# Static vs Kinetic Friction in Rotating Systems

## Main Question or Discussion Point

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.

-Nick

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berkeman
Mentor
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.... ?

rcgldr
Homework Helper
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?

berkeman
Mentor
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?