Static friction in two block problems

• Niklaus15
In summary: Static friction will act as long as the relative motion is zero. If the relative motion increases, Kinetic friction takes over and will act until the relative motion decreases again to zero.
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.

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• IMAG0436.jpg
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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?

Niklaus15 said:
Why doesn't it change to kinetic when the object is accelerating?
Because the friction force doesn't exceed μ*N.

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?

Anyone?

Somebody please just check if I got it right

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.

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.

Alright got it now thanks

What is static friction?

Static friction is the force that prevents two surfaces from sliding against each other when there is no relative motion between them. It is a type of friction that occurs when an object is placed on a surface and is not moving.

How is static friction different from kinetic friction?

Static friction is the force that keeps an object at rest, while kinetic friction is the force that opposes the movement of an object. In other words, static friction occurs when there is no relative motion between two surfaces, while kinetic friction occurs when there is relative motion between two surfaces.

What factors affect the magnitude of static friction?

The magnitude of static friction is affected by the nature of the two surfaces in contact, the normal force between the surfaces, and the roughness of the surfaces. It also depends on the coefficient of static friction, which is a constant value for a given combination of surfaces.

How is static friction calculated in two block problems?

In two block problems, the force of static friction is calculated using the equation Fs = μsN, where Fs is the force of static friction, μs is the coefficient of static friction, and N is the normal force between the two blocks.

Can the force of static friction be greater than the force applied?

Yes, the force of static friction can be greater than the force applied. This occurs when the force applied is not strong enough to overcome the force of static friction and cause the objects to move. In this case, the objects will remain at rest and the force of static friction will be equal in magnitude to the applied force.

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