ATAR Notes: Forum
Archived Discussion => 2011 => Mid-year exams => Exam Discussion => Victoria => Physics => Topic started by: Lols123 on June 14, 2011, 04:17:42 pm
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Question 10
Shouldnt part of the beam be under tension at the bottom, since its a cantilever?
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How can on one side half be in tension and the other half be in compression? It can't. Imagine if the block at B wasn't there.
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what...thats how cantilevers normally work...
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Not necessarily true. Repeat the question, but stick a piece of rope in rather than a plank. The rope would sag between A and B.
edit: directed at doppler.
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I imagined the question like a real life diving board, when you apply a force on the end the whole top of the material is in tension.
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Chemology 2011 also had a diving board question.
It asked which is true of the forces on the diving board:
A. The board is under compression
B. The board is under tension
C. The top of the board is under tension and the bottom is under compression
D. The bottom of the board is under tension and the top is under compression
The answer was C as the board will bend under Bella's weight. Thus, steel rods should be placed entirely at the top which is under tension.
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That's where I remember doing this question before.
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that was a simple question.
because the bottom will compress the top must be in tension THE WHOLE WAY
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if i lost 3 marks, is that 50 material?
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that was a simple question.
because the bottom will compress the top must be in tension THE WHOLE WAY
Just had a chat to an assessor, he went with D. Most likely though they'd accept both.
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How the fuck do you get to have a chat with an assessor?
sorry, D as in the whole of the top was in tension?
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He's one of our teachers at school....
Oh and whoops i just realised, I shouldn't have said "just," i had the chat straight after the exam.
No, D as in the ___------------ answer.
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Well, thushan, I can't agree with that, about the pivot point, it is clear that directly to the left and right will be in tension, so yeh...
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Fair enough, we'll see.
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It's definitely A, the centre of mass of diving board was past the pole thing so the wall thing had to exert a downwards force to balance the structure!!
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Centre of weight to the right of post B, plus the weight of the diver, very to different to past VCAA questions like this. Think about it if structure A wasnt holding it in place, it would rotate clockwise, but when A holds the very end of the left side of the plank, it causes a bending motion upwards, causing tension along the top. There is no physical nor theoretical way that option D could have been correct in my opinion.. though I've made a mistake once before...
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Hey, richard.sw, are you who I think you are? :P
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You know it tush! On a quest to retrieve my 2 marks haha ;)
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ahahah talk to dr bhargava
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the answer is d, just try this experiment right now, but with a piece or paper, and the paper would sag between a and b
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it may also depend if it was physically attached to points a and b, but the question states "mounted" which is ambigous.
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I put A for my answer.
The way I saw it was:
If we look at support A, if we remove that bottom block of support A, then the setup will still be fine, and the top of AB would be in tension because of A pushing downwards, and B pushing upwards.
If we remove the top block of support A, then the entire beam will just spin clockwise.
Therefore, we need the top block of support A, but the bottom block isn't really required.
So we have the top block of support A, as well as support B - so the top of AB will be in tension.
I dunno, that was my thought process in the exam. Not sure if it makes sense.
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Confirmed. It is A.
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I put A for my answer.
The way I saw it was:
If we look at support A, if we remove that bottom block of support A, then the setup will still be fine, and the top of AB would be in tension because of A pushing downwards, and B pushing upwards.
If we remove the top block of support A, then the entire beam will just spin clockwise.
Therefore, we need the top block of support A, but the bottom block isn't really required.
So we have the top block of support A, as well as support B - so the top of AB will be in tension.
I dunno, that was my thought process in the exam. Not sure if it makes sense.
thankyou
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Just a question guys, why would a diving board be made out of reinforced concrete?
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Just a question guys, why would a diving board be made out of reinforced concrete?
So that it's supah strong!
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Just a question guys, why would a diving board be made out of reinforced concrete?
So that it's supah strong!
Fair enough.
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confirmed answer is d
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Kidding? Personally I put D, but I think in hindsight it is A. How did you confirm it's D?
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I did cantilevers as my physics project SAC EPI. The top part is definitely all under tension. We saw the part between the blocks clearly bend with the top in tension during our prac.
Just a question guys, why would a diving board be made out of reinforced concrete?
Why would you have a 300kg cyclist going at 15 m/s at a fairly steep incline?
Answer: Its physics, anything is supposed to be possible
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two teachers at my school are the assesors, and they both agree it is d. the question states "mounted"which implies the slab is physically attached to b, hence from a to b it is sagging down, therefore answer must be d. they said a has to wrong becuase it is impossible under the given conditions.
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english dictionary says, "mount- to fix securely to a support", thus d.
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But the centre of mass of the beam is to the right of support B, so we have a clockwise torque if you take the pivot point at B. So to balance that out, support A must push down, anticlockwise, which means AB will be in tension
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OK guys...as we can see this question is quite contentious and depends highly on the wording (if it were mounted then tension only occurs from pivot etc etc). The assessors will have a meeting over this and there will be a fight. I'll give you word when the verdict is given. Personally, because of ambiguity of the question, both A and D should be paid IMO.
