Accelerating wedge with a block at rest on it

[Delta2]

Yes ok I made up my own problem because this thing puzzles me...

 

[rudransh verma]

sorry I don't understand what's wrong with that statement?

I think the wedge can move with constant velocity but how can the block move with constant velocity if the net force on it is not zero.

 

[Delta2]

I think the wedge can move with constant velocity but how can the block move with constant velocity if the net force on it is not zero.

Hm, right but if there is friction between block and wedge this can hold true.

 

[haruspex]

Hm, right but if there is friction between block and wedge this can hold true.

Yes, but without information on the coefficient there is no way to answer the question. That only leaves the option of assuming no friction and a non constant velocity.

 

[jbriggs444]

I believe that a frictionless surface is implicit in the problem statement (emphasis added by me). From the OP:

That says that the ground beneath the block is frictionless but says nothing about the wedge/block interface. Since that interface is not mentioned as "smooth" while the ground interface is, I read the problem to be including friction for the wedge/block interface.

If it were a complete problem, context could make interpretation easier.

 

[Mark44]

Yes, but what is missing is that there is no friction between block and wedge.

That says that the ground beneath the block is frictionless but says nothing about the wedge/block interface.

In retrospect, I agree that the problem is silent on whether there is friction between the block and wedge.

If it were a complete problem, context could make interpretation easier.

So, since the problem is poorly stated, it seems to me that the only recourse for the OP is to contact the instructor. The instructor could then either throw out the problem entirely or provide additional an assumption on whether there is friction between the block and wedge.

If this is a problem that was made up by the instructor, it's not the first time in history that a poorly conceived problem was given.

 

[phinds]

If this is a problem that was made up by the instructor, it's not the first time in history that a poorly conceived problem was given.

Sure, but the OP could have solved the issue in advance, if he understood the dangers of being OK with poor problem statements, by including something along the lines of "the problem statement is incomplete in that it does not specify any coefficient of friction between the block and the wedge, so I am going to assume that it is zero and solve the problem under that assumption."

 

[Mark44]

Sure, but the OP could have solved the issue in advance

Or maybe not. It takes a lot of experience and confidence to recognize that just because something is written down on paper, it's not necessarily the gospel truth, and could contain errors or lack necessary information. I don't think we should blame @rudransh verma here for not recognizing that the problem was incomplete.

 

[kuruman]

$\dots~$so I am going to assume that it is zero and solve the problem under that assumption."

One can do better than that. One can examine all the possible cases and come up with answers.

1. The block is glued with gorilla glue on the wedge. In this case the coefficient of static friction is infinite and the block will not slide relative to the wedge for any P.
2. The coefficient of static friction between the block and the wedge is finite. In this case there is a range of forces P with an upper limit. Furthermore, if the angle of the incline is greater than the angle of repose, then there is a lower limit to the pushing force in addition to the upper limit.
3. There is no friction between the block and the wedge. In this case the pushing force is single-valued.

 

[phinds]

Or maybe not. It takes a lot of experience and confidence to recognize that just because something is written down on paper, it's not necessarily the gospel truth, and could contain errors or lack necessary information. I don't think we should blame @rudransh verma here for not recognizing that the problem was incomplete.

Actually, I agree, BUT ... when I pointed out to him that it was incomplete he argued that it didn't matter. I don't find that acceptable.

 

[kuruman]

Actually, I agree, BUT ... when I pointed out to him that it was incomplete he argued that it didn't matter. I don't find that acceptable.

I agree. I listed all the possibilities hoping that this summary will end the discussion before the mentors close the thread.

 

[phinds]

I agree. I listed all the possibilities hoping that this summary will end the discussion before the mentors close the thread.

Ha. These Rudransh Verma threads never seem to end, we just keep rehashing the same stuff over and over.

 

[phinds]

Hm ... I see he's lined out again. I wonder is this yet another 10 day or is it permanent this time?

 

[kuruman]

Ha. These Rudransh Verma threads never seem to end, we just keep rehashing the same stuff over and over.

Yup. I intended post #41 to be a gentle hint to the mentors.