Conservation of energy with rotational motion problem

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Homework Help Overview

The discussion revolves around a problem related to the conservation of energy in the context of rotational motion. Participants are examining the role of rotational kinetic energy and its relevance to a scenario involving a person on a ride, specifically questioning the assumptions made regarding the center of gravity and the treatment of the person in relation to the bar.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the significance of rotational kinetic energy and question why it might be neglected in the solution. There are discussions about the implications of the center of gravity and whether assumptions about the person's position and motion are valid.

Discussion Status

The conversation is active, with participants providing insights and questioning the assumptions made in the problem. Some guidance is offered regarding the treatment of the person and the bar, but there is no explicit consensus on the interpretation of the rotational kinetic energy in this context.

Contextual Notes

Participants note that the problem states to 'neglect the size of the passenger,' which raises questions about the implications for the moment of inertia and the overall energy calculations. There is also mention of the person being seated in a way that may affect their rotational motion.

theBEAST
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Homework Statement


Alright so I am confused about why the solution does not include the kinetic energy due to rotational energy from I_g2 as indicated in red in the following image. I don't understand why they would neglect it... I calculated the value and it makes a big difference.

pUlvS.png
 
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The rotational kinetic energy measures energy of the body due to rotation around its center of gravity.

The person is not spinning around his center of gravity (located somewhere in his stomach); presumably he stays upright during the ride.
 
aralbrec said:
The rotational kinetic energy measures energy of the body due to rotation around its center of gravity.

The person is not spinning around his center of gravity (located somewhere in his stomach); presumably he stays upright during the ride.

But the person spins around the pivot? OH is it because the center of gravity for the bar INCLUDES the person on it?

Are we suppose to make an assumption?
 
Last edited:
theBEAST said:
But the person spins around the bar? OH is it because the center of gravity for the bar INCLUDES the person on it?

I just saw the question says 'neglect the size of the passenger' which means IG2 = 0. Notice that IG2 << IG1 anyway.

But just to continue... the person and bar are definitely treated separately here. If the person were rigidly attached to the bar so that he does not move with respect to the bar, you would be right to say the person has rotational kinetic energy 0.5*IG2*w^2. This is because the person would be like he is part of the bar and would posses the same w.

I assumed that this ride is like most rides where the person is seated in a carriage that is pinned to the end of the arm so that the person always stays upright. Then the person does not spin and has no rotational kinetic energy.
 

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