Does sweeping before a stone in curling increase the speed?

Answer 1

Maybe about 8 feet. It depends on how good at sweeping your sweepers are (how much pressure they can put on the ice, how well they can keep up fast strokes all the way down the ice, etc), and also the ice conditions.

Answer 2

Sweeping doesn't actually increase the rock's speed. Thinking in terms of physics, a rock traveling down the ice after released can only have its speed decreased from friction; it can't increase its speed unless it gets pushed. Sweeping does, however, decrease the friction between the rock and the ice, which makes the rock's speed decrease not as fast as normal, so the rock has more time to travel, and thus travels farther when swept.