Two blocks and wall system

In summary, the problem is about a system in equilibrium with rough surfaces. The direction of friction on block B due to block A is upwards, and to balance the forces in all directions, only upward friction forces are needed on each block. A free body diagram can help clarify this concept and show that the weight of block B is balanced by friction from block A and the wall. The key is to remember that the force of friction is always in the opposite direction of motion, and for Newton's third law, the forces are equal and opposite.
  • #1

rudransh verma

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Homework Statement
All the surfaces shown are rough and the system is in equilibrium. The direction of friction on B due to A is-
Relevant Equations
F=ma
This problem is similar to what I have done before here. I think since the system is in equilibrium, that is both bodies are at rest, net force on each should be zero. So to balance the forces in all directions we need only friction forces on each in upward direction. So the force on B due to A is upwards.
 

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  • #2
As a free body diagram will make clear ! :smile:

## \ ##
 
  • #3
rudransh verma said:
Homework Statement:: All the surfaces shown are rough and the system is in equilibrium. The direction of friction on B due to A is-
Relevant Equations:: F=ma

I think since the system is in equilibrium, that is both bodies are at rest, net force on each should be zero.
So far, so good.
rudransh verma said:
Homework Statement:: All the surfaces shown are rough and the system is in equilibrium. The direction of friction on B due to A is-
Relevant Equations:: F=ma

So to balance the forces in all directions we need only friction forces on each in upward direction.
What about Newton's third law?
rudransh verma said:
Homework Statement:: All the surfaces shown are rough and the system is in equilibrium. The direction of friction on B due to A is-
Relevant Equations:: F=ma

So the force on B due to A is upwards.
Don't guess. Work it out step by step. As @BvU advises, start with a free body diagram. Do a diagram for each block.
 
  • #4
Doc Al said:
What about Newton's third law?
I don't think its correct since there is more force downwards than upwards.
 

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  • #5
' For each block ' is pretty unambiguous!

That way you can untangle the mess you now have between A and B.

And perhaps you need a FBD for the wall too :wink:

## \ ##
 
  • #6
rudransh verma said:
I don't think its correct since there is more force downwards than upwards.
Do you seriously think Newton's 3rd law doesn't apply here? Draw a free body diagram for each block.
 
  • #7
Doc Al said:
Do you seriously think Newton's 3rd law doesn't apply here? Draw a free body diagram for each block.
 

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  • #8
For block B, describe each force you show. On the left of the block, you show a force going up and a force going down. What are they supposed to be?
 
  • #9
Doc Al said:
For block B, describe each force you show. On the left of the block, you show a force going up and a force going down. What are they supposed to be?
I am unable to decide which body applies friction up and which body applies down.
 
  • #10
rudransh verma said:
I am unable to decide which body applies friction up and which body applies down.
Just pick a direction and see what Newton says about it. You know each body is in equilibrium. Focus on block B.
 
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  • #11
Doc Al said:
Just pick a direction and see what Newton says about it. You know each body is in equilibrium. Focus on block B.
I will start from the wall . All is balanced for A. Now due to weight of B it applies downward force on A and inreturn A applies upwards on B.
 

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  • #12
rudransh verma said:
I will start from the wall .
OK, but you'll find the answer you need quicker if you start with B.
rudransh verma said:
All is balanced for A.
OK. Forces are balanced for both A and B.
rudransh verma said:
Now due to weight of B it applies downward force on A and inreturn A applies upwards on B.
Careful. The weight of B (the force of gravity) acts only on B. There is a force that B exerts on A (friction) and A exerts an equal and opposite friction force on B. (They are 3rd law pairs.)

But your diagram is much better now! 👍
 
  • #13
Doc Al said:
Careful. The weight of B (the force of gravity) acts only on B. There is a force that B exerts on A (friction) and A exerts an equal and opposite friction force on B. (They are 3rd law pairs.)
It doesn’t feel right though. A has more downward force than upwards. How is it going to stay in equilibrium?
 
  • #14
rudransh verma said:
A has more downward force than upwards.
Why do you think that? Just because there are two forces acting down and only one acting up?
rudransh verma said:
How is it going to stay in equilibrium?
All the forces must balance. What does that tell you?
 
  • #15
Doc Al said:
All the forces must balance. What does that tell you?
Well it tells that friction of wall is much greater than weight and force from B.
There should be a beginning point in these kind of problems otherwise I feel I will get lost. Like here I started with wall.
 
  • #16
rudransh verma said:
Well it tells that friction of wall is much greater than weight and force from B
Really? So you think A is accelerating up the wall? (If you think the upward force is greater than the downward force, that's what you must conclude.)

What it should tell you is that the upward friction force from the wall must just balance the downward forces acting on A.
 
  • #17
Here is a thought: The blocks don't know (or care to know) whether they are up against a wall or another block. So the force of friction exerted by A on B is in the same direction as the force of friction exerted by the wall on A. Does this help you sort out the directions of Newton's 3rd law pairs?
 
  • #18
How about doing the exercise without ##F## and with glue instead of friction ?
What Would the FBDs be ?

##\ ##
 
  • #19
Doc Al said:
Careful. The weight of B (the force of gravity) acts only on B. There is a force that B exerts on A (friction) and A exerts an equal and opposite friction force on B. (They are 3rd law pairs.)
I have a doubt which I didn’t sort and ignored.

The force of weight acts downward on B which will tend to pull it down and this will be opposed by friction from A. So, an upward force acts on B by A. And B exerts an equal and opposite force on A.
Same result but this explanation seems right.

Is there a starting point here like not starting from wall but from body B when drawing FBD?
 
  • #20
rudransh verma said:
Is there a starting point here like not starting from wall but from body B when drawing FBD?
Of course. All you need to answer the question is a FBD of body B.

That's what I tried to get you to do when I said above:
Doc Al said:
Focus on block B.

If you start from the wall, you're doing unnecessary work. But it's good to do anyway to deepen your understanding.
 
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1. What is a "Two blocks and wall system"?

A "Two blocks and wall system" is a construction method where two blocks are stacked on top of each other with a layer of mortar in between to create a sturdy wall. This system is commonly used in building projects for its simplicity and strength.

2. How does the "Two blocks and wall system" work?

The "Two blocks and wall system" works by interlocking the blocks and bonding them together with mortar. The weight of the blocks and the friction between them creates a stable structure that can support heavy loads.

3. What materials are needed for a "Two blocks and wall system"?

The materials needed for a "Two blocks and wall system" include concrete blocks, mortar mix, and tools such as a trowel and level. Optional materials may include reinforcement bars and waterproofing materials for added strength and protection against water damage.

4. What are the advantages of using a "Two blocks and wall system"?

The advantages of using a "Two blocks and wall system" include its simplicity, cost-effectiveness, and strength. This method also allows for easy customization and can be used for various types of walls, such as retaining walls, garden walls, and foundation walls.

5. Are there any limitations to using a "Two blocks and wall system"?

While the "Two blocks and wall system" is a popular construction method, it does have some limitations. It may not be suitable for very tall walls or structures that require intricate designs. It also requires proper training and expertise to ensure the wall is built correctly and can withstand external forces.

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