02-13-2013, 02:33 AM
The sled does not move easier with skis because of less surface area. Rather, the ski's are made with a material that has a low coefficient of friction. Lower than virtually all of the sleds us fisherman use to haul around our gear (cheap plastic). As has been mentioned, you can lower that coefficient of friction by waxing the ski's - further reducing the friction force. The surface area has nothing to do with the friction force involved.
The best way I can describe this coefficient of friction is this... sandpaper would have a very high coefficient of friction and an ice rink would have a very low coefficient of friction. The friction force is equal the coefficient of friction times the normal force (perpendicular to the ice in this case) aka weight force. So even with ski's and wax, if your sled is loaded with a lot of weight the friction force will be bigger. So going lighter helps too.
Hope it helps. If you doubt me, try this. Set up a wooden ramp that you can adjust the height of (angle). Then cut a rectangular piece of wood (shoe box shaped/ book shaped). Then see what angle it requires for the block to begin to slide down the ramp when laid on the ramp,with the most surface area possible (the "flattest" side --> most surface area). Once you have that angle measured, do the same test for the side which has the least surface area the "narrowest/thinnest/shortest side --> least surface area. The weight is the same (same block), the coefficient of friction is the same and the angle that the block slides regardless of surface area is the same. [:p] (and technically the grains in the wood can affect the coefficient of friction of the wood too, depending on which direction they are the block may slide at a slightly different angle).
And one last thing: If you make the skis that support the sled super thin (very small in surface area)... the sled is putting more stress on the ice because the weight is the same but the surface area is much smaller. I could use an ice analogy to describe it... when people fall through the ice and get everyone recommends they should crawl on the ice not stand up right where they fell through... because when you crawl your weight is distributed over more surface area and that results in less pressure on the ice. I don't know about you guys but I often load my sled pretty heavy early in the year and it makes me wonder but I do not use ski's so all the weight is spread over a wide area... maybe very heavy sleds with very thin skis could be dangerous on early ice?
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The best way I can describe this coefficient of friction is this... sandpaper would have a very high coefficient of friction and an ice rink would have a very low coefficient of friction. The friction force is equal the coefficient of friction times the normal force (perpendicular to the ice in this case) aka weight force. So even with ski's and wax, if your sled is loaded with a lot of weight the friction force will be bigger. So going lighter helps too.
Hope it helps. If you doubt me, try this. Set up a wooden ramp that you can adjust the height of (angle). Then cut a rectangular piece of wood (shoe box shaped/ book shaped). Then see what angle it requires for the block to begin to slide down the ramp when laid on the ramp,with the most surface area possible (the "flattest" side --> most surface area). Once you have that angle measured, do the same test for the side which has the least surface area the "narrowest/thinnest/shortest side --> least surface area. The weight is the same (same block), the coefficient of friction is the same and the angle that the block slides regardless of surface area is the same. [:p] (and technically the grains in the wood can affect the coefficient of friction of the wood too, depending on which direction they are the block may slide at a slightly different angle).
And one last thing: If you make the skis that support the sled super thin (very small in surface area)... the sled is putting more stress on the ice because the weight is the same but the surface area is much smaller. I could use an ice analogy to describe it... when people fall through the ice and get everyone recommends they should crawl on the ice not stand up right where they fell through... because when you crawl your weight is distributed over more surface area and that results in less pressure on the ice. I don't know about you guys but I often load my sled pretty heavy early in the year and it makes me wonder but I do not use ski's so all the weight is spread over a wide area... maybe very heavy sleds with very thin skis could be dangerous on early ice?
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