_BarMagnets.xml
Exactly why I hate homework. XD
The answer is simple, Sandra. Have someone else with tools take the bars off the table.
Either pole of the magnetic rod will attract and adhere to any point on the non-magnetic rod’s surface. The non-magnetic rod will be attracted and adhere to the magnetic rod only at the magnetic poles.
Clarissa Corelli, of Firebrand Investigations: Duh! The first one I point to is magnetic, the other one is non-magnetic. Not only do my reality distorting powers make it true, but my teachers accept it out of respect… and desire not to have things catch fire. Now, I’m taking over this case, and *girlish sigh* this client.
Sandy South: Aha! Not so fast, my mob-connected rival! You’ve just told me you have motive… and means and opportunity can’t be far behind. *draws water pistol* Put your hands up!
Clarissa Corelli: *mirthless chuckle* Alright Landon. *nods* *nods again* Now, Landon. Yes, now. Go on! Just hit her with it! Ugh, for crying out loud! That’s what I get for sending a nerd to do a jock’s job! *shakes head, grumbles*
If the bars are made of the same material, the end of the magnet will stick to the middle of the non-magnet, whereas the end of the non-magnet will not stick to the middle of the magnet. This is because magnetic attraction is greatest at the poles and weakest at the center.
Otherwise, just call Larisa and have her set fire to the table. At 770 degrees C the magnetized bar will degauss, at which point you can conclude that neither bar is magnetic.
Pour iron filings onto the table in order to trace out the magnetic field lines.
The magnetic rod killed my Pa!
MIRACLES!
well, one day Something similar happened to me….but…IN class
The teacher gave us a sheet of paper with questions and I started to use the book ..he then said
“You remember this is a test..right?!”
“so I can’t use the book and answer at the same time ??”
“Panic “
I see a few others have posted the accepted answer so I will confine myself to pointing out problems with alternative answers. 🙂
@Crestlinger:
It’s almost certainly a non-metallic or at least non-magnetic table. Besides, there’s no rule against touching the bars, only against lifting them from the table.
@Petah-Petah: You get a D- (instead of an F) for inventiveness, but I think that technically this would count as having someone else solve the problem for you, which is cheating. :-J
@myth buster: Iron filings would count as a tool.
@Frozenwolf150: Alternate answer might count as having someone else solve the problem for you, and would certainly count as using tools (whatever Larissa uses to set the fire), but it’s still an awesome idea! 😀
Oh, and @Sambo: The friction from rubbing against the table would overwhelm the relatively small effect of the Earth’s magnetic field, so the magnet wouldn’t end up pointing north-south any more often than the non-magnetic bar. But I’ll give you a E+ for innovation. 🙂
My first reaction to the problem mentioned: What situation has happened to cause this? Is this the devious plot of some evil genius bent on world destruction, which the only way to stop a doomsday device requiring that the rod used is either magnetic or not… and a trap on the table to make sure you only have long enough to use one of them? …I then read that it was physics homework, and felt oddly disappointed.
My second reaction: If you can’t use tools, and the rods can be a tool for testing which one is magnetic, can you then not use the rods? This seems trickier than before!
My third reaction: Wait, in that case, anything used would be the tool to determine this. The answer would have to result from an independant action.
My final answer: Since anything used to determine which rod is magnetic would then be the tool used to determine that, this is a trick question. You cannot determine the answer without using a tool unless some random event happens to reveal the answer. *Somehow, my teachers always hated my homework.*
This particular strip suddenly puts several of our previous strips in perspective. 😀
Ah. So it actually means “both bars are magnetic, but only one is magnetized.” That’s different! Much easier!
Who taught her how to be a detective, Professor Layton?
My answer:
Step 1: Remove boot. It matters not which one.
Step 2: Hold it next to each bar, toe closest to the bar.
Step 3: The bar that moves toward the boot is magnetic.
I only ever wear steel-toed boots. I only own those and a pair of thongs, and the thongs almost never leave the house.
@ …:
Sadly, steel is diamagnetic. And boots will count as tools. http://alturl.com/waw29
@ Melkior:
Technically it says Sandra has no tools. It says nothing about Larissa having or not having tools. And when would our favorite pyromaniac not have implements for lighting a fire?
Setting priorities is important, and Cloud can’t give her a bad grade for failing to solve his little conundrum…
JDad wrote:
I don’t know how school works elsewhere, but I distinctly remember I started physics in sixth grade. Czech Republic, for your information.
Not that it helps me to figure this out now…
Greenwood Goat wrote:
Greenwood Goat wrote:
Slight issue… second rod will imediately magnetize after such attempt… (technicaly there exists no unmagnetized metal on the scope of this planet since it has magnetic field and any paramagneticum in it is SLIGHTLY magnetized.
Well it will magnetize during such attempt. therefore in the end it will be impossible to make decision… what you have posted would work in ideal universe where there would be no reaction.
JDad wrote:
I do not know about rest of the Europe and America but in Czech Republic we have mandatory physics as subject from 6th grade… 7th, 8th and 9th follows… it is mandatory at all technical and grammar high schools for another 4 years.
Generaly everyone leaving such schools should be able to do elementary differential and integral calculus. (Albeit I admit infinitesimal amount of people understand it at that point).
But I must admit even after this 1/3 of the populace is incapable of understanding law of conservation of energy and general impossibility of storing any reasonable amount of electric charge in medium. (which sadly produces only direct current which is more or less useless on aby but local basis… alternation throws away half the capacity).
Get a 3rd metal object.
