Static equilibrium, trusses/section method

[J-dizzal]

Homework Statement

Calculate the forces in members BC, BE, and EF. Solve for each force from an equilibrium equation which contains that force as the only unknown. The forces are positive if in tension, negative if in compression.

Homework Equations

Sum of forces and moments equal zero.

The Attempt at a Solution

I'm on force BE, i cannot see what is wrong with my moment equation. what am i missing here? thanks.

 

[Dr. Courtney]

How many torques are acting on B?

 

[J-dizzal]

How many torques are acting on B?

B has two torques acting on it.

 

[Dr. Courtney]

Are you sure? Why only two?

 

[J-dizzal]

Are you sure? Why only two?

because i made a cut between B and C. and the only torques are: $$Dy(5) and 2.5EFcos∅$$
im sure i think its only two. but I am often wrong

 

[J-dizzal]

Are you sure? Why only two?

Should i include G(2.5)? if yes then why because i cut it out?

 

[TSny]

Your distance between D and P doesn't look correct.

 

[J-dizzal]

Your distance between D and P doesn't look correct.

I created point P to be 2.5m from point D. was i wrong to do that?
edited

 

[TSny]

I thought you chose P so that P lies on the extension of segment FE. Then force F_EF would not produce any moment about P.

If P is not on the extension of EF then you will need to include the moment due to F_EF when summing moments about P.

 

[J-dizzal]

I thought you chose P so that P lies on the extension of segment FE. Then force F_EF would not produce any moment about P.

If P is not on the extension of EF then you will need to include the moment due to F_EF when summing moments about P.

Oh ok i get the trick now. that makes much more sense. let me try again. thanks

 

[J-dizzal]

Oh ok i get the trick now. that makes much more sense. let me try again. thanks

so how do i find the length of DP if i only know the angles and one side of a similar triangle CED?
Is using similar triangles the correct way to solve for DP?

 

[TSny]

Consider triangle CEP. What is the angle at vertex P? What is the length of side CP?

 

[J-dizzal]

Consider triangle CEP. What is the angle at vertex P? What is the length of side CP?

the angle at P is 17.74 degrees. I am still thinking about how to find length CP

 

[J-dizzal]

Consider triangle CEP. What is the angle at vertex P? What is the length of side CP?

CP is 5.8 using the law of sines.

 

[TSny]

the angle at P is 17.74 degrees. I am still thinking about how to find length CP

OK

 

[TSny]

CP is 5.8 using the law of sines.

I don't think this is correct. I don't see how you are using the law of sines.

If you know the angle at P and the length of the side CE, you should be able to get side CP.

 

[J-dizzal]

I don't think this is correct. I don't see how you are using the law of sines.

If you know the angle at P and the length of the side CE, you should be able to get side CP.

Im struggling to see how. I used the law of sines knowing two angles of CEP and one side.
$$sin(72.26)/CP = sin(17.74)/1.7$$

 

[TSny]

Im struggling to see how. I used the law of sines knowing two angles of CEP and one side.
$$sin(72.26)/CP = sin(17.74)/1.7$$

OK. That will work. But I don't get 5.8 for CP using this formula.

I was thinking of just using 1.7/CP = tan(17.74) to find CP.

 

[J-dizzal]

OK. That will work. But I don't get 5.8 for CP using this formula.

I was thinking of just using 1.7/CP = tan(17.74) to find CP.

Yea i made an algebra mistake...i should of taken an algebra class this summer instead.