Structures and Materials Question

[Thelimz]

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.

[Thelimz]

english dictionary says, "mount- to fix securely to a support", thus d.

[Milkshake]

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

[thushan]

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.

[yawho]

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

[yawho]

anyway, it is a silly question, 80 kg for a 4-m concrete diving board.

[Thelimz]

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.

[thushan]

Come on guys no need to argue!

[Thelimz]

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.

[Romperait]

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.

[Thelimz]

why would a be right?

[Milkshake]

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

[Thelimz]

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.

[Milkshake]

Then what is canceling out the torque on the right side?

[Thelimz]

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.