I'm a computer programmer that never studied physics in school and now it's coming back to bite me a bit in some of the stuff I'm being asked to program. I'm trying to self study some physics and I've got a few open source intro physics books and understanding it for the most part but I'm a bit confused on this statement I've stumbled upon in a section about static friction.
It's already gone over the formula for static friction and so forth. It gets into a section explaining that the weight of a train increases static friction between the wheels and the tracks. Alright, makes sense. But then it says this:
The reason locomotives are built to be so heavy is for traction. The upward normal force of the rails on the wheels, FN, cancels the downward force of gravity, FW, so ignoring plus and minus signs, these two forces are equal in absolute value, FN = FW. Given this amount of normal force, the maximum force of static friction is Fs = sFN = sFW. This static frictional force, of the rails pushing forward on the wheels, is the only force that can accelerate the train, pull it uphill, or cancel out the force of air resistance while cruising at constant speed. The coefficient of static friction for steel on steel is about 1/4, so no locomotive can pull with a force greater than about 1/4 of its own weight. If the engine is capable of supplying more than that amount of force, the result will be simply to break static friction and spin the wheels.
- "Newtonian Physics", Light and Matter - Book 1, p158 B. Crowell http://www.lightandmatter.com/bk1.pdf
I'm confused as to how static friction is the only thing that can move the train forward. I thought static friction was what kept it in place in the first place. There's another force - that I can't think of the name of, but I've heard of somewhere - that I thought was more what they're describing here, where the weight of the wheels pushing down and forward slightly on the tracks causes the tracks to push up and forward (from the opposite side).
Can someone explain to me what this is saying?