My answer (posted below) was deleted from the following question: Calculating mass of unknown object using center of mass. The deleter's comment read,

"I'm deleting this in accordance with our homework policy. Please do not give complete or near-complete answers to homework-like questions."


You don't really need torque equations. Just treat the object as a point mass, and use the following equation for CoM: $$x_{\rm CoM}=\dfrac{mx+MX}{m+M}$$ where m, x, M and X represent the mass of the stick, position of the stick's CoM, the mass of the object, and the position of the object's CoM (all positions relative to 1 fixed point).

All I did was provide an equation that is present on the first page of every textbook section relating to the center of mass of two points. And yet this is a 'complete' or 'near complete' answer.

The OP's question read, "How exactly would you go about solving this. I know the answer should be roughly about 108 grams but am unsure how to set up the equation."

OP did not ask for an answer. OP asked for a) general approach, and b), setting up the equation.

The general approach is the CoM method for a), and for b), I gave out the standard textbook equation. Furthermore, this problem is of interest to more than OP, as the approach is pretty standard in these introductory problems.

There is still quite some work for OP to do; associate the positions of the masses correctly relative to some origin which is yet to be identified, figure out the connection between a rigid body's CoM and how it plays into a system of particles, and, of course, solve for the mass. If I were to provide a complete answer to this question, mine would start with the provided equation.

Thus, I don't see how my answer constitutes a 'complete' or 'near complete' solution. I thought I left enough work for OP.

What makes the answer complete?


1 Answer 1


I can't say why the mod who did the deletion left that particular comment rather than some other wording. But that homework-like question is not really about a concept (other than "I'm stuck") and should probably have been closed rather than answered.

In the past, moderators removing such answers would emphasize that the deletion was temporary, and invite folks who disagreed with deletions to wait a couple of weeks and then flag their answers for undeletion. The idea was to encourage answers that are helpful in the long term, while adding friction for "help vampires" who would like drive-by help getting numerical answers to their homework. However I restore such answers perhaps twice a year, so I have stopped writing "temporarily deleted" in my comments unless I think the homework-like question is particularly likely to survive the roomba.


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