How Is the Accelerating Force Resolved at the Axle of a Wheel?

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Discussion Overview

The discussion revolves around the resolution of accelerating forces at the axle of a wheel, particularly in the context of a massless wheel with a concentrated mass at the axle. Participants explore the implications of torque, force application, and the concept of inertia in this scenario.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where a tangential force acts on a massless wheel with a concentrated mass at the axle, questioning how to resolve the accelerating force at the axle.
  • Another participant suggests that torque may not be useful due to the mass not being distributed and proposes thinking in terms of levers to determine the acceleration of the mass.
  • A subsequent reply reiterates the lever analogy, stating that the force at the axle is twice the applied force, leading to an acceleration of 2F/m, while noting the absence of a torsion reaction force at the contact point.
  • One participant introduces the concept of the point of contact being treated as an instantaneous lever in the ground's reference frame, discussing the relationship between the motion of the wheel's center and the top point's speed.
  • A philosophical note is made regarding the nature of inertia and its independence from perception, suggesting a broader context for the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the application of torque and the interpretation of inertia, indicating that multiple competing perspectives remain without a consensus on the resolution of the accelerating force at the axle.

Contextual Notes

Participants mention the limitations of the model, such as the assumption of a massless wheel and the treatment of forces and inertia, which may depend on specific definitions and conditions not fully explored in the discussion.

Eugbug
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In the diagram attached, a force acts tangentially on the edge of a wheel. To make things easier, the wheel is massless, so moment of inertia doesn't need to be taken into account. However there is a concentrated mass M at the axle(It could be taken as the mass of the axle). Two equal but opposite forces can be introduced at the point of contact with the surface without changing the situation. This then results in a couple whose magnitude is the torque T and also a force F at the surface. The reaction to the force F is the rolling friction force Rf which prevents the wheel from slipping.
How can an accelerating force be resolved at the axle? Do I split the torque T up again into forces? I'm going around in circles thinking about this and it's doing my head in!
 

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I can see why you'd have trouble - torque won't help you because the mass is not distributed.
To use this description, you want to think in terms of levers. If you apply a force twice as far from the fulcrum as the mass, what is the acceleration of the mass?
 
Simon Bridge said:
I can see why you'd have trouble - torque won't help you because the mass is not distributed.
To use this description, you want to think in terms of levers. If you apply a force twice as far from the fulcrum as the mass, what is the acceleration of the mass?

Well the force at the axle is 2F since the clockwise moment = the anticlockwise moment. So the acceleration is 2F/m. There is no torsion reaction force at the point of contact between the wheel and ground since the "lever" is free to move. So the reaction is simply the inertia of the mass. Inertia can be thought of as a reaction, can't it? (Here I go splitting hairs again!)
 
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In the reference frame of the ground - the point of contact is stationary for an instant. Thus it is a common model to treat it as an instantaneous lever with the fulcrum in the ground.

Same reference frame:
The top point moves twice the instantaneous speed of the center - it got to that speed in the same time.

Consider - the force is being provided by someone pulling on a (very thin) string that is wrapped around the rim of the wheel. If the unrolled length of string increases by length L, the center of the wheel has moved a distance ...

... you can actually conduct an experiment and check if you still doubt this approach.

Note:
Objects are usually considered to have inertia whether they are reacting to something or not.
I think it's a philosophical position to do with the independence of reality from perception... there's a different forum for that sort of discussion though.
 

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