Engineering Dynamics, weight is missing, simple problem

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SUMMARY

The forum discussion addresses a common issue in engineering dynamics regarding the omission of weight in free body diagrams (FBDs) for blocks in a system. Specifically, the weight of block B was not included in the FBD when applying Newton's Second Law, which is a critical oversight that impacts the solution. The discussion clarifies that while the weights of both blocks are relevant, their omission does not affect the overall dynamics due to the negligible friction in the system. This highlights the importance of accurately representing forces in FBDs for correct problem-solving.

PREREQUISITES
  • Understanding of Newton's Second Law
  • Familiarity with Free Body Diagrams (FBDs)
  • Basic knowledge of engineering dynamics
  • Concept of tension forces in mechanical systems
NEXT STEPS
  • Study the principles of Free Body Diagrams in engineering mechanics
  • Learn about the implications of friction in dynamic systems
  • Explore case studies on common mistakes in applying Newton's Laws
  • Review the effects of mass versus weight in mechanical systems
USEFUL FOR

Students and professionals in mechanical engineering, particularly those focusing on dynamics and statics, will benefit from this discussion. It is also valuable for educators teaching engineering principles related to forces and motion.

bigu01
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Homework Statement


It is given in the photo


Homework Equations


They are all given in the photo

The Attempt at a Solution

I solved exactly the same as in the solution manual, except for the FBD of the slider B, when they write the 2nd Law, they do not input the weight there, only the Tension force by the rod.Why did they do that, that was the part I did not understand. Note: For the B block they use the x-direction as vertical.
 

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bigu01 said:

Homework Statement


It is given in the photo


Homework Equations


They are all given in the photo

The Attempt at a Solution

I solved exactly the same as in the solution manual, except for the FBD of the slider B, when they write the 2nd Law, they do not input the weight there, only the Tension force by the rod.Why did they do that, that was the part I did not understand. Note: For the B block they use the x-direction as vertical.
good question, they forgot to include the weight of the block B in the free body diagram. In fact, they didn't include the weight of block A in its free body diagram, although that doesn't affect the solution. But the omission of block B weight was an apparent oversight that does affect the result.
 
The sliders move with negligible friction in the horizontal slots shown.
The picture is a top view. The mass of both blocks is relevant, but their weights are not.
 

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