Some of our highly respected physics teachers say A, others say D.
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english dictionary says, "mount- to fix securely to a support", thus d.
which dictionary did you use?
mount means put or fix in position
the question said the board is mounted with two supports at A and B, meaning the board is put or fixed in position with two supports at A and B, not what you tried to say
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anyway, it is a silly question, 80 kg for a 4-m concrete diving board.
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i asked both of my highly respected physics teachers, and they said the trick in the question was to realise that the slab is fixed physically at b, hence ab will sag, they also said a must be marked wrong. i know what ur thinking, but option a will only work if the slab was not mounted at b, but it says it is, and only one correct answer will be given.
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Come on guys no need to argue!
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it's not an argument thushan, it is just that i'm trying to make sure a lot of people don't think they're wrong in saying d is the answer, and to make sure a lot of peole don't think they're right in saying a is the answer.
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Although I'd love it if D was considered correct, I cannot see the examiners marking D correct AND marking A wrong at the same time.
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why would a be right?
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I know that it is mounted to B, but that doesnt change the fact that there is a clockwise torque around pivot point B. Support B must push upwards, making the beam be in tension along the top. I've already posted my explanation before, so I'm just repeating myself now
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i don't think you fully understand milkshake, any torque from the right hand side of the beam abslutely does not effect the section a to b, so you treat a to b isolated, hence a to b will sag, no matter what the torque from the other section of the beam. just think about it, and that's the explanation two of my vce assessors used.
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Then what is canceling out the torque on the right side?
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ok, basically you can think of the 4m beam in two sections. one 1 metre section and one 3 metre section. if the 1 metre section of attached at a and b, it will sag, the second section is also attached to b, but not to the 1 metre section, hence any torque generated by the 3 metre section is opposed bu the "mount"of that section onto b.
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Just some insights from my dad who is knowledgeable in these type of engineering qns:
D is correct IF the vertical plank is MOULDED to both the beam AND the ground, because the torque exerted by the weight of the beam + that of the diver is counteracted by the torque in the plank (as the plank and the beam are the same object).
If either (or both) the plank acts as a pivot, or it is not fixed to the ground, A is correct.
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question clearly says "mounted", which means d.
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ok, basically you can think of the 4m beam in two sections. one 1 metre section and one 3 metre section. if the 1 metre section of attached at a and b, it will sag, the second section is also attached to b, but not to the 1 metre section, hence any torque generated by the 3 metre section is opposed bu the "mount"of that section onto b.
Won't support A have to contribute as well? Because if you remove support A, then since the centre of mass is to the right of B, then the structure will just topple over.
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no because the plank is actually attacked to b, removing a means both sides sag
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meant to say attached
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Think you guys are wasting time now. Even if you can prove A or D is actually correct in a real-world or theoretical approach, at the end of the day, VCAA will determine what is write, probably in a way that doesn't make sense to anybody.
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LOL it's like VCAA chooses C... :P
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i hated the m&s questions... got 3 wrong
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ok, basically you can think of the 4m beam in two sections. one 1 metre section and one 3 metre section. if the 1 metre section of attached at a and b, it will sag, the second section is also attached to b, but not to the 1 metre section, hence any torque generated by the 3 metre section is opposed bu the "mount"of that section onto b.
The fault in this explanation is 3m section is not attached to the 1m section. The question did not say this, and the board appeared as one piece in the picture.
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it's A...
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support B must be deep into the ground and very strong against bending in order to counteract the clockwise torque of the separate 3m board so to keep it in place
would an engineer design such a structure?
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english dictionary says, "mount- to fix securely to a support", thus d.
which dictionary did you use?
mount means put or fix in position
the question said the board is mounted with two supports at A and B, meaning the board is put or fixed in position with two supports at A and B, not what you tried to say
@thelimz
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i still don't think people get it, you can treat the board as 2 sections because it's mounted at b, think about it, and i wouldn't disagree with the actual assessors. if the board is mounted at b, one side does not affect the other. not that hard. therefore d.
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at the end of the day i don't really care if you get it or not, but it needs to be clarified that a is not the answer, and that d is correct
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I'm guessing u chose D?...
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yes, the correct answer. i'm guessing you chose a?
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somebody is in denial??
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http://www.itute.com/board/viewtopic.php?f=6&p=3911#p3911
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how about we wait until the examiners report and then it may be clear to you.
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well i hope you are right Thelimz cuz i picked :D
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http://www.itute.com/board/viewtopic.php?f=6&p=3911#p3911
Yeah, and that "helper" guy is a physics guru so....
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Itute are the least qualified people on the net, this is evident in their practice exams that they write. They are of the lowest quality, just because Itute thought it was A does not mean it was correct.