Well, This is my personal solution, tell me what you think pls:
The magnetic bar must have a north and a south pole, near the north pole the magnetic field lines enters in the bar and in the south they exit from the bar. so there is a surface where the magnetic field is null:
you should recognize it by spinning slowly a bar near the other, there would be an angle where the attraction of the two bars is lower and another angle where it is maximum.
I thought women were supposed to be good at multitasking. Guess not!
No mention of the type of table is given. Assume it is smooth plastic.
Spin the two rods.
The one that is magnetic will slow first, due to distruption of the earth’s magnetic field.
@ Bill:
Non-magnetic means exactly that: non-magnetic. Putting the end of a magnet near the middle of a non-magnetic rod will have no reaction, just as putting the end of a non-magnetic rod near the middle of a magnet would not.
Spinning the rods DOES have a strong effect. Try it some time. One magnetic field fighting against another is very effective, and will slow the rod’s rotation quickly.
@ Sambo:
On a nearly frictionless table, that would work. On a normal wooden table, you could spin each bar a hundred thousand times and write down all the outcomes and still need at least two semesters of undergraduate-level statistics to be able to decide if you’d learned anything useful.
@ Crestlinger:
It’s not reasonable to assume the table is metal if the problem does not specifically say so. Most tables are made of non-ferromagnetic materials. Of course, it is undeniable that the question is badly written (among other things, it says that one bar is “magnetic” without specifying which of the several meanings of the word “magnetic” is intended — ferromagnetic is the most likely meaning, but it’s also conceivable that in saying “similar looking” the problem intends both bars to be made of the same presumably ferromagnetic material and that one of them is a permanent magnet), so it’s possible you may be right.
@ …:
For using the word “thong” in a manner that would be rather amusingly misinterpreted by young people, you are charged with being over the age of fifty. How do you plead?
@ Chino:
Any diamagnetism that steal might exhibit would in this situation be overwhelmed by its ferromagnetism. If you hold it up to a permanent magnet, it’ll stick. (The fact that the steel is encased in leather means the permanent magnet would have to be polarized in a section larger than the thickness of the leather. For a traditional bar magnet that’s a non-issue, but a lot of refrigerator magnets won’t magnetically interact with a steel-toed boot at all, because the leather is thicker than their field.)
@ Paeris Kiran:
In America, physics is not called “physics” until high school. (Prior to that we just call it “science”.) But yes, we study science in every grade, and this includes topics that would technically be classified as physics. I think I first studied magnetism (at a very simple level) in third grade.
What do you mean you have no tools, Sandra? Your coat buttons are ferromagnetic!
In the transcription, you spelled Woo’s name as “Qoo”.
@ DHGM:
It’s part of the story arc they’re in right now, so technically it’s correct.
@ Petah-Petah:
Or you can take the whole table somewhere….
Got it! The one with N and S stamped at either end is the magnetic one. The question only says they look similar, not identical.
ROLL THEM TOGETHER! Whichever one rolls towards the other one faster is the non-magnetic one.
@Melkior and @Jonadab
aw. oh well, guess i’m in grade 10 and can’t do grade 7 physics.
She doesn’t have any tools, but nothing says she can’t go and get a tool, and then use it. And nothing says she can’t carry the table down to the local gym and see if one of their iron weights is attracted to one of the rods.
(We don’t know if the non-magnetic rod is made out of a metal that is attracted to a magnet.)
Simple take a quarter or a pencil (not a tool, just crud in your pocket) and put it betwix the rods, whichever rolls towards the metal is the magnet
Choose one bar. Pretend it is a magnet and try to magnetize the other bar by rubbing what would be the pole along the other bar several hundred times. Now, see if either of the “poles” of the “magnetized” bar repels the “pole” you’ve been rubbing it with. If one does, the bar you’ve been rubbing with was a magnet to begin with and the other bar was not (but now is) a magnet. If neither repels, then the bar you chose was not the magnet and the other bar is a magnet.
@Melkior – Extending the Larisa idea to not use “tools” or friends. Assuming a wooden table, one can break the legs off (while carefully holding the magnets to ensure that they do not fall off the table) then rub one of the legs on some hard surface (another leg if necessary) until both of its legs are worn into a point. Another leg can be used to gouge a dent in the table top and in a different leg (maybe the leg you were using as a rubbing stick). This process should have produced a lot of wood dust. Fill the table dent with the dust. Now rip your clothes into strips and braid a rope which you tie to the two ends of the gouging leg and twirl around the middle of the pointed leg (one twist). Put the duller of the points in the table dent and stand the leg up. Put the dented leg on the other point. Now pull on the gouging leg to rapidly twirl the pointed leg in the dent. After a long time friction should provide enough heat to ignite the dust and that should ignite the table and the pointed leg.
A simpler way to demagnitize (also using the broken table legs) would be to line the bars up facing east-west and bang on them repeatedly with a broken table leg – eventually the magnetic domains will be scrambled by the earth’s magnetic field. (Of course, I’m assuming we’re doing this on earth, not on that infinite frictionless plane with uniform charge that appears so often in physics books.)
The metal filings idea can also be implemented without “tools”: rub the two bars together to scrape off filings. Or (if the chips coming off are magnetized) scrape the bars on the edge of the table: one bar will leave filings that will clump and the other’s wont. Use the non-clumpy filings to display the field lines of the other bar.
Technically a trick question as you have to use the rods as tools to determine which is a magnet.
otherwise you take one and use the edge to scrape the other and use the resulting fillings to determine which has a magnetic field.]
alternatively, find someone with something metal in their body (metal plate, screws, etc…) and have them hold whatever piece of their body has metal in it near both rods, whichever one moves toward them is a magnet