# Science Question - Two masses suspended from a pulley **URL:** \ **Category:** Factual Questions\ **Created:** [August 6, 2013, 10:38am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484 "2013-08-06T10:38:50Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 7, 2013, 2:05am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/81 "2013-08-07T02:05:48Z")
> [@Darth\_Panda](#): > > There really is no sarcasm there There was plenty in my post, trust me. I asked you an example test question and your strategy failed abysmally, as it would with most real test questions. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [August 7, 2013, 2:08am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/82 "2013-08-07T02:08:10Z")
> [@Princhester](#): > > There was plenty in my post, trust me. I know, but I don’t care. I think it’s great that you’re this interested in your son - a lot of people aren’t. > [@Princhester](#): > > I asked you an example test question and your strategy failed abysmally, as it would with most real test questions. I don’t think that question is representative of what would actually be on any test. As for real tests, I’ve done far better than I deserve, I’m sure. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 7, 2013, 2:30am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/83 "2013-08-07T02:30:07Z")
> [@Darth\_Panda](#): > > I don’t think that question is representative of what would actually be on any test. As for real tests, I’ve done far better than I deserve, I’m sure. Sure it is. I’ll dress it up a bit more: "When standing on a ship facing the bow, which side is “port” and which side is “starboard”? A/ The side to your left is port and the side to your right is starboard B/ The side to your right is port and the side to your left is starboard C/ The side of the cabin to your left is port and the side of the ship’s wheel furthest from you is starboard D/ The side of beef is port and the sideboard is to starboard" You simply cannot answer this based on the question. You have to have subject knowledge. If you haven’t done tests where most questions were of this nature, you frankly just haven’t done substantial tests (I doubt that). If you’ve done well on real tests, I’m sure you had good subject matter knowledge. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [August 7, 2013, 2:39am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/84 "2013-08-07T02:39:20Z")
> [@Princhester](#): > > Sure it is. I’ll dress it up a bit more: > > "When standing on a ship facing the bow, which side is “port” and which side is “starboard”? > > A/ The side to your left is port and the side to your right is starboard > B/ The side to your right is port and the side to your left is starboard > C/ The side of the cabin to your left is port and the side of the ship’s wheel furthest from you is starboard > D/ The side of beef is port and the sideboard is to starboard" > > You simply cannot answer this based on the question. You have to have subject knowledge. If you haven’t done tests where most questions were of this nature, you frankly just haven’t done substantial tests (I doubt that). If you’ve done well on real tests, I’m sure you had good subject matter knowledge. I still don’t think anyone would ask that question on a test, but I get your point. Yes, you need subject matter knowledge for some tests. Is that something that really needs to be stated? You need to read between the lines at least a little. I stated that “for this question,” I would “fall back” on a general idea to help me out, and it would work. If you’d rather focus on debating an obviously way-too-literal reading that becomes a gotcha, then I suppose that’s your call, but I can’t believe that you really think that’s more important than finding a way to help your son deal with questions that have some vagueness (and, in my experience, most do). I did well on real tests - I had good subject matter knowledge, but I have no doubt that I outperformed many people who knew the material better than I did. eta: I did much better on non-subject material tests (LSAT, GMAT, etc.) ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 7, 2013, 2:39am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/85 "2013-08-07T02:39:47Z")
Here we go, have a look at this .pdf (since I’m sticking to a nautical theme): > **[mar\_rb\_workbook5.pdf](https://www.transport.wa.gov.au/mediaFiles/mar_rb_workbook5.pdf)** > > 121.13 KB Go to page 20. You wouldn’t be able to answer a single one of the sample questions there. Real world sample questions and your strategy fails. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [August 7, 2013, 2:46am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/86 "2013-08-07T02:46:22Z")
> [@Princhester](#): > > Here we go, have a look at this .pdf (since I’m sticking to a nautical theme): > > [Page Not Found (404)](http://www.transport.wa.gov.au/mediaFiles/mar_rb_workbook5.pdf) > > Go to page 20. You wouldn’t be able to answer a single one of the sample questions there. Real world sample questions and your strategy fails. Are you actually reading my posts? 🙂 Or are you just after internet points? I’ll give you 4, if you think that’s fair. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 7, 2013, 2:50am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/87 "2013-08-07T02:50:04Z")
