# Finding two masses as one rotates about an axis, no givens [closed]

My Physics C class was given a lab today with little explanation, and many of us are having issues determining a solution.

We are given this apparatus:

We must find the masses of both objects m and M. We aren't allowed spring scales, springs, or scales. Using cameras, timers, and rulers, we must find the masses.

I've attempted to derive multiple equations for each mass with the kinematic equations, rotational kinematic equations, and energy equations, however when I use them in a system of equations, the masses always cancel out.

My next idea is to determine rotational momentum, convert the rotational momentum to linear momentum, and take the derivative to find force. I'm not really sure what force this is, but if I got that far I believe I could figure it out.

Another issue is that the masses aren't uniform, and are in the shape of a conical frustrum:

What am I missing? This seems like an odd problem or a trick question. Our physics teacher is known to throw stuff at us like this without a prepared solution, so I'm worried this is one of those times.

Thank you!

## closed as off-topic by Aaron Stevens, Kyle Kanos, Jon Custer, ACuriousMind♦Oct 20 at 22:19

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Aaron Stevens, ACuriousMind
If this question can be reworded to fit the rules in the help center, please edit the question.

• I've attempted to derive multiple equations for each mass with the kinematic equations, rotational kinematic equations, and energy equations, however when I use them in a system of equations, the masses always cancel out. No one can say if you are doing anything wrong here if you don't show what you are trying to do and which physics concepts are tripping you up. Using N2L you should easily be able to at least determine the ratio $m/M$ – Aaron Stevens Oct 17 at 19:07
• @AaronStevens the free-body diagram for this system doesn't provide any useful information. The bottom mass's net force is zero because the only two forces acting on it are gravity and tension. The top mass's net force cannot be found because the horizontal component of its net force is tension which includes mass, which is unknown – Charles Averill Oct 17 at 19:14
• I am just trying to help. I think it is a great problem, but typically questions like these are closed as being off-topic for this specific site. Usually a question is better received here where work is shown and the focus is on a specific physics concept. – Aaron Stevens Oct 17 at 19:23
• I think all you can do here is determine $m/M$, but I will not post an answer saying this since I don't have a somewhat rigorous proof of it. And perhaps I am just not being creative enough about it. – Aaron Stevens Oct 17 at 20:08
• Hi and welcome to physics.SE!Please note that homework-like questions and check-my-work questions are generally considered off-topic here. We intend our questions to be potentially useful to a broader set of users than just the one asking, and prefer conceptual questions over those just asking for a specific computation. – ACuriousMind Oct 20 at 22:19