(Original post by SillyEddy)
I'm really not sure about the question being asked... Can you quote the exact passage that you're referring to?
The wheels move backwards? I can't quite see why they would
Exerts a force on the track? Yep, because there is weight, there will be a force on the tracks
Track does exert an equal and opposite force - This is the reaction
I don't see why the reaction is responsible for causing motion in the roller coaster though. Friction opposes motion, it doesn't encourage motion.
Indeed, friction is not wanted. It will slow down on the tracks. That's why they use circular wheels instead of square ones - They can roll easily with minimal friction.
In mechanics, which you'll have to say if you've covered or not in maths, F=μR (Friction is equal to the coefficient of friction multiplied by the reaction). So a rougher surface creates more friction. When you're working out motions, you want to consider the sum of all the forces acting on a system. So for a roller coaster there is weight (mg), there is a reaction, there is force forwards (either by the chain lift, or because it's going downhill) and there's air resistance (usually negligible in physics experiments) and friction. Friction opposes motion, so less friction is better for the travel of the roller coaster.
I'm not entirely sure about traction though, I've never really covered it and can't say for certain what it actually is. I would've thought that it just be the normal reaction between the track and the wheels.