Balls. The answer is D for the same reason a siphon works, same reason this works. The test question was obviously contrived by someone who had to turn to writing vague science test questions that erode brighter, sharper children’s self confidence because he was turned down by the police academy. Probably works at the middle school level because he hates small kids and isn’t sharp enough to teach high school.
No offense inteded to cops and teachers on The Straight Dope as they are clearly a cut above the stereotypes of their respective groups.
I would actually say that it’s right and proper for a physics test to test, in part, for student’s ability to do this. The problem is not that you shouldn’t be assuming that minor variables are negligible, the problem here is that it’s not clear which one of two variables should be ignored and which one shouldn’t, in a situation where it is correct to ignore only one of them.
Consider a different question: “What’s the ratio of forces pulling down on the left and right sides in the ‘displaced’ configuration?” No problem: it’s 1.0, and we can get that with a merely light string without the need to invoke a massless string. We also get 1.0 is we assume a massless string, but that’s neither here nor there since we actually have a light string. But then…
“What’s the ratio of forces if the test masses are removed?” Since we have a light string, we need to know the exact lengths of string on the two sides in the displaced configuration. Since we don’t know this, we can’t answer the question. But wait! In the above scenario, we got the same answer whether we assumed a massless string or a light string. Why not do that here? If we try, we get mathematical trouble: zero-divided-by-zero. This isn’t a problem since we already know the answer in this case is “not enough information”. The fact that an erroneous change in the problem’s specification happens to break the math is neither here not there. But its erroenous-ness jumps out mathematically for this scenario.
For the OP’s problem, the behavior of the system is governed entirely by the “small” quantities. The gut-reaction to make these quantities zero happens (in this case) to give a specific numerical answer rather than something broken like zero-divided-by-zero, but that doesn’t make it any less erroneous. The only correct solution is “not enough information”.
The onus is on the test writer not to allow unanswerable questions to appear, not on the student (11 years old or otherwise) to make a fallacious logical step just because that same fallacious step happens to give an equivalent answer in an entirely different problem (e.g., a scenario where the test masses are more different in weight than the weight of the string or the friction in the pulley).
The question was written by the University of New South Wales. I suspect the problem is the examinor has imbibed so deep of physics problem conventions they have forgotten that’s what they are: conventions.
I agree except that conventions are called conventions and not natural laws for a reason: you can’t figure them out without being told about them. And this wasn’t an examination of taught knowledge.
I agree 100% with Pasta and I feel that Pasta has done a fantastic job explaining why the answer is either B or D and neither one is more correct than the other. The point about “light string” vs “easily moved” being negligible only when other factors are dominant in the system is spot on. When the masses are the same, the “light string” and the “easily moved” pulley are in fact the only important factors.
If static friction in pulley is greater than extra weight of string on the longer side, then the answer is B.
If static friction in pulley is less than the extra weight of string on the longer side, then the answer is D.
It’s a badly worded question and I’d throw it out if I gave it on a test as physics teacher without reading it closely first.
-drewtwo99, former physics teacher and physics major
See, I think I’d fall back on my test-taking theory that every answer is contained within the question itself. A little amateur psychology will reveal what the person who wrote the question was meaning to test - and once you know that, the correct answer (not necessarily the right one) becomes obvious.
What was really in your heart when you asked the question? Was it port or starboard or were you thinking about what you’d hoped I’d say? The problem with your question is that it wasn’t honest. You didn’t ask what you wanted to know.
What I can say is that it would work in the present question. Once you realize that the teacher wasn’t concerned about the friction of the pulley or the weight of the string, the answer is obvious. (and the reason you can tell they didn’t care is because they didn’t spend any time on that part of the question, yet they went to all that trouble detailing the masses, and positions, drawing diagrams, etc.)
You are completely missing the point. You proposed a strategy about answering exam questions. I gave an example exam question which you couldn’t answer using your strategy. Would your answer “You’re right, it doesn’t always work”
get you a mark in an exam? No it would not; your strategy seldom works.
If your point is that the OP question is one that can be answered with knowledge of physics question conventions and the psychology of teachers (and that is your point, really) we are in furious agreement.
If the OP question was one designed to test physics knowledge, it was a piss poor question.
A farmer once asked me for help figuring out how to produce more milk at a lower cost. I told him to imagine a spherical, mass-less cow and scale that up to a few fields. He’s still not talking to me.
Make sure the kid does still understand that knowing the science in a thinking and logical sense is important. In grad school, the instructor is far more interested in confirming that you can think about the topic and try to make sense of what you are asking and discuss potential problems and potential solutions (1) than just parrot off an answer.
(1) Johnson and O’Toole (1995) proposed a model for defining the expectations and underlying assumptions of physics problems for small children. Also see Smith (1975) for a similar discussion of math word problems.
You did not give an example exam question. Everything else you said is built on this premise, and therefore irrelevant.
We don’t live in an ideal world. My point is that my strategy addresses real world concerns, like the one your son encountered. I’m glad you think it would work in this instance, you’ll find many more like it.
And, to be clear - yes, you need varying amounts of knowledge of the topic at hand in order to be able to answer the question. But most of the time, the differentiating factor is usually test taking skills, not pure knowledge.
No problem. I’d honestly be happy to help with any related or non-related issue that I can. There really is no sarcasm there (I can’t think of a way to type this to have it sound non-sarcastic, stupid reliance on visual cues and context…)
