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Hello all, first post.

I have a mechanics test coming up and a the lecturer gave a very strong hint that a question like this with a scissor truss will be on the exam so how would you go about solving a question like this (finding force in all members).

I worked out the reaction forces as:

[tex]\sum[/tex]F(y) = 0 = A(y) + F(y) - 500 - 500 ; 1000 = A(y) + F(y) [eq1] .

[tex]\sum[/tex]F(x) = 0 = A(x)

[tex]\sum[/tex]M(A) = 0 = 15[F(y)] - (5x500) - (10x500) ; 15[F(y)] = 7500 ; F(y) = 500

Subbing F(y) = 500 into eq1 gives ; A(y) + 500 = 1000 ; A(y) = 500

So A(y) = F(y) = 500 and A(x) = 0 , with all forces being in kilo newtons.

I know that it has to be done using the method of slices because theres no joint with only two unknowns.

Now my question is where do I take the slice? Because the only two places I can think of where there are only three unknown forces are long the dotted lines in the picture but that gives the problems of not being able to do a sum of moments equation. The reason for this is that all three unknowns would be concurrent from the same joint so no moment equation can be done around that point (please refer to the FBD I have so eloquently drawn).

Please help

I have a mechanics test coming up and a the lecturer gave a very strong hint that a question like this with a scissor truss will be on the exam so how would you go about solving a question like this (finding force in all members).

I worked out the reaction forces as:

[tex]\sum[/tex]F(y) = 0 = A(y) + F(y) - 500 - 500 ; 1000 = A(y) + F(y) [eq1] .

[tex]\sum[/tex]F(x) = 0 = A(x)

[tex]\sum[/tex]M(A) = 0 = 15[F(y)] - (5x500) - (10x500) ; 15[F(y)] = 7500 ; F(y) = 500

Subbing F(y) = 500 into eq1 gives ; A(y) + 500 = 1000 ; A(y) = 500

So A(y) = F(y) = 500 and A(x) = 0 , with all forces being in kilo newtons.

I know that it has to be done using the method of slices because theres no joint with only two unknowns.

Now my question is where do I take the slice? Because the only two places I can think of where there are only three unknown forces are long the dotted lines in the picture but that gives the problems of not being able to do a sum of moments equation. The reason for this is that all three unknowns would be concurrent from the same joint so no moment equation can be done around that point (please refer to the FBD I have so eloquently drawn).

Please help