Statics problem free body diagram

[lc99]

Homework Statement

Homework Equations

The Attempt at a Solution

why are my reactions at A considered wrong? I know that AC and DC are two force-members

 

[lc99]

I meant to post this on the physics homework forum if anyone can move it!

 

[lc99]

Here is more of my drawings...

 

[anorlunda]

I meant to post this on the physics homework forum if anyone can move it!

I think it is fine here. No need to move it. If you don't get any answers soon, click on my name and send me a message.

 

[haruspex]

why are my reactions at A considered wrong?

You are mixing up forces on different members. The reaction between the hinge at A and AB is independent of the reaction between that hinge and AC.

 

[lc99]

You are mixing up forces on different members. The reaction between the hinge at A and AB is independent of the reaction between that hinge and AC.

What do you mean by independent? I'm not sure if i am understanding. :(

For the reaction between hinge A and AB, you have Ax and Ay. for the reacting between hinge A and AC, you have Fac from the two force member.

are both of my drawings wrong?

 

[haruspex]

For the reaction between hinge A and AB, you have Ax and Ay. for the reacting between hinge A and AC, you have Fac from the two force member.

Yes, but you have included Fac on the free body diagram for AB.

 

[lc99]

Yes, but you have included Fac on the free body diagram for AB.

isnt that right? the member AC has force on AB

 

[haruspex]

isnt that right? the member AC has force on AB

No, this is my point about their independence. Each spar is tied to the joint independently of the other. It would be exactly the same if they were tied to different joints that happened to be at the same point.

 

[lc99]

No, this is my point about their independence. Each spar is tied to the joint independently of the other. It would be exactly the same if they were tied to different joints that happened to be at the same point.

so it should just be Ax and Ay?

 

[haruspex]

so it should just be Ax and Ay?

Yes.

 

[lc99]

Yes, but you have included Fac on the free body diagram for AB.

Would it be wrong if i leave Ax and Ay off except Fac? If it is, then whenever i see a two force member and a pin together on one joint, i only draw the Ax and Ay?

Do i leave Fac off because Fac is a reaction force to the pins Ax,Ay?

 

[haruspex]

Would it be wrong if i leave Ax and Ay off except Fac? If it is, then whenever i see a two force member and a pin together on one joint, i only draw the Ax and Ay?

Do i leave Fac off because Fac is a reaction force to the pins Ax,Ay?

There is not (or does not need to be) any force between AB and AC. They just happen to be connected to the same hinge. So the free body diagram for one should ignore the presence of the other.