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gtfitzpatrick
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Homework Statement
Verify stokes theorem where F(xyz) = -yi+xj-2k and s is the cone [itex]z^2 = x^2 + y^2[/itex] , 0≤ Z ≤ 4 oriented downwards
Homework Equations
[itex]\oint_{c} F.dr = \int\int_{s} (curlF).dS [/itex]
The Attempt at a Solution
Firstly the image of the widest part of the cone on the xy plane is the circle ofradius 4
we parametize this circle using the parameters x=4cost , y=4sint , z=0 and also dx=-4sintdt , dy=4costdt
so we get
[itex] \oint_{c} F.dr = \int^{2\pi}_{0} -ydx + xdy -2dz [/itex]
[itex] \oint_{c} F.dr = \int^{2\pi}_{0} (-4sint)(-4sint) + (4cost)(4cost)dt [/itex]
[itex] = 16 \int^{2\pi}_{0} (sin^2 t) + (cos^2 t)dt [/itex]
[itex] = 16 \int^{2\pi}_{0} dt = 16(2\pi- 0) = 32\pi[/itex]
next curlF = 2k
so [itex]= \int\int_{s} (curlF).dS = \int\int_{D} (2).dA [/itex]
=[itex] 2\int^{4}_{0}\int^{2\pi}_{0} rdtdr = 2\int^{4}_{0} 2\pi r dr [/itex]
=[itex] 2(\pi r^2)^{4}_{0} = 2(16\pi) =32\pi [/itex]
ok so i got the same answer both ways which verifies Stoke but the thing I am not sure about is the question says the cone is oriented downward so should i have reversed the limits as in 0≥r≥-4 and because it is downward orientated should i have put a minus infront of each side? but then the would have canceled each other out anyways?