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rob
5:53 AM
Just to make the transcript easier to follow: we're discussing this question:
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Q: Is this the correct approach to find acceleration of the wedge?

blueConsider a block sliding without friction on a wedge as in the following picture: Given the height of the block of mass $m$ as a function $h(t)$ of time $t$ with initial height $h(0) = h_0$, is the following method to determine the acceleration of wedge $M$ correct? Using conservation of ener...

 
rob
6:10 AM
I have to admit that I think it's a great example of an on-topic homework-like question, and also a great example of why accepting homework questions at all is tricky for us.
The question says "Here's this problem; I tried this approach for these reasons [emphasis mine] and I'm getting the wrong answer; am I making a conceptual error or doing something silly, because it's hard for me to tell at this point?" And the answer turns out that the asker was conceptually correct, but was making a hidden and incorrect assumption.
I consider it a high-effort homework question, and the responses by the asker to commenters and answerers show willingness to engage in a conceptual discussion; however it turned out that the hidden assumption wasn't as conceptual as it might have been.
Here are some now-deleted comments that show how the question went from being off-topic to being on-topic:
@ACuriousMind Any way to make this question on-topic? I'm not really asking someone to the calculations for me. I just want to know if I am making any conceptual error. — blue yesterday
As said in the linked meta post, the way to make such questions on-topic is to find the step that produces the error and ask why it is not applicable here. But if you are not even sure if this is right or wrong, then we can't really help you - we don't check your work, since if you haven't made a mistake, there's really nothing to write in an answer. — ACuriousMind ♦ yesterday
@ACuriousMind I think I've identified the equation which is causing the incorrect result. The second equation looks fine to me (as it is simply momentum conservation). I want to know "why" my first equation is wrong. And, about your second point: I'm pretty sure that this technique gives an incorrect result because I've checked it with another very similar problem (which I've linked). — blue yesterday
I still think there is too little information about what you actually did: Without clicking on the link to the picture one doesn't actually know what the problem is you're trying to solve, and it's not exactly clear what you did to eq. (1) to obtain your solution: Did you solve for $v_2(t)$, then differentiate to obtain the acceleration? Be specific! — ACuriousMind ♦ yesterday
@ACuriousMind Yes, I solved for $v_2(t)$ and then found out $dv_2/dt$. I've edited to add that point. By the way, thanks for your edit. Is the question open-able now? — blue yesterday
It looks like you saw the question after that clarifying discussion was removed, @sammygerbil.
At the same time: it's similar enough to a low-effort homework question that it attracted not one but two photographs of complete solutions from new users. Which is kind of a downer.
But all in all I think the decision to close and the decision to re-open were both reasonable.
 

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