# Determining $L_{o}$: Finding Angular Momentum of System

• vcsharp2003
In summary, In this conversation, the concept of angular momentum is discussed in relation to a system of two disks rotating about an axis passing through O and perpendicular to the plane of the disk. The reasoning is that since there is no torque acting on the system, the only necessary factors to determine the angular momentum are the moment of inertia of the smaller disk about the axis through O and the angular velocity of the system about O. The system is given an initial angular velocity about a vertical axis passing through O', and both disks are considered to be a part of the system. However, the setup of the system is unclear and not fully defined. The original question from the exam is also confusing and unclear, making it difficult to determine the exact parameters and variables
Delta2 said:
And if the bodies in interest are rigid, a linear motion of the CoM implies the same linear motion for every point of the rigid bodies.
You mean the same translation motion for each point of the rigid body.

Delta2
haruspex said:
It is always valid to represent a motion as a sum of motions, and it often helps to represent a rotation about a point other than the CoM as a rotation about the CoM plus a linear motion of the CoM.
I am assuming that the disks are in a horizontal position due to the mentioned vertical axis in problem statement. I am hoping that was also your assumption.

Delta2 said:
And if the bodies in interest are rigid, a linear motion of the CoM implies the same linear motion for every point of the rigid bodies.
So, when determining angular momentum about it's center axis, the translation motion part will not contribute to the angular momentum, only rotational part will contribute to angular momentum. Is that right?

vcsharp2003 said:
I am assuming that the disks are in a horizontal position due to the mentioned vertical axis in problem statement. I am hoping that was also your assumption.
If it is floating in space there is no vertical or horizontal. The reference to a vertical axis is strange anyway since it had not stated the disks were horizontal. I feel it was just a clumsy way of trying to say an axis normal to the plane of the disks.

vcsharp2003 and Delta2
haruspex said:
If it is floating in space there is no vertical or horizontal. The reference to a vertical axis is strange anyway since it had not stated the disks were horizontal. I feel it was just a clumsy way of trying to say an axis normal to the plane of the disks.
But, what you said in post#27 and 29 wouldn't change even if the disks were horizontally aligned?

haruspex said:
It is always valid to represent a motion as a sum of motions, and it often helps to represent a rotation about a point other than the CoM as a rotation about the CoM plus a linear motion of the CoM.
Based on the fact about translation motion plus rotational motion, I reached the following conclusion. The smaller disk is considered a point mass.

Steve4Physics said:
As previously noted (Post #12) the question is from this exam paper:

It wasn't too hard to find the official solutions. They are here:
Wow, you're a master in Google search. I spent many hours to search for original test paper and also it's solutions, but I couldn't get your results even after using your suggestion in an earlier post.
Very few people in today's world will have your skills for Google search.

Steve4Physics
haruspex said:
If it is floating in space there is no vertical or horizontal. The reference to a vertical axis is strange anyway since it had not stated the disks were horizontal. I feel it was just a clumsy way of trying to say an axis normal to the plane of the disks.
Please see my final working based on your inputs in post #41. After @SteveForPhysics posted the solution link, I cross checked my answer with problem#13 in that solution sheet and it exactly matches. Wow, what an effort. This was after all a good problem and not an impossible problem.

haruspex and Lnewqban
vcsharp2003 said:
Very few people will have your skills for Google search.
jee advanced 2019 paper 2 "03-02-2019" solutions

Not that clever, though I had to put the date in quotes as a refinement. The answer was the 4th match. Try it for yourself.

Lnewqban and vcsharp2003
Steve4Physics said:
jee advanced 2019 paper 2 "03-02-2019" solutions

Not that clever, though I had to put the date in quotes as a refinement. The answer was the 4th match. Try it for yourself.
For some reason, I'm not getting the 4th listing as the match. I'll look more into this. I'm not getting anything like an answer sheet, but I'll keep searching.

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