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frameworks problem (again)

i have another frameworks related problem. also i haven't any idea how to set this answer out to obtain the marks.

i'm just unsure where to begin with forces and such because i simply haven't been taught this. i know it usually boils down to some trig using the angles of the rods but i just dont know where to apply it. if anyone could put me in the right direction for how to answer these generally with internal forces etc that would be great.

question 4 a ii on this paper http://www.mei.org.uk/files/papers/m205ju_ftnd74.pdf

thanks a lot
(edited 11 years ago)
Reply 1
Avatar for delllboy
delllboy
OP
8
bump someone help pleease
Reply 2
Avatar for mathz
mathz
8
from pic
in triangle ABR where R is where the force R line in the diagram would meet AC

you have sides

AR=BR=2..so what can you say about cos and sin A?

now resolve at A vertically and horizontally. so you have the weight down and the T form AC and the T' from AB

dont worry about if they are thrust or tensions, define a direction and the signs will come out from the equations so you will know if it is thrust or tension. so just put them down as tension for the mo.


now consider triangle BCR

BR=2 and CR=3 so what can we say about sin C and cos C?

once again resolve only down as we know T (from AC) so only want equation involving T'' (force at BC).
Reply 3
Avatar for delllboy
delllboy
OP
8
Original post by mathz
from pic
in triangle ABR where R is where the force R line in the diagram would meet AC

you have sides

AR=BR=2..so what can you say about cos and sin A?

now resolve at A vertically and horizontally. so you have the weight down and the T form AC and the T' from AB

dont worry about if they are thrust or tensions, define a direction and the signs will come out from the equations so you will know if it is thrust or tension. so just put them down as tension for the mo.


now consider triangle BCR

BR=2 and CR=3 so what can we say about sin C and cos C?

once again resolve only down as we know T (from AC) so only want equation involving T'' (force at BC).


thanks so much for helping but i dont get what you ment by "now resolve at A vertically and horizontally. so you have the weight down and the T form AC and the T' from AB"
the weight isnt at A. did you mean use the force downwards of V being 1.5N down?

i dont see how i calculate from the force of V when there are two pieces of rod experiencing that force. or do i assume AC doesnt take any of that force?
Reply 4
Avatar for mathz
mathz
8
yes,sorry meant the downward force V.

V acts at A and that is where we are resolving. i am resolving at the joint A not for the bar AC or AB.

this joint,at A,experiences the thrust/tension from AC AB and the downward force V. resolve these vertically and horizontally

joint at C experiences vertically thrust/tension from BC and mass

or maybe im wrong if this is not making sense to you? only only just learning this stuff myself :frown:
Reply 5
Avatar for delllboy
delllboy
OP
8
Original post by mathz
yes,sorry meant the downward force V.

V acts at A and that is where we are resolving. i am resolving at the joint A not for the bar AC or AB.

this joint,at A,experiences the thrust/tension from AC AB and the downward force V. resolve these vertically and horizontally

joint at C experiences vertically thrust/tension from BC and mass

or maybe im wrong if this is not making sense to you? only only just learning this stuff myself :frown:


well to be honest our teacher didn't cover frameworks so i don't know whats right and whats wrong.
whats confused me with this one is that each force is acting on multiple pin joints so i'm unsure how to calculate these things.

does your method give you correct answers in the past papers?
Reply 6
Avatar for mathz
mathz
8
have you resolved as i suggested? does it give the correct answer? if it does then we are ok i think.

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