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This is the question: enter image description here

the answer to this question is A and B. That means C and D are correct.

I see that in this question option C has already been answered here but what I don't understand is option D. How does friction being static and kinetic depend on the frame of reference?

My approach:

I understand that since the coefficient of friction isn't given in the question we can't say that the friction is static or kinetic. Since we don't know what force the man is applying on the plank and whether it exceeds/does not exceed the static friction force. But even if the reference frames are changed the magnitude of the forces are going to be the same in every frame of reference, only their direction can vary.

Case 1: In this case I am observing with respect to ground

Case 1: In this case I am observing with respect to ground

Case 2 : In this case I am observing with respect to the plank

Case 2 : In this case I am observing with respect to the plank

(In both of these diagrams f - refers to the frictional force; a(m) - refers to the acceleration of the man; a(p) - refers to the acceleration of the plank)

So according to my Case 2 diagram here frame of reference should only decide which direction the relative motion is occurring and hence the direction of the frictional force and nothing about it being kinetic or static.

Kindly shine some light on this question and also if I might be going wrong somewhere. Any help is greatly appreciated!

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  • $\begingroup$ The question asks to pick incorrect answers. Is this a typo or indeed you have to select the incorrect ones? $\endgroup$
    – nasu
    Commented Sep 10, 2021 at 10:48
  • $\begingroup$ @nasu I just now realized. I have edited my question. But my query is still stands. The incorrect options were given A and B. So my question indirectly says the correct options are C and D. $\endgroup$
    – Tanya
    Commented Sep 10, 2021 at 11:03
  • $\begingroup$ Now it makes sense. And I agree that D is not right. $\endgroup$
    – nasu
    Commented Sep 10, 2021 at 12:46

1 Answer 1

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Option D is incorrect.

If the man's foot is stationary with respect to the plank then the friction between the man and the plank is static friction. If the man's foot is not stationary with respect to the plank (i.e. it is sliding along the plank) then the friction between the man and the plank is kinetic friction. In either case, the type of friction depends on the relative motion between the man's foot and the plank, which is independent of the reference frame that is being used.

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  • $\begingroup$ I thought so too. I saw no relation between type of friction and frame of reference. Thanks for the answer. $\endgroup$
    – Tanya
    Commented Sep 10, 2021 at 10:30
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    $\begingroup$ I use the word “slipping” to describe the difference. If the surfaces are slipping then it is kinetic and if they are not slipping then it is static. All frames agree on slipping. $\endgroup$
    – Dale
    Commented Sep 10, 2021 at 11:03

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