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can someone explain me why in this scenario (picture below) we have a friction force Ftr12 in the same direction as movement (left part of the picture, case 1 ) and how these directions of friction force match the ones on the right case ...

One remark: I understand the case on the right, but not completely the on the left, so I need clarification with it ...

Ftr12 means the force which body 1 acts on body 2, correct ? And for Ftr21 means the opposite ?

Here is the picture

enter image description here

P.S. My theory is that when the body 2 starts moving, body 1 goes along with it and is affected from resistance force of air which wants to move it to the left...So Ftr12 is the reaction to that force, that's why it is intent to the direction of movement.

Also if we let these bodies move indefinitely, would the body 1 slide off from body 2, given the V1>V2 ?

Also, additional question, what would be the case with direction of friction forces if V2>V1 and would the body 1 slide off backwards from body 2 ?

Thanks in advance people :D

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1 Answer 1

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One remark: I understand the case on the right, but not completely the on the left, so I need clarification with it ...

One hugely important thing about the case on the left is that the speed of box 1 is stated to be faster than the speed of box 2, both of which are presumably being measured relative to the ground. That means that relative to box 2, box 1 is moving to the right, just as it is in the right picture. If the speed of box 1 were less than the speed of box 2, then box 1 would still be moving "objectively" to the right, but would be moving left "relative to box 2," which would flip the direction of the drag force.

Ftr12 means the force which body 1 acts on body 2, correct ? And for Ftr21 means the opposite ?

Generally, yes, the numbers $A_{mn}$ mean "The $A$ with which body $m$ acts on body $n$", for whatever the $A$ is.

Also, if we let these bodies move indefinitely, would the body 1 slide off from body 2, given the V1>V2 ?

I mean, not necessarily. The whole point of friction is that it's trying to make the two things have zero relative velocity. If friction can do that before the boxes fall off of each other, then they never will.

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