Why is Block B not experiencing a normal force in this static friction problem?

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Homework Help Overview

The discussion revolves around a static friction problem involving three blocks with different weights. The original poster questions why block B does not experience a normal force when it is assumed to slip upwards while block A remains stationary.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of block B slipping upwards and the resulting absence of a normal force from the ground. There is a focus on the relationship between the blocks and the conditions under which they might move or remain stationary.

Discussion Status

Participants are actively questioning the assumptions made about the movement of the blocks and the forces acting on them. Some suggest that if block B is lifted, it cannot have a normal force, while others consider the possibility of the blocks moving together as a unit. The conversation reflects a range of interpretations regarding the dynamics of the system.

Contextual Notes

There is mention of specific weights for the blocks and coefficients of static friction, but the original problem statement was not fully clear at the outset. Some participants express uncertainty about the implications of block A's movement on the overall system.

OmniNewton
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Homework Statement


How come for the following image when I assume block B slips up, block A does not move. The free body diagram at block B does not have a normal from the ground. This is a static friction problem.
c8159d667350baaca3628c32559c52fb.png
[/B]

Homework Equations


No relevant equations all theory[/B]

The Attempt at a Solution


This should be the Free body diagram but why does B have no normal from the ground when this assumption is made?
9125af555fd21b6e87a783fb1a808f7c.png
 
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OmniNewton said:

Homework Statement


How come for the following image when I assume block B slips up, block A does not move. The free body diagram at block B does not have a normal from the ground. This is a static friction problem.
c8159d667350baaca3628c32559c52fb.png
[/B]

Homework Equations


No relevant equations all theory[/B]

The Attempt at a Solution


This should be the Free body diagram but why does B have no normal from the ground when this assumption is made?
9125af555fd21b6e87a783fb1a808f7c.png

It's not clear what the original problem statement was, since you didn't provide it.

When blocks A and C are forced together, block B is going to slide upwards on the sloped face of block C. Once Block B is just a minute distance off the ground, there is no more normal force on the block from that direction.
 
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I would assume if block B is slipping up, it is no longer in contact with the ground and therefore there cannot be a normal force.
 
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The original problem statement was:
The three stone blocks have weights of 60lb 150lb and 40 lb respectively Determine the smallest horizontal force P that must be applied to block C in order to move this block. The coefficient of static friction between the blocks is 0.3 and between the floor is 0.5
 
Based on the question, the only way to move C, is to lift B. Therefore, no normal.
 
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rpthomps said:
Based on the question, the only way to move C, is to lift B. Therefore, no normal.
Excellent, thank you that is what I thought. I did the question originally assuming the normal and the system was statically indeterminate I was unsure how to solve until I was told this FBD did not contain a normal but I was unsure as to why
 
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rpthomps said:
Based on the question, the only way to move C, is to lift B. Therefore, no normal.
Is that from checking with the actual numbers? With some combinations of weights etc., the three blocks would slide horizontally, no lifting.
 
haruspex said:
Is that from checking with the actual numbers? With some combinations of weights etc., the three blocks would slide horizontally, no lifting.

No, I didn't use numbers. I was trying to visualize the block moving without lifting...I couldn't see it.
 
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rpthomps said:
No, I didn't use numbers. I was trying to visualize the block moving without lifting...I couldn't see it.
Despite the way the blocks are drawn, block A is much lighter than block B. If it's light enough it will slide instead of B rising. It's easy to visualise - the three blocks slide as if one unit.
Note that the clause "block A does not move" in the OP is not part of the problem specification; it is an observed consequence of the assumption that B slides up.
 
  • #10
haruspex said:
Despite the way the blocks are drawn, block A is much lighter than block B. If it's light enough it will slide instead of B rising. It's easy to visualise - the three blocks slide as if one unit.
Note that the clause "block A does not move" in the OP is not part of the problem specification; it is an observed consequence of the assumption that B slides up.

I guess I just latched onto the phrase Block A does not move. I am not sure if that was a part of the problem originally but that was the assumption I was basing my thoughts on. If it can move, I agree, there is a possibility that the whole unit could move together. In that case, yes I can visualize Block B traveling horizontally. :)
 
  • #11
rpthomps said:
I guess I just latched onto the phrase Block A does not move. I am not sure if that was a part of the problem originally but that was the assumption I was basing my thoughts on. If it can move, I agree, there is a possibility that the whole unit could move together. In that case, yes I can visualize Block B traveling horizontally. :)
The actual problem statement appears to be in post #4.
 

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