Static vs Kinetic Friction in Rotating Systems

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Discussion Overview

The discussion revolves around the concepts of static and kinetic friction in the context of rotating systems, specifically focusing on the behavior of railroad cars during the initial movement and the implications of rolling versus sliding friction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes a technique used by railroad operators to start a train by moving it backwards first, suggesting it helps in overcoming static friction one car at a time rather than all at once.
  • Another participant questions the necessity of this technique, wondering if kinetic friction is relevant when the wheels are rolling and not sliding on the rails.
  • A different participant introduces the concept of bearing friction in axle assemblies, indicating that this might also play a role in the discussion.
  • One participant expresses confusion about the difference between pushing back and pulling forward, considering the effects of coupling play in the train cars.
  • Another participant mentions that backing up increases play in the couplings and discusses the torque capabilities of diesel-electric locomotives at 0 rpm.
  • A participant seeks clarification on whether static friction is the only type at play when a wheel or ball rolls without dragging across a surface.
  • Another participant reiterates the question about static friction and introduces the topic of bearing friction in the context of wheels.

Areas of Agreement / Disagreement

Participants express differing views on the role of static versus kinetic friction in rolling objects, with some questioning the relevance of kinetic friction while others introduce additional factors like bearing friction and coupling play. The discussion remains unresolved regarding the exact nature of friction in these scenarios.

Contextual Notes

There are assumptions about the definitions of static and kinetic friction that may not be fully explored, and the relationship between rolling motion and friction types is not definitively established. The discussion also touches on mechanical aspects of train operation that may influence the frictional forces at play.

reiternick
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I don't know if you guys have heard that, when a railroad car is about to start moving, railroad operators sometimes have the train go backwards a little bit at first, that way when they move forward the force of the engine car only has to overcome the Static friction of one cart at a time, as opposed to starting from rest and having to overcome the static friction of all carts at the same time (which would be harder than using the trick mentioned above, since coefficient of static friction is always greater than the coefficient of kinetic friction for any 2 surfaces).

Yet, when I was thinking about this technique, I wondered why it would make any difference at all. Is kinetic friction coming into play at all? The wheels are not being dragged along the rails, they are rolling. With respect to the metal surface of the rail, the wheel is not moving. So why would this make a difference if the railroad operators do their nice trick as mentioned above?

In other words, when anything round is rolling, is kinetic friction at work or is it solely static friction since the rolling object isn't sliding.

Thanks for your help.

-Nick
 
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I think they are referring to bearing friction in the axle assemblies.

And I don't get why pushing back first is any different from pulling first. Oh, wait, if there is play in the couplings, then you want to start in the direction that uses the play to isolate each car start. Seems like if the train stopped under braking, though, the cars would all be pushed forward, so the couplings would click one at a time if you just started forward... ?
 
Backing up would increase the play in the couplings. Regarding braking, generally the brakes for the cars brake about the same, so the play in couplings after braking would be somewhat random. With a diesel eletric locomotive, the electric engines connected to the driven wheels can generate huge amounts of torque at 0 rpm (without overheating). Older technology locomotives spun the driven wheels in order to start up.
 
Am I at least right in saying that for any kind of wheel or ball, there is only static friction at play since nothing is being dragged across another surface?
 
reiternick said:
Am I at least right in saying that for any kind of wheel or ball, there is only static friction at play since nothing is being dragged across another surface?

What about bearing friction in the wheel case?
 

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