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

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SUMMARY

The discussion centers on the correct representation of forces in a free body diagram for a middle board compressed between two other boards, weighing 95.5N. Participants debate the inclusion of two forces pointing towards the middle board, specifically the normal force (N) and the compression force (F). It is concluded that while both forces are equal in magnitude, they are conceptually distinct; however, they can be represented as a single force in the diagram. The importance of accurately depicting these forces in free body diagrams is emphasized for clarity in understanding the mechanics involved.

PREREQUISITES
  • Understanding of free body diagrams in physics
  • Knowledge of forces: normal force and compression force
  • Basic principles of friction in mechanics
  • Familiarity with Newton's laws of motion
NEXT STEPS
  • Study the principles of free body diagrams in physics
  • Learn about the relationship between normal and compression forces
  • Explore the role of friction in static and dynamic systems
  • Review Newton's laws of motion and their applications in mechanics
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Students studying physics, educators teaching mechanics, and anyone interested in understanding the dynamics of forces in free body diagrams.

Romain Nzebele
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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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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!
 

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