My friends who haven't watched Curling much often ask "what is the sweeping good for?" and the very short answer is "about 12 feet."
The player delivering the stone is supposed to release it with the correct turn and the proper weight.
The turn determines generally the direction the stone will curl (throw with "no handle" and the rock will curl, but the direction is random), and the weight determines how far the stone goes (unless it encounters another rock, which is also the subject of a later article).
Sweeping burnishes and slightly warms the ice. The stone slides more easily on the smoother ice (or the tiny bit of water on top of it) and that makes it slide farther.
The curved path (the curl) is caused by the difference in friction between one side of the rock (turning toward the direction of travel) and the other side (turning away) and reducing the friction makes the stone go a little straighter.
Trouble is, sweeping does both at the same time.
Also, sweeping doesn't make the rock go faster, it makes it slow down less. You can't reverse the curl if the rock is thrown poorly.
As an example, take the opening rock in an end.
The opening rock is thrown by the Lead. The Second and Vice-Skip sweep, while the Skip is stationed in the house, watching the stone as it comes down the ice.
The lead can see the rock well at the start, and the Second and Vice are over the rock. The lead might call for sweeping if the rock doesn't have enough weight, but the Skip is in the best position as the rock approaches the house.
The Skip may call "off" if the weight and line look good (or if the rock is too hard -- sweeping can only make that worse). He might say "sweep for line" meaning light sweeping just to straighten things up a little. Or, he might yell "Hard! Hurry!" for vigorous sweeping.
The opposing team may sweep your rock once it reaches the tee line, which runs across sheet through the middle of the house. Sweeping hard can take a rock that would have scored, and keep it moving right on out the back -- and then it does not score.
This is another reason why Curling is a team sport. Every member of a rink contributes to every shot.
No curling this weekend. It's the Summer Blockbuster Bonspeil in Hollywood.
Not related to curling: today (May 25th) is Towel Day. Douglas, enjoy your lunch, wherever you are.