Free Body Diagram Explained: Why is Right FBD Upward?

In summary, the free body diagram for each member is drawn based on Newton's 3rd law, with equal and opposite forces acting on each other. The horizontal forces are intuitive, while for the vertical forces, one is chosen as down and the other must be up. The book mentions 3 unknown forces on each member, but since the coefficients of friction are given, one of the forces can be eliminated, leaving only two unknown forces. These forces are likely the components bx and by, and the normal force Na.
  • #1
Drain Brain
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Can you explain why the free body diagram of each members are as shown? I kind of understand why the FBD of the member on the left is drawn as shown. But the FBD on the right is confusing, Why does it have a component in Y (upward) direction?
thanks!
 

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  • #2
Drain Brain said:
Can you explain why the free body diagram of each members are as shown? I kind of understand why the FBD of the member on the left is drawn as shown. But the FBD on the right is confusing, Why does it have a component in Y (upward) direction?
thanks!

Hi Drain Brain!

It's Newton's 3rd law: whenever a body pushes against another body, that body pushes back with an equal and opposite force.
For the horizontal forces that should be intuitive.
For the vertical forces we pick one down, meaning the other must be up.
 
  • #3
I like Serena said:
Hi Drain Brain!

It's Newton's 3rd law: whenever a body pushes against another body, that body pushes back with an equal and opposite force.
For the horizontal forces that should be intuitive.
For the vertical forces we pick one down, meaning the other must be up.

Hi I Like Serena! Now I understand!

There's another thing I want to ask, my book says that there are 3 unknown forces on each member, but I see 4 unknowns. Can you tell me why that is?
 
  • #4
Drain Brain said:
Hi I Like Serena! Now I understand!

Good!

There's another thing I want to ask, my book says that there are 3 unknown forces on each member, but I see 4 unknowns. Can you tell me why that is?

They're probably jumping a bit ahead.
Since the coefficients of friction are given, that gives us the extra equation $F_A=\mu_A N_A$, effectively eliminating $F_A$ as an unknown.
 
  • #5
I like Serena said:
Good!
They're probably jumping a bit ahead.
Since the coefficients of friction are given, that gives us the extra equation $F_A=\mu_A N_A$, effectively eliminating $F_A$ as an unknown.

Do you mean to say that the 3 unknown forces my book is referring to are the components bx, by and the normal force Na?
 
  • #6
Drain Brain said:
Do you mean to say that the 3 unknown forces my book is referring to are the components bx, by and the normal force Na?

That's what I expect yes.
 

Related to Free Body Diagram Explained: Why is Right FBD Upward?

1. What is a free body diagram (FBD)?

A free body diagram is a visual representation of the forces acting on an isolated object or system. It is a simplified drawing that shows the forces as arrows pointing in the direction of their action, with their magnitudes and directions labeled.

2. Why is the right FBD upward?

The right FBD is upward because it follows the conventions of drawing a free body diagram. The upward direction is usually chosen as the positive direction, and all forces acting in the upward direction are labeled with a positive sign, while those acting in the downward direction are labeled with a negative sign.

3. How does drawing a FBD help in solving physics problems?

Drawing a free body diagram helps to visualize the forces acting on an object or system, making it easier to understand and analyze the problem. It also helps to identify all the forces involved and their directions, which are essential for applying the laws of motion and solving the problem.

4. Can there be multiple free body diagrams for one system?

Yes, there can be multiple free body diagrams for one system, depending on the different forces or objects that need to be considered. For example, if a box is pushed along a horizontal surface by a person and also experiences friction, there can be two separate FBDs - one showing the forces acting on the box and the other showing the forces acting on the person.

5. Are there any limitations to using free body diagrams?

Free body diagrams are a simplified representation of the forces acting on an object or system, and they have limitations. They do not take into account the size or shape of the object, and they assume that all forces are acting at a single point. Also, they do not consider any non-contact forces, such as magnetic or electric forces.

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