What is the science behind sweeping a curling stone? Why does it effect the rocks travel? I know it will go farther and straighter when swept, but not sure why.
I presume because the sweeping smooths the ice by creating friction, thereby melting the top layer of ice crystals which then refreeze a lot smoother. That’s just a theory of mine…I have no idea whether it’s true or not, but it sounds good. 
Your questions about sweeping answered.
As a slight aside, sweeping the rock “illegally,” that is, sweeping in a manner where your broom only sweeps a particular side of the rock, can affect the curl of the rock in somewhat. For example, a rock with a “dead” handle (no rotation) can be given one by cheating a little to one side or the other. However, in spite of urban legends I’ve heard over the years of sweepers being able to curl a rock in a snake-like pattern down the ice by cheating, I’ve never seen a pair of sweepers good enough or strong enough to make that kind of difference (and I’ve played against a couple former world champs).
P.S. Stilla: Welcome aboard. May your stay at the SDMB be a pleasant one.
wow, I was right!
Many janitors are fans of curling. It’s the only sport I know of where a guy with a broom is a hero.
I’m always struck by how slowly they spin the rocks. It would seem that getting more curl would be useful some times. Does spinng the rocks faster give less control, or does it just not give the rocks more curl?
Spinning the rock faster reduces accuracy and curl. If I have my physics right, the faster speed reduces the difference in relative friction betweem the inside and outside of the running surface, so the rock curls less (observation confirms the more spin, less curl thing). Of course, too little spin and the rock might stop spinning altogether in mid-ice. I’ve been taught over and over that the optimal spin is one-and-a-half rotations for the length of the sheet – enough to ensure the rock keeps spinning, but not too much to sacrifice curl.