# Understanding Free Body Diagrams: Explaining the Forces on a Compressed Board

• Romain Nzebele
In summary, the body diagram shown in the solution manual is incorrect because it omits the forces of friction.
Romain Nzebele
New user has been reminded to use the Homework Help Template when starting schoolwork threads at the PF
Hello everyone. I find the following problem confusing.

A board sandwiched between two boards in the figure below weighs 95.5N. If friction exists between the middle and the compressing boards, draw the free body diagram of the middle board.

Below I attached the diagram found in the solution manual but I don't agree with it. Since the middle board is compressed by 2 boards, I believe the diagram should include two forces pointing toward the middle board in addition to the normal forces. Can someone explain to me why the solution manual omitted them? I will appreciate it very much.

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• fullsizeoutput_b73.jpeg
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Please show us your version of the free body disgram.

Hello, here is the body diagram I believe is correct. In my drawing Ff represents the force of friction, F represents the compression force, N represents the normal force, and Fg represents the weight of the middle board.

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What makes you think that N and F are separate and distinct entities? Why isn't the normal force N just the compression force F?

The normal force results from the compression force. If there was no compression force, there will be no normal force. Since we have two boards compressing the middle block, it seems correct to me to represent these forces. Although I understand that the normal force and the compression force are equal in magnitude in this example.

Romain Nzebele said:
The normal force results from the compression force. If there was no compression force, there will be no normal force. Since we have two boards compressing the middle block, it seems correct to me to represent these forces. Although I understand that the normal force and the compression force are equal in magnitude in this example.
There is only one force. You can call it a compression force or you can call it a normal force, but either way, there is only one. They are one and the same thing.

Okay, thank you for the feedback. I appreciate it.

Romain Nzebele said:
Okay, thank you for the feedback. I appreciate it.
My pleasure. By the way, Welcome to Physics Forums!

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

A free body diagram is a visual representation of all the forces acting on an object. It is a simplified drawing that includes the object and all the forces acting on it, with each force represented by an arrow indicating its direction and magnitude.

## 2. Why are free body diagrams important in problem-solving?

Free body diagrams help to identify all the forces acting on an object, making it easier to analyze and solve problems involving forces. They also help to visualize the interactions between different forces and how they affect the motion of an object.

## 3. How do I draw a free body diagram?

To draw a free body diagram, start by identifying the object and drawing it as a simple shape. Then, identify all the forces acting on the object and draw them as arrows originating from the object's center. Label each force with its name and direction. Finally, make sure all the forces are drawn to scale, with the length of the arrows representing the magnitude of the force.

## 4. Can a free body diagram be used for objects at rest?

Yes, free body diagrams can be used for objects at rest as well as objects in motion. In fact, they are particularly useful in solving problems involving objects at rest, as they help to identify all the forces that must be balanced for the object to remain at rest.

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

While free body diagrams are a powerful tool for analyzing forces, they do have some limitations. They assume that all forces are acting at a single point on the object, which may not always be the case. They also do not take into account the effects of friction or air resistance, which can complicate real-world scenarios. Additionally, free body diagrams are only useful for analyzing forces, and cannot be used to determine other properties such as an object's velocity or acceleration.

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