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Motion when Kinetic Friction is more than Static Friction

  • #1
navneet9431
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Homework Statement


IMG_20181023_102302.jpg

How would the motion of the block look like when the Force of kinetic friciton is more than the Limiting value of static friction?

Homework Equations


Kinteic Friciton=μN

The Attempt at a Solution


I think the block after overcoming the limiting value of static friction will move for some time and then it would deaccelerate because of the force of static friction.
 

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  • #2
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How would the motion of the block look like when the Force of kinetic friciton is more than the Limiting value of static friction?
That seems like a strange question. Friction is a reaction force. It can only oppose some applied force. It doesn’t make sense to set the force of kinetic friction against the force of static friction. I think that must not be what you meant, but I’m not sure what you did mean.
 
  • #3
haruspex
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How would the motion of the block look like when the Force of kinetic friciton is more than the Limiting value of static friction?
I'm guessing you mean "when the coefficient of kinetic friction is greater than that of static friction".
It's a logical impossibility. A force insufficient to overcome kinetic friction will not succeed in moving the object, so it would be considered not to have overcome static friction.
 
  • #4
navneet9431
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That seems like a strange question. Friction is a reaction force. It can only oppose some applied force. It doesn’t make sense to set the force of kinetic friction against the force of static friction. I think that must not be what you meant, but I’m not sure what you did mean.
I meant - ""when the coefficient of kinetic friction is greater than that of static friction,then what would the motion of block look like?"
 
  • #5
CWatters
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I meant - ""when the coefficient of kinetic friction is greater than that of static friction,then what would the motion of block look like?"
Do you mean in that problem or on general?

In the problem it still won't move.

In general ... Are you worried about a paradox when the applied force is greater than the max static friction (so it should move) but less than the force of kinetic friction (so it should slow to a stop)?

That just tells you there is a problem with the model. The coefficient of kinetic friction can't be greater than static friction (at least not at velocities approaching zero). Note that the coefficient of kinetic friction doesn't have to be independent of velocity. It's just that problems are easier to solve when we model it that way.
 
  • #6
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I meant - ""when the coefficient of kinetic friction is greater than that of static friction,then what would the motion of block look like?"
Ah, then haruspex has your answer in post 3. The static friction coefficient is determined by the amount of force it takes to break loose and start moving by definition. If it is moving, it must have accelerated so the net force must be greater than 0 by Newton’s law. Therefore the kinetic friction cannot be greater than the applied force, so logically the coefficient of kinetic friction cannot be greater than the coefficient of static friction.
 
  • #7
ZapperZ
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Homework Statement


View attachment 232624
How would the motion of the block look like when the Force of kinetic friciton is more than the Limiting value of static friction?

Homework Equations


Kinteic Friciton=μN

The Attempt at a Solution


I think the block after overcoming the limiting value of static friction will move for some time and then it would deaccelerate because of the force of static friction.
The problem here is also in interpreting the question. The way I read it is this:

It tells you indirectly what μk is from the situation where the block is in motion at constant velocity. However, it doesn't say that the block is in motion at the start of the problem. It just said that IF the block is already in motion, and the applied force is 3.5 N, the block will move with constant velocity.

Then, the question asks you what would happen if 3 N is applied. It did not say that the block is already in motion. So from my perspective, the block is just sitting there.

From what is given, we know that μk < μs, and this is true in most situation like this, where it requires a greater force to start something in motion, than the force required to keep it at constant velocity once it is already sliding. So obviously, if the block did not start while in motion, then it will not move when 3 N is applied to it. This makes (c) the answer that I would have picked, after reading and re-reading the question a few times.

Zz.
 
  • #8
haruspex
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The problem here is also in interpreting the question. The way I read it is this:

It tells you indirectly what μk is from the situation where the block is in motion at constant velocity. However, it doesn't say that the block is in motion at the start of the problem. It just said that IF the block is already in motion, and the applied force is 3.5 N, the block will move with constant velocity.

Then, the question asks you what would happen if 3 N is applied. It did not say that the block is already in motion. So from my perspective, the block is just sitting there.

From what is given, we know that μk < μs, and this is true in most situation like this, where it requires a greater force to start something in motion, than the force required to keep it at constant velocity once it is already sliding. So obviously, if the block did not start while in motion, then it will not move when 3 N is applied to it. This makes (c) the answer that I would have picked, after reading and re-reading the question a few times.

Zz.
My reaction on reading the actual question is that it perhaps intended to specify that the kinetic coefficient is .3 and a force of 3.5N is applied, making the answer (2).
 
  • #9
ZapperZ
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My reaction on reading the actual question is that it perhaps intended to specify that the kinetic coefficient is .3 and a force of 3.5N is applied, making the answer (2).
How did you get any of these?

μk is 0.35 if you use the values given while it is moving at constant velocity. The APPLIED force in the very last line of the question said "3N". None of what you said here matches any of these.

Zz.
 
  • #10
haruspex
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How did you get any of these?

μk is 0.35 if you use the values given while it is moving at constant velocity. The APPLIED force in the very last line of the question said "3N". None of what you said here matches any of these.

Zz.
No, I understand that. I am suggesting that the questioner may have intended to highlight what happens when static friction is exceeded and the kinetic coefficient is lower. To do that, the applied force would need to be the 3.5N and the kinetic friction force 3N. In the question as stated, those are the other way around. This makes some of the data irrelevant.

Of course, it may be the questioner was deliberately reversing it from what may be a more common form of question to check that the student is paying attention.
 
  • #11
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The next to last sentence is ambiguous. They either mean : “... is applied to the STATIONARY block ...” or they mean “... is applied to the ALREADY MOVING block ...”. They are both reasonable questions, so I don’t think there is any authoritative way to determine which they meant short of tracking them down and asking.

For my money “stationary” makes a better and more instructive problem. In the case of “already moving” we can quantify the deceleration and postulate an initial velocity and say that the block comes to a halt in blah blah blah. Fun for us, maybe, but not an obvious illustration of a fundamental point. On the other hand “stationary” would have been more instructive if the applied force had been between 3.5 and 4 N. Missed opportunity?
 

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