Static friction in two block problems

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Discussion Overview

The discussion revolves around the concept of static friction in the context of two blocks moving together. Participants explore how static friction can remain effective even when the applied force exceeds the limiting friction, particularly in scenarios where relative motion is zero. The conversation includes theoretical aspects and clarifications regarding the conditions under which static and kinetic friction operate.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how static friction can remain effective when the applied force exceeds limiting friction, questioning the conditions under which this occurs.
  • Another participant clarifies that the maximum value of static friction applies only to the friction force and not to the applied force, suggesting that static friction can adjust to match the applied force up to its maximum limit.
  • There is a discussion about the transition from static to kinetic friction, with some participants asserting that kinetic friction only takes over when relative motion occurs, while others emphasize that static friction can still act even when the applied force exceeds its maximum value.
  • Participants debate the conditions under which static friction remains effective, with one stating that static friction acts as long as relative motion is zero, regardless of the situation.
  • Clarifications are sought regarding the relationship between applied force, static friction, and acceleration, with some participants noting that exceeding static friction while maintaining zero relative motion is not possible.

Areas of Agreement / Disagreement

Participants generally agree that static friction acts as long as there is no relative motion between the blocks. However, there is disagreement regarding the implications of exceeding limiting friction and the conditions under which static friction transitions to kinetic friction, indicating that the discussion remains unresolved.

Contextual Notes

Participants reference specific values for limiting friction and discuss the need to consider both coefficients of friction between the blocks and the ground, highlighting the complexity of the problem.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics who are grappling with concepts of friction, particularly in multi-body systems, and those seeking clarification on the nuances of static versus kinetic friction.

Niklaus15
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Doubt - Force cannlt exceed limiting friction for a block to remain at rest on ground.

However for to blocks to move together, it becomes more than limiting friction, yet only static friction is acting on the blocks? Which moves the lower block. How can it be more in this case? Relative motion is zero in both cases.

I am missing some crucial point here but I am unable to find the flaw.

A direct answer or hint whatever is given would be much appreciated.

Image of the problem is attached. It's not a homework problem, it's a solved example alongside another situation I created to show what doubt I am facing.
 

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Just in case it isn't clear in first case lim. Friction is 120 N
 
Niklaus15 said:
Force cannlt exceed limiting friction ...
Which force? If the blocks accelerate together, the static friction between them is different from the force applied to the upper block.
 
Ok figured out most of the stuff,read some other threads. One guy had almist same doubt as me. Only confusion I now have is how can friction remain static even F exceed f limiting.

An answer to this question exists on the boards but I could not understand it

mr.physics said: ↑
But when F is greater than F(fr) doesn't that mean the friction is no longer static?
No the maximum value for static friction (that is
μSN
) applies only to the friction force (the tangential component of the constraint force) not to the force applied to move the object
If the object is not moving the applied force (or better its component parallel to the friction direction) and friction have the same magnitude but if the object is moving -as in your case since friction occurs with another moving body- they will differ by ma
>
 
A.T. said:
Which force? If the blocks accelerate together, the static friction between them is different from the force applied to the upper block.
The force being applied on block.
 
Niklaus15 said:
The force being applied on block.
No, as the quote you found states : "...the maximum value for static friction (that is μ*N) applies only to the friction force..."
 
I couldn't understand that fully as I said earlier. Could you explain what he was saying in simpler english, preferably with an example.

As far as i knew when the force moving the object became greater than maximum static friction the friction changes to kinetic.. .
 
Niklaus15 said:
As far as i knew when the force moving the object became greater than maximum static friction the friction changes to kinetic.. .
Only if the object isn't accelerating.
 
Why doesn't it change to kinetic when the object is accelerating?
 
  • #10
Niklaus15 said:
Why doesn't it change to kinetic when the object is accelerating?
Because the friction force doesn't exceed μ*N.
 
  • #11
Sorry for being irritating...

So as far as I understand, static friction will act so long as relative motion is zero, doesn't matter the situation - on ground or between blocks. Max value of static value starts acting when the applied force exceeds limiting friction. Then if the magnitude of applied force increases, friction stays at its max value so long as relative motion is zero.

As soon as the bodies move relatively, static friction ceases to act and kinetic friction starts acting with a fixed value which is not self adjusting.

Am I right?
 
  • #12
Anyone?
 
  • #13
Somebody please just check if I got it right
 
  • #14
Niklaus15 said:
So as far as I understand, static friction will act so long as relative motion is zero, doesn't matter the situation - on ground or between blocks. Max value of static value starts acting when the applied force exceeds limiting friction. Then if the magnitude of applied force increases, friction stays at its max value so long as relative motion is zero.

Well, if the applied force exceeds the force from static friction the object begins to accelerate. You cannot exceed the force of static friction and still have zero motion.
 
  • #15
Niklaus15 said:
Somebody please just check if I got it right
To say what will happen from a static state, as you increase the applied force, you have to know both coefficients of friction: between blocks, between bottom block and ground.
 
  • #16
Alright got it now thanks
 

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