Why is the friction force at A not considered in forces in equilibrium?

[Faiq]

Homework Statement

[moderator note: Post edited to make problem statement images visible]

https://www.physicsforums.com/attachments/question-jpg.113056/

My question is when forces are resolved horizontally, why is the friction force at A not taken into account?

The Attempt at a Solution

Find the value of u (in terms of W)

Note:- The W next to u in the picture is actually Reaction at D, R(d)

Given solution:
https://www.physicsforums.com/attachments/forces-answer-jpg.113057/

In the given solution, why is the friction force at A not taken into account?

 

[gneill]

Moderator note: Thread cleaned up after amending the original post to clarify the problem.

 

[Faiq]

I just want to know when they resolved the forces horizontally, why does their answer makes sense even though they didnt include the frictional force at point A

 

[gneill]

I just want to know when they resolved the forces horizontally, why does their answer makes sense even though they didnt include the frictional force at point A

If you read the problem statement (which is now visible ) , how is the rod joined to the plane at point A?

 

[Faiq]

By a hinge

 

[gneill]

By a hinge

Right. The hinge will be attached (probably bolted) to the floor. No sliding friction involved, the rod is firmly attached.

 

[Faiq]

Oh but there will still be a reaction at the hinge and we have to take account the horizontal component of that

 

[gneill]

Oh but there will still be a reaction at the hinge and we have to take account the horizontal component of that

Since it is fixed its horizontal reaction will be equal to and opposite of any horizontal force applied to it.

My interpretation of the shown solution is that they were concentrating on the block's FBD, where the local friction has to counter the other external forces applied to the block.

 

[Faiq]

So even if there was a resultant horizontal force at A, it won't effect the block?

 

[gneill]

So even if there was a resultant horizontal force at A, it won't effect the block?

Not if the system is in equilibrium. Only forces applied directly to the block affect the block.

 

[Faiq]

But the force at A will effect the rod which will effect the block

 

[gneill]

But the force at A will effect the rod which will effect the block

The rod does not move. Nothing moves. How can the force at A affect how the rod presses on the block? That was determined by taking moments about A.

It is enough to know that the rod cannot slip at A.

 

[Faiq]

oh Okay thank you. Can you also help me in another question

 

[gneill]

oh Okay thank you. Can you also help me in another question

Start a new thread for it and I or someone else will be happy to help.