Is it possible to create linear acceleration using nothing but different moments of inertia beteen a wheel and axle?

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Discussion Overview

The discussion revolves around the possibility of generating linear acceleration using a system involving a character named Axel, who is attached to tractor wheels. Participants explore the mechanics of how different moments of inertia between Axel and the wheels might allow for movement, particularly focusing on the role of the center of mass and torque.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that if Axel's center of mass is perfectly aligned with the axis of rotation of the wheels, he may still achieve some movement if his moment of inertia is significantly greater than that of the wheels.
  • Another participant proposes that if Axel can flex his body to keep his center of mass forward of the rotation axis, he could roll, although he would flip end for end, likening the mechanism to that of a Segway.
  • There is a mention of the need for a motor to enable movement, as seen in the game where Axel is depicted as motorized.
  • A later reply draws a parallel to a tightrope walker, suggesting that similar principles of torque could apply to Axel's situation, allowing him to accelerate his center of gravity by exerting torque against the wheels.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of movement for Axel, particularly regarding the conditions under which movement is possible. There is no consensus on whether linear acceleration can be achieved solely through the described system without external forces.

Contextual Notes

Participants reference the need for a motor and the implications of the center of mass being on or off-axis, indicating that the discussion is contingent on these factors. The mechanics of torque and moment of inertia are central to the exploration but remain unresolved in terms of definitive outcomes.

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TL;DR
Is it possible to create linear acceleration using nothing but different moments of inertia beteen a wheel and axle?
The Twisted Metal video game series includes a character aptly named Axel, whose vehicle is just himself strapped to a pair of tractor wheels. This is the guy:

axel.jpg

Most of my friends assume that in real life, he probably wouldn't be able to drive anywhere unless his feet were incredibly heavy, so that gravity would apply torque as his lower half swung forward--similar mechanics to a monocycle.

But one of us claimed that even if Axel's center of mass lay perfectly on the axis of rotation of the big wheels , he would still be able to move a little bit if he were so heavy that the moment of inertia of himself was significantly greater than the moment of inertia for his wheels. Axel would still be spinning sickeningly, but his contraption could move about at least a little bit.

Is this correct? Can two very-different wheels applying torque to one another via an axle generate linear motion from friction, even in the absence of any external forces tangential to the axle? We were skeptical, but couldn't figure out how to set up the system to test the math.
 
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If he's rigidly attached to the wheels and he can flex his body to keep his center of mass forwards of the rotation axis then sure he can roll. He'd flip end for end as he does it, though, and I'm guessing the vehicle acts more like a Segway.

Obviously Segways are possible, but you need a motor.
 
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Welcome!
What are those things attached to the shoulders?
 
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Lnewqban said:
What are those things attached to the shoulders?
Machine guns or missiles, I forget which.

Ibix said:
If he's rigidly attached to the wheels and he can flex his body to keep his center of mass forwards of the rotation axis then sure he can roll. He'd flip end for end as he does it, though, and I'm guessing the vehicle acts more like a Segway.

Obviously Segways are possible, but you need a motor.
Yes, in game he's motorized like a Segway. In-game he's also perfectly upright at all times, which is not terribly realistic...

The crux of my question is, if motorized and if his CoM is always on-axis, is movement possible for Axel. If he has a CoM off-axis, then he clearly can (in any number of ways), and I know how to draw up the free-body diagram to analyze it.
 
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BishopBrain said:
The crux of my question is, if motorized and if his CoM is always on-axis, is movement possible for Axel.
Sure, but the wheel hubs (with him) will spin opposite to the wheels.
 
There is some similarity with a tight rope walker carrying a pole.

The walker can accelerate his center of gravity rightward or leftward by exerting torque against the pole while his feet exert a counter torque on the rope.