Statics problem free body diagram

In summary: 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.
  • #1
lc99
161
3

Homework Statement


upload_2018-4-16_14-50-21.png


Homework Equations

The Attempt at a Solution


upload_2018-4-16_14-54-16.png


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

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  • #2
I meant to post this on the physics homework forum if anyone can move it!
 
  • #3
Here is more of my drawings...

upload_2018-4-16_15-19-41.png
 

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  • #4
lc99 said:
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.
 
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  • #5
lc99 said:
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.
 
  • #6
haruspex said:
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?
 
Last edited:
  • #7
lc99 said:
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.
 
  • #8
haruspex said:
Yes, but you have included Fac on the free body diagram for AB.
isnt that right? the member AC has force on AB
 
  • #9
lc99 said:
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.
 
  • #10
haruspex said:
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?
 
  • #11
lc99 said:
so it should just be Ax and Ay?
Yes.
 
  • #12
haruspex said:
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?
 
  • #13
lc99 said:
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.
 

1. What is a free body diagram?

A free body diagram is a visual representation of the forces acting on an object in a statics problem. It is a simplified drawing that shows all the external forces acting on the object, without including the object's internal forces or any other irrelevant forces.

2. Why are free body diagrams important in statics problems?

Free body diagrams are important because they help us understand and analyze the forces acting on an object in a statics problem. By drawing a free body diagram, we can identify and label all the forces acting on the object, and use them to solve for unknown variables.

3. How do you draw a free body diagram?

To draw a free body diagram, start by identifying the object you want to analyze and isolating it from its surroundings. Then, draw a simple sketch of the object and label all the external forces acting on it, including the direction and magnitude of each force. Make sure to include all relevant forces, such as weight, normal force, and applied forces.

4. Can a free body diagram be used for moving objects?

No, free body diagrams are only used for analyzing the forces acting on an object in static equilibrium. Moving objects require dynamic analysis, which takes into account the object's acceleration and the changing forces acting on it.

5. Are free body diagrams only used in physics?

No, free body diagrams are also used in engineering, specifically in the field of statics. They are used to analyze the forces acting on structures and to design and test new structures.

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