A shallow segment of a sphere is a close approximation to a piece of a parabaloid. And a parabaloid doesn't focus only on the axis. If the Sun is off axis a little to one side then there will be a focal point a little off axis on the opposite side - in this case the wall of the house. Brent On 5/4/2013 1:35 PM, Henry Baker wrote:
The optimum shape would be a portion of a parabola with a focus on the area you want to burn.
But this wouldn't make for a very good dog dish, unless the sun were overhead, and you wanted to burn some protruding member.
Would it be more plausible if the sun first hit some other reflector, then the dog dish?
Or the sun goes through a half-silvered glass plate, hits the dog dish, hits the back of the glass plate, and then hits the house.
Perhaps some amateur mathematician on the Santa Rosa Fire Dept. wants to check out this story more carefully...
Now if the dog dish were _rotating_, then the surface of the water in the dish would form a parabola & possibly reflect the sunlight better.
To make this more efficient, put molten mercury in the rotating dog dish. But then the hazmat people would show up with their hazmat suits.
At 12:40 PM 5/4/2013, Robert Baillie wrote:
not that i want to try this, but what shapes would work?
bob baillie ---
Henry Baker wrote:
FYI -- So much for MythBusters... I wonder what Arthur C Clarke would say about this: http://zebu.uoregon.edu/~probs/therm/sunstrok/sunstrok.html http://sanfrancisco.cbslocal.com/2013/05/02/shiny-dog-bowl-blamed-in-santa-r... Shiny Dog Bowl Blamed In Santa Rosa Fire May 2, 2013 11:59 PM SANTA ROSA (CBS SF) A dog’s water bowl was being blamed for starting a fire at its owner’s Santa Rosa home, according to fire officials. North Bay firefighters called it is one of the most unusual fire calls they’ve ever had. Perry and Shay Weisbrich said they had to call 9-1-1 after their dog Toby’s metal bowl reflected a concentrated beam of sunlight onto the side of their house — a beam so powerful, the wood started to burn. “I just looked outside a minute and saw what i thought was steam.” said Shay Weisbrich. “A big plume of it through the glass door.. So I went over there thinking, ‘that’s not right, not normal.’ And it was smoke!” The Bennett Valley Fire Department responded before the fire got out of control. As for Toby, the Weisbrichs said he’s getting a new dish: a plastic bowl.
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