Volume between cone and sphere of radius $sqrt2$ with surface integral












1












$begingroup$


Consider the cone $z^2=x^2+y^2$ between $z=0$ and $z=1$. Find the volume of the region above this cone and inside the sphere of radius $sqrt2$ centered at the origin that encloses the cone.




The straightforward approach to this problem would have been a triple integral in spherical coordinates, but for practice I tried using the cone as a surface and using a surface integral to find the volume between the cone and the sphere. I first parameterized the surface using cylindrical coordinates $x=rcostheta$, $y=rsintheta$, $z=r$ (since $z^2=x^2+y^2$), and then found the intersection points using the equations $z=r$ and $r^2+z^2=2$:
$$2z^2=2Rightarrow z=1Rightarrow r=1$$
Now that the bounds of integration have been found I used the function $z=sqrt{2-r^2}$ as the function to integrate and set up my surface integral as:
$$iint_S sqrt{2-r^2}Vertvec{r_r}timesvec{r_theta}Vert,text{d}S$$
I then computed the cross product, for $vec{r}(r,theta)=left<rcostheta,rsintheta,rright>$:
$$begin{vmatrix}
hat{imath}&hat{jmath}&hat{k}\
costheta&sintheta&1\
-rsintheta&rcostheta&0
end{vmatrix}=hat{imath}(-rcostheta)-hat{jmath}(-rsintheta)+hat{k}(rcos^2theta+rsin^2theta)$$
So the surface integral becomes
$$int_0^{2pi}int_0^1sqrt{left(2-r^2right)left(r^2cos^2theta+r^2sin^2theta+r^2right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{left(2-r^2right)r^2left(cos^2theta+sin^2theta+1right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{2r^2(2-r^2)},text drtext dtheta$$
$$=2sqrt2piint_0^1rsqrt{2-r^2},text dr$$
