(This post is in response to Wednesday’s Post, Railroads)
I spent a great part of my time that I should have spent preparing for this post, reading chapter 12 of Fundamentals of Physics by Halliday, Resnick, a book that every Indian studying for engineering entrance exams would recognize well. This time’s question involved friction, force, torque, horse-power and honestly, I didn’t do my physics in class 11 and 12 very well (something I regret till date). I still havent understood the question to the depth that I would have liked to, but I realized the answer couldn’t wait any longer. Apologies for the delay in putting up this post.
As usual, this time’s reading brought up some very interesting details. Turns out my friend wasn’t entirely wrong. There are trains that work on the following mechanism:
These are called rack railways. In India, the Nilgiri Mountain Railway, in the Indian state of Tamil Nadu is a rack railway. These tracks are usually laid out in mountains, for the precise reason that trains slip on sleep slopes in regular railways. There is also a railway system called funicular which is also used in mountains. The funicular is like a pulley, where a train descending pulls a train ascending.
Coming back to the question of our usual railways- some one did give a very crisp answer to the question on facebook, didn’t comment on the website but the answer was correct anyway. The conventional railways that are question is about are called adhesion railway. (“Adhesion is the tendency of dissimilar particles or surfaces to cling to one another”)
The adhesion is directly proportional to the coefficient of friction and the weight on the wheel. Now steel has a certain fixed coefficient of friction. So there is a certain limit to the force that can be applied on the wheels; if the force on the wheel exceeds the adhesion limit, the train doesn’t move. Instead what we get, is a wheelspin.
If the torque applied is too much, the turning force exceeds the friction force and the wheels of the train slip i.e- the wheels rotate, but there is no forward motion. This is what a wheelslip in cars looks like:
There are a couple of train wheelslip videos on youtube as well, but none suitable for demonstrative purposes.
If the engine power is too low, then there is a forward motion without any rotation. And this is what it looks like:
Delay in the running of trains, due to leaves on the line is rather common in Europe. In autumn, the fallen leaves compact into a hard slippery lignin coating. (Remember lignin, from the post on the science of paper, where it was used for smoothening the paper, and making it less absorbent). With this lignin coating, the friction is greatly reduced. Since friction is reduced, the adhesion is reduced, and a train will begin slipping at a lower engner force. Simply put, it won’t be able to move as fast.
There is a lot more physics behind when a train can topple which I am not going to get into. The question revealed that trains are a fixation for a lot of people. A friend shared this- a 25 year old club for Indian Railway fans. :-O