Why is sweeping in curling necessary? Isn’t the ice-sheet clean enough?
In: Physics
It might sound obvious to say, but curling shots curl. That is, you send the shot at a target, but it stops 4-6 feet to the left or right of the target. A curling playing sheet is also about 120ft from release to stopping point. You are looking to make the rock stop at a particular spot, 120ft down the sheet and ~5 feet from the target, and have a tolerance somewhere between 2 feet and 0 inches.
Sweeping changes how the rock interacts with the ice. Although the exact physical mechanism is unknown and somewhat disputed, the physical effect of sweeping is known. It makes the rock (1) slow down less quickly, and (2) depending on the angle of sweeping and the fabric of the brushes, curl (change direction) less, or (less frequently) more.
Sweeping controls the speed and the path of the rock as it travels. The sweepers can see most accurately how quickly the rock is traveling, and know where it *should* stop. So they want to sweep the right amount to get there. The skip (team captain; stationed on the other end) is watching the line (left/right travel). The skip and sweepers communicate how the rock needs to travel at different points along the shot, and might sweep more or less, or differently, to achieve the desired result.
Edit: I should add that the ice is *not necessarily* clean enough. Humans shed stuff all the time, and dust, hair, bits of rubber, or clothing fibers, settle on the ice. If a rock runs over one of those, it can “pick,” changing direction wildly and ruining the shot. If you hear a curler calling “clean!” they want the sweeper to brush the ice to remove any debris that could pick, but not so hard as to affect the rock strongly.
Edit2: Many people are saying that sweeping melts the ice. This is very probably not true. Melting ice requires a lot of energy and you’re not generating that much on a sweeping pass. You may be *heating* the ice a bit. You might also be making micro scratches on the surface that the running band of the rocks interact with. But you’re not melting it.
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