The discussion centers on the dynamics of a block resting on a wedge, specifically analyzing the forces acting on the block when a force P is applied. Participants emphasize the importance of including friction in the problem statement, as it significantly affects the block's motion. The correct equations derived include ##F_N \cos \beta = mg## and ##F_N \sin \beta = ma##, leading to the acceleration of the block being ##a = g \tan \beta##. The conversation highlights the necessity of clear problem statements in physics to avoid assumptions that can lead to incorrect conclusions.
PREREQUISITESStudents of physics, educators designing problem sets, and anyone interested in the dynamics of systems involving inclined planes and forces.
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 said:
Hm, right but if there is friction between block and wedge this can hold true.rudransh verma said:
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.Delta2 said:
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.Mark44 said:
haruspex said:
In retrospect, I agree that the problem is silent on whether there is friction between the block and wedge.jbriggs444 said:
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.jbriggs444 said:
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 said:
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.phinds said:
One can do better than that. One can examine all the possible cases and come up with answers.phinds said:
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.Mark44 said:
I agree. I listed all the possibilities hoping that this summary will end the discussion before the mentors close the thread.phinds said:
Ha. These Rudransh Verma threads never seem to end, we just keep rehashing the same stuff over and over.kuruman said:
Yup. I intended post #41 to be a gentle hint to the mentors.phinds said:Ha. These Rudransh Verma threads never seem to end, we just keep rehashing the same stuff over and over.