Static vs Kinetic Friction in Rotating Systems

AI Thread Summary
Static friction is crucial in the operation of railroad cars, particularly when starting movement, as it is typically greater than kinetic friction. Railroad operators sometimes reverse the train slightly to reduce the static friction that needs to be overcome when moving forward, allowing each car to start moving one at a time. The discussion raises the question of whether kinetic friction is involved when wheels roll on rails, as they do not slide. It is clarified that while rolling, static friction is primarily at play, but bearing friction in the axle assemblies also contributes to the overall resistance. Understanding these friction types is essential for optimizing the efficiency of train operations.
reiternick
Messages
2
Reaction score
0
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
 
Physics news on Phys.org
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?
 

Thread 'Is calling fictitious forces not real just about terminology?'

Hi there, im studying nanoscience at the university in Basel. Today I looked at the topic of intertial and non-inertial reference frames and the existence of fictitious forces. I understand that you call forces real in physics if they appear in interplay. Meaning that a force is real when there is the "actio" partner to the "reactio" partner. If this condition is not satisfied the force is not real. I also understand that if you specifically look at non-inertial reference frames you can...
View full post »

Thread 'Derivation of Hamilton's Principle: Questions'

I have recently been really interested in the derivation of Hamiltons Principle. On my research I found that with the term ##m \cdot \frac{d}{dt} (\frac{dr}{dt} \cdot \delta r) = 0## (1) one may derivate ##\delta \int (T - V) dt = 0## (2). The derivation itself I understood quiet good, but what I don't understand is where the equation (1) came from, because in my research it was just given and not derived from anywhere. Does anybody know where (1) comes from or why from it the...
View full post »

Similar threads

B Why is static friction necessary for pure rolling?
Replies
4
Views
3K
I What friction causes objects to decelerate?
Replies
21
Views
2K
B Static friction needed for rolling without slipping
Replies
4
Views
2K
B Friction and Newton's laws in car braking systems
Replies
10
Views
2K
Difference between static and kinetic friction
Replies
10
Views
3K
Rolling friction and static friction....
Replies
2
Views
2K
I Torque required from powered roller to rotate cylinder
Replies
19
Views
2K
I Understanding Net Internal Torque and Kinetic Friction in a System
Replies
9
Views
2K
I Direction of friction of each wheel and total moment when a car turns
Replies
6
Views
2K
What happens when a car turns?
Replies
16
Views
2K

Hot Threads

Recent Insights

Back
Top