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Dynamic Change
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NB. first time using Latex so apologies if something came out wrong, I've done my best to double check it.
Consider the curve [itex]y = \frac{1}{x}[/itex] from [itex]x=1[/itex] to [itex]x=\infty[/itex]. Rotate this curve around the x-axis to create a funnel-like surface of revolution. By slicing up the funnel into disks with [itex]r=\frac {1}{x}[/itex] and thickness [itex]dx[/itex] (and hence volume ([itex]\pi r^2 dx[/itex])) stacked side by side, the volume of the funnel is
[itex]V = \int_1^\infty \frac{\pi}{x^2} dx=- \frac{\pi}{x}\mid\int_1^\infty = \pi,[/itex]
which is finite. The surface area however involves the circumferential of the disks, which is [itex](2\pi r)dx[/itex] multiplied by a [itex] \sqrt{1+y'2}[/itex] factor accountng for the tilt of the area. The surface of the funnel is therefore
[itex]A = \int_1^\infty \frac{2\pi\sqrt{1+y'2}}{x} dx > \int_1^\infty \frac{2\pi}{x}dx[/itex]
which is infinite. As the volume is finite but the area is infinite, the funnel can be filled with paint but you can't paint it. Which appears to be a paradox since one should be painting the inside surface when filling up the funnel. But the inside surface=outside surface given the funnel has no thickness.So the question asks me to basically make sense of this paradox, I've done a lot of headscratching over it but thinking logically I simply can't find find our what's going on here, since , having checked over the equations theoretically the statements posed by the question are logical and I see no disconnect, so seems possible but impossible in reality given the paint should be at a constant (visible) thickness which I assume.
Consider the curve [itex]y = \frac{1}{x}[/itex] from [itex]x=1[/itex] to [itex]x=\infty[/itex]. Rotate this curve around the x-axis to create a funnel-like surface of revolution. By slicing up the funnel into disks with [itex]r=\frac {1}{x}[/itex] and thickness [itex]dx[/itex] (and hence volume ([itex]\pi r^2 dx[/itex])) stacked side by side, the volume of the funnel is
[itex]V = \int_1^\infty \frac{\pi}{x^2} dx=- \frac{\pi}{x}\mid\int_1^\infty = \pi,[/itex]
which is finite. The surface area however involves the circumferential of the disks, which is [itex](2\pi r)dx[/itex] multiplied by a [itex] \sqrt{1+y'2}[/itex] factor accountng for the tilt of the area. The surface of the funnel is therefore
[itex]A = \int_1^\infty \frac{2\pi\sqrt{1+y'2}}{x} dx > \int_1^\infty \frac{2\pi}{x}dx[/itex]
which is infinite. As the volume is finite but the area is infinite, the funnel can be filled with paint but you can't paint it. Which appears to be a paradox since one should be painting the inside surface when filling up the funnel. But the inside surface=outside surface given the funnel has no thickness.So the question asks me to basically make sense of this paradox, I've done a lot of headscratching over it but thinking logically I simply can't find find our what's going on here, since , having checked over the equations theoretically the statements posed by the question are logical and I see no disconnect, so seems possible but impossible in reality given the paint should be at a constant (visible) thickness which I assume.
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