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

[OmniNewton]

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.

[/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?

 

[SteamKing]

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.

[/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?

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.

 

[rpthomps]

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.

 

[OmniNewton]

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

 

[rpthomps]

Based on the question, the only way to move C, is to lift B. Therefore, no normal.

 

[OmniNewton]

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

 

[haruspex]

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.

 

[rpthomps]

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.

 

[haruspex]

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.

 

[rpthomps]

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. :)

 

[haruspex]

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.