Statics free body diagram concept question

[xzibition8612]

Homework Statement

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Homework Equations

The Attempt at a Solution

In the explanation attachment, I don't get the last paragraph "Note that force Fce acts downward on bar...compression."

Since from the figure Fce is pointing down, and Fbd is pointing up, shouldn't bar BD be in tension and bar CE in compression? I know intuitively just by looking at the figure, yes, obviously BD is in compression and CE is in tension, but I'm confused by the free body diagram the book draws and its subsequent explanation. I must be missing some concept here. Thanks.

 

[hotvette]

The horizontal member pushes down on member BD, placing it in compression. In turn, member BD pushes up on the horizontal member (equal and opposite). Thus, F_BD points up in the bottom picture, which is a free body diagram of the horizontal member. A free body diagram of member BD will have an force pointing down at the top and a force pointing up at the bottom (ground reaction). That's how free body diagrams work.

 

[xzibition8612]

ah so the figure is a free body diagram of the *bar ABC* and not member BD, so it only displays the forces relative to how they effect ABC? Thanks a lot.

 

[hotvette]

Correct.

 

[DeeNos]

I would like to clarify the concept of tension and compression in the context of statics free body diagrams. In this particular example, the free body diagram shows two bars connected at a joint, with forces acting on them. The force Fce is acting downward on bar CE, while the force Fbd is acting upward on bar BD.

Now, when we talk about tension and compression in a structural system, we are referring to the internal forces that are created within the bars due to an external load. In this case, the external load is the force Fce and Fbd acting on the bars. Tension refers to the force that pulls the two ends of a bar away from each other, while compression refers to the force that pushes the two ends of a bar towards each other.

In the free body diagram, the force Fce is acting downward on bar CE, which means it is pulling the two ends of the bar away from each other. This creates tension in the bar CE. On the other hand, the force Fbd is acting upward on bar BD, which means it is pushing the two ends of the bar towards each other. This creates compression in the bar BD.

So, in conclusion, the free body diagram and its explanation are correct. The force Fce is creating tension in bar CE, while the force Fbd is creating compression in bar BD. I hope this clarifies your confusion and helps you understand the concept of tension and compression in statics free body diagrams.