Static friction in two block problems

[Niklaus15]

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.

 

[Niklaus15]

Just in case it isn't clear in first case lim. Friction is 120 N

 

[A.T.]

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.

 

[Niklaus15]

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
>

 

[Niklaus15]

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.

 

[A.T.]

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..."

 

[Niklaus15]

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.. .

 

[A.T.]

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.

 

[Niklaus15]

Why doesn't it change to kinetic when the object is accelerating?

 

[A.T.]

Why doesn't it change to kinetic when the object is accelerating?

Because the friction force doesn't exceed μ*N.

 

[Niklaus15]

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?

 

[Niklaus15]

Anyone?

 

[Niklaus15]

Somebody please just check if I got it right

 

[Drakkith]

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.

 

[A.T.]

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.

 

[Niklaus15]

Alright got it now thanks