Frictional force in a free-body diagram

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Homework Help Overview

The discussion revolves around understanding the direction of the frictional force in a free-body diagram related to a problem from Engineering Mechanics. Participants are examining the implications of friction in a scenario involving a block held by a wire and its interaction with another block below it.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are exploring the reasoning behind the direction of the frictional force, questioning the assumptions about motion and the conditions under which friction acts. There is an attempt to reconcile the behavior of the blocks and the implications of Newton's laws.

Discussion Status

The conversation is ongoing, with participants actively questioning the setup and the role of friction. Some guidance has been offered regarding the application of Newton's laws, but there is still exploration of different interpretations of the free-body diagrams.

Contextual Notes

Participants are working with a specific problem context that includes a visual aid (a free-body diagram) and are discussing the effects of friction in a system where one block is held by a wire and another block is positioned below it. There is a focus on the conditions of motion and the assumptions about the blocks' interactions.

zeralda21
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Homework Statement


I am having trouble understanding the direction of a friction force in a sample problem from Engineering Mechanics, Meriam & Kraige. I will attach a picture for no confusion.

http://i.imgur.com/0EdCofi.jpg



The Attempt at a Solution



Consider only the top free-body diagram. Why is the friction force in that direction?

"The friction forces are assigned in the directions to oppose the relative motion which would occur if no friction were present." So where would the object move if there was no friction? Well, it would not since it is attached to a wire. If not attached, it would(or tendency) to move downward and therefore an frictional force in the opposite direction drawn. So, from my own conclusion I would have chosen:

1) Upward force or 2) No friction force at all since it would not move if there was no friction.

Why am I wrong?
 
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The object held by the wire would not move. But the object just below would.
 
voko said:
The object held by the wire would not move. But the object just below would.

Let's say that the 50kg-block would move. Then I assume it would move downward and therefore there is a friction force F_1 in the upward direction, opposite the motion, exactly as in the SECOND free-body diagram. But that doesn't explain why the friction force in the TOP f-b diagram is in the downward direction. Right?
 
Apply Newton's third law.
 
voko said:
Apply Newton's third law.

Alright, I think I see it now. Thanks voko.
 
zeralda21 said:
Let's say that the 50kg-block would move. Then I assume it would move downward and therefore there is a friction force F_1 in the upward direction, opposite the motion, exactly as in the SECOND free-body diagram. But that doesn't explain why the friction force in the TOP f-b diagram is in the downward direction. Right?

No.

The friction opposes relative motion between the surfaces in contact. If the 50 kg block moved it would leave the top block behind without friction. So the force of friction between the blocks tends to move the top block together with the 50 kg one downward.

ehild
 

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