Force = mass * acceleration. A question about gears.

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Discussion Overview

The discussion revolves around the mechanics of bicycle gearing, specifically how different gear ratios affect acceleration despite a constant force applied to the pedals. Participants explore the relationship between force, torque, and acceleration in the context of cycling.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that the force applied to the pedals remains constant regardless of the gear used, questioning why acceleration differs when starting in low versus high gear.
  • Another participant challenges the idea of constant force, noting that lower gears feel harder to pedal, suggesting a difference in perceived effort.
  • A third participant explains that while the force on the pedals may be constant, the torque applied to the wheel changes with the gear ratio, impacting acceleration.
  • It is noted that the rotational inertia of the driven wheel also varies with the gear ratio, which contributes to the ability to accelerate faster in lower gears.
  • A later reply emphasizes that the relevant forces for acceleration include external forces acting on the bike, which are affected by the gear ratio.

Areas of Agreement / Disagreement

Participants express differing views on the constancy of force across gear ratios, with some agreeing that torque and external forces play a significant role in acceleration, while others maintain that the force applied to the pedals does not change.

Contextual Notes

There is a lack of consensus on the definitions of force and how it relates to acceleration in this context, as well as varying interpretations of the mechanics involved in gear ratios and their effects on performance.

pletharoe
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This question is about push bikes, but could be applied to anything with a versatile engine and a gearing mechanism.

My bike is propelled by pushing on the pedals. Whether I use a high or low gear, the force remains the same. If the force and mass remain constant, how come I can accelerate faster by starting in a low gear and working up in stead of just starting in in the high gear?
 
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When you pedal, you feel a constant force between a low gear and a high gear? I feel that the lower gears are harder to pedal...
 
The force on your pedals may remain the same for any gear, but the torque applied to your wheel varies with the overall gear ratio.
Also, the rotational inertia of the driven wheel, as seen at the driving gear, varies with the gear ratio squared.
All this transpires to make it easier to accelerate faster in low gear.

Cheers,
Terry
 
pletharoe said:
Whether I use a high or low gear, the force remains the same.
Which force? What matters for the acceleration of you+bike are the external forces on you+bike, like the force between road and wheel. In lower gears the same internal force on the pedal creates a greater external force between road and wheel, and therefore more acceleration.
 

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