Came up with a problem that I can't solve

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Discussion Overview

The discussion revolves around a physics problem involving a hoop with mass M and radius R, on which a point mass m is placed. The problem explores the conditions under which the point mass m leaves the hoop as it rolls without slipping. Participants examine the application of high school physics principles, conservation laws, and various approaches to solve the problem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the problem and questions whether it can be solved using high school physics, noting the complexity of the equations involved.
  • Another participant requests the original poster to share their equations for better understanding.
  • A participant outlines three key principles used to derive their equations, including conservation of energy, escape conditions for the point mass, and relative motion considerations.
  • Another participant introduces the concept of degrees of freedom in the system and suggests that the Lagrangian approach could be useful, although they acknowledge the complexity of the problem.
  • One participant expresses that they solved the problem in a class setting, prompting requests for the solution details.
  • Another participant mentions finding a fourth equation related to momentum and angular impulse, contributing to the discussion of the problem's dynamics.
  • Some participants discuss the appropriateness of the problem for high school students, with differing opinions on its complexity and suitability for physics olympiad preparation.
  • Several participants share insights on the effects of mass and friction on the motion of the hoop and point mass, with examples comparing different materials and their behavior on slopes.
  • A participant describes their numerical approach to the problem, later admitting to an error in their assumptions, indicating the iterative nature of problem-solving in physics.
  • Another participant elaborates on the conditions under which the point mass will leave the hoop, detailing the forces and accelerations involved in the system.

Areas of Agreement / Disagreement

Participants express a range of views on the solvability of the problem using high school physics, with some believing it is too complex for that level, while others have successfully tackled it in educational settings. There is no consensus on the best approach or the appropriateness of the problem for high school students.

Contextual Notes

Participants note various assumptions and conditions that affect the problem, such as the role of static friction, the relationship between angular and linear motion, and the complexities introduced by different mass distributions. Some equations presented are dependent on specific definitions and may not be universally applicable.

Who May Find This Useful

This discussion may be of interest to students and educators in physics, particularly those exploring mechanics, as well as participants in physics competitions seeking to understand complex dynamics involving rolling objects.

  • #31
JorisL said:
The angle is equivalent to that position, see the picture in post 3.
Post #3 does not have a picture. Thus, angle to what? Horizontal radian at point of release? Or vertical radian? An angle of 2 what then? 2 degrees above or below horizontal or what?
 
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  • #32
Alright I made a mistake, post 4.
But have you read the entire thread?

As to finding a solution, perhaps going purely mathematical could work (understanding it).
First try to find the EOM for ##\phi## and ##\psi## (at first sight they look equivalent) using the Lagrangian formalism.
Next suppose you have an initial condition with angle ##\phi=\delta \phi## where ##\delta\phi## is infinitesimal.
Then one could check using an iterative method what the solution could be (and if it's feasible).
 
  • #33
JorisL said:
Alright I made a mistake, post 4.
But have you read the entire thread?

As to finding a solution, perhaps going purely mathematical could work (understanding it).
First try to find the EOM for ##\phi## and ##\psi## (at first sight they look equivalent) using the Lagrangian formalism.
Next suppose you have an initial condition with angle ##\phi=\delta \phi## where ##\delta\phi## is infinitesimal.
Then one could check using an iterative method what the solution could be (and if it's feasible).

A little confused because you are now communicating under a different name. Does not matter, Assume associate. My education is old and not a complete college degree. But, I do know and used extensive math on several different professions. Know nothing of Lagrangian and never learned the complete greek alphabet nor used it much. I use what I know from experience combined with education and experimentation.

Regardless of the comparative density and mass weights and comparative sizes of m and M, without "push off" force, I cannot believe the speed will increase enough for centrifugal force to enter the equation in a calculable amount. Thus my answer in an earlier post of point of leave being at gravitational level radiant. If m is too heavy, M will slip. If M is too heavy or large in diameter it will not continue from even a push off.

Any way you look at the problem with only presented stats, it is not solvable mathematically nor with just establishing an equation.
 
  • #34
I wasn't arguing against your explanation, it's how I would explain it as well.

The proposition was how I would approach the problem if I was trying to find an explicit solution :-)
I might follow through on the approach if I find the time (not before the end of June)

Odd that you see my name changing, I've had it since way back in 2011.
 
  • #35
JorisL said:
I wasn't arguing against your explanation, it's how I would explain it as well.

The proposition was how I would approach the problem if I was trying to find an explicit solution :-)
I might follow through on the approach if I find the time (not before the end of June)

Odd that you see my name changing, I've had it since way back in 2011.
I guess you replied to a reply of mine to another poster and I did not recognize that since the system does not show who the replies are to.
 
  • #36
golfrmyx said:
The friction would not be the same as the weight of gold increases friction in contact with sloping surface.
Friction is also proportional to mass. If we neglect air resistance, all forces and energies are proportional to mass, and acceleration is independent of it. A simple dimensional analysis shows the same.
 
  • #37
mfb, I see my comment was not clear for some reason. The gold friction on the sloping surface would be more than that of glass that is more than plastic. The increased friction I was referring to would be the air resistance that is too large to ignore. I do not mean increased. Not sure why I used that word. The marbles I was mentioning must be the same diameter so mass would be the same and thus air friction the same. Gravitational pull is the major force.
 

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