> [@Darth\_Panda](#): > > If you’d rather focus on debating an obviously way-too-literal reading that becomes a gotcha, then I suppose that’s your call… All I did was point out by way of an example that your initial post on this subject was an overstatement. Here’s a clue: if you don’t want to go on debating an indefensible point till you have been ground into the dirt, concede early. Don’t go on and on and then blame me for defending an obviously correct position. > [@](#): > > …but I can’t believe that you really think that’s more important than finding a way to help your son deal with questions that have some vagueness (and, in my experience, most do). Ohmygosh, you’re right. That’s exactly my attitude. As I’ve literally said in this thread several times, I think it’s more important to debate you on this point than help my son with questions that have some vagueness. There is no excluded middle here. It’s either one thing or the other. I need to give up debating you, otherwise providing my son with guidance on this subject becomes impossible. You are right. What a fool I have been. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 7, 2013, 2:51am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/88 "2013-08-07T02:51:22Z")
> [@Darth\_Panda](#): > > Are you actually reading my posts? 🙂 Not before you post them. It was a simulpost. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [August 7, 2013, 2:53am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/89 "2013-08-07T02:53:50Z")
> [@Princhester](#): > > All I did was point out by way of an example that your initial post on this subject was an overstatement. Here’s a clue: if you don’t want to go on debating an indefensible point till you have been ground into the dirt, concede early. Don’t go on and on and then blame me for defending an obviously correct position. You’re kidding right? Anyone who thinks that either a) they’ve been ground into dirt on the internet or b) they’ve ground someone else into dirt on the internet, has much bigger issues than the correct answer to a question. I concede nothing, other than the 4 internet points already given. ---
**Author:** ![TokyoBayer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tokyobayer/32/13989_2.png) [@TokyoBayer](https://boards.straightdope.com/u/TokyoBayer)\ **Post date:** [August 7, 2013, 3:52am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/90 "2013-08-07T03:52:24Z")
> [@Darth\_Panda](#): > > 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. Unfortunately, you’re getting flack for this, but it’s actually good advice. Not a magic formula, but it helps clarify your thinking into what should I be expected to know. The counter example fails because it’s a situation where one has zero knowledge of a situation, while it’s obvious that the son had some understanding of physics. It was a piss poor test, so it’s not the son’s fault, I can see myself going through the same logic. Anyway, I like your advice. I’ll try to remember to share it with my kids when they start to get problems like this. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [August 7, 2013, 3:58am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/91 "2013-08-07T03:58:36Z")
> [@TokyoBayer](#): > > Unfortunately, you’re getting flack for this, but it’s actually good advice. Not a magic formula, but it helps clarify your thinking into what should I be expected to know. > > The counter example fails because it’s a situation where one has zero knowledge of a situation, while it’s obvious that the son had some understanding of physics. It was a piss poor test, so it’s not the son’s fault, I can see myself going through the same logic. > > Anyway, I like your advice. I’ll try to remember to share it with my kids when they start to get problems like this. Thanks. The flack is no big deal, it’s just a too-literal interpretation. I’m pretty sure the OP gets the point, regardless of his posts. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [August 7, 2013, 6:19am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/92 "2013-08-07T06:19:46Z")
> [@The Great Sun Jester](#): > > Balls. The answer is D for the same reason a siphon works, same reason [this](http://www.youtube.com/watch?v=6ukMId5fIi0) 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. The reason a siphon works is that the friction of the water in the pipe is indeed negligible while the mass of the water is not. The only way this is going to work with the string and the pulleys is when for some unstated reason you can ignore the friction in the pulley but not the weight of the string. If you’re ignoring them both then the system is in neutral equilibrium no matter the height of the weights, if you’re ignoring neither then the difference in the weight of the strings is trivial compared to the friction in the pulley. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 7, 2013, 11:12am UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/93 "2013-08-07T11:12:32Z")
If you’re ignoring both you’re wrong, because you can’t have each one small compared to the other, and if you’re ignoring neither then you likewise don’t know which one is bigger. ---
**Author:** ![nate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nate/32/13085_2.png) [@nate](https://boards.straightdope.com/u/nate)\ **Post date:** [August 7, 2013, 1:42pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/94 "2013-08-07T13:42:40Z")
I initially thought D, but upon reading who the intendend audience is, I will go with B. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [August 7, 2013, 2:29pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/95 "2013-08-07T14:29:28Z")
> [@Princhester](#): > > Thanks for your answers. The official answer was B. As an experienced sitter of physics exams, that would have been my answer too. However… > > To give some context, this is an optional voluntary national exam. The children taking it have not studied most of the material covered by the exam at all. The first time my 11 yr old had seen a problem of this nature was on the exam, and the same would have been true of the others. The children would not have known there was some particular principle that the examiner was seeking to demonstrate. They were answering based purely on their ability to figure stuff out. > > My son answered D. I asked him why and he said obviously the two masses are the same so that won’t make either side go down, but the weight of the string is now greater on one side, so it will go down on that side. So he got punished for knowing the physics and being meticulous. > > I struggle to see any reasonable basis upon which his answer was wrong\*. It’s only wrong in one of two ways. One can take the \*\*Malacandra \*\*approach and conclude that probably light string isn’t going to provide enough mass to overcome pulley friction (which involves assumptions that the examinees shouldn’t have to make). Or one can take the “convention” approach and say that the “convention” in physics problems is to ignore minor variables. In which case the problem is in signficant part testing “exam convention” knowledge and not physics ability. Which I doubt was the examinor’s intent (and IMHO should certainly not have been the examinor’s intent). > > It’s a piss poor problem. \*\*Pasta’s \*\*second last paragraph I agree with. It’s a riddle, not a question about physics. And the problem could have been rectified so easily by just saying “ignore the mass of the string”. I would be more forgiving if the question was one set by some local schoolteacher but this was set at a national level by people who I think one has a right to expect to have a frickin’ clue. > > \*It may not be right (because on one view B is right) but I struggle to see his answer as wrong. I also answered B and did that because of the very light string insertion. I am a certified crane operator and know all about line weight, but there is also a need for movement to be initiated in very low weight situations. Water surfaced tension is what comes to mind when the thought of movement initiation comes up. I do believe there should have been a disclaimer on the front cover of the test booklet such as, Don’t M I T it, Don’t I B M it, Don’t assemble a panel of Rhodes Scholars! OR there should have been a comment box to rationalize the answer just as your son did. I sure hope your son shakes off this missed question easier than we parents tend to when our loved children miss a Mark. 😉 ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [August 7, 2013, 4:03pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/96 "2013-08-07T16:03:44Z")
> [@Malacandra](#): > > The reason a siphon works is that the friction of the water in the pipe is indeed negligible while the mass of the water is not. The only way this is going to work with the string and the pulleys is when for some unstated reason you can ignore the friction in the pulley but not the weight of the string. Silk thread, teflon-coated pulley. That sucker is sliding off the pulley with or without a weight. 😃 ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [August 7, 2013, 8:45pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/97 "2013-08-07T20:45:51Z")
_Your Mom_ is sliding off the pulley. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [August 7, 2013, 10:13pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/98 "2013-08-07T22:13:25Z")
😃 dude… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 7, 2013, 10:31pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/99 "2013-08-07T22:31:51Z")
> [@](#): > > Quoth **GBro** : > > I am a certified crane operator and know all about line weight, but there is also a need for movement to be initiated in very low weight situations. This is because static friction is usually somewhat larger than kinetic friction. So you might have a load that’s smaller than the static friction, and hence could stay put indefinitely, but larger than the kinetic friction, and so could continue moving (and accelerating) indefinitely if it just got started. ---
**Author:** ![slm2955](https://avatars.discourse-cdn.com/v4/letter/s/c6cbf5/32.png) [@slm2955](https://boards.straightdope.com/u/slm2955)\ **Post date:** [August 7, 2013, 10:36pm UTC](https://boards.straightdope.com/t/science-question-two-masses-suspended-from-a-pulley/665484/100 "2013-08-07T22:36:32Z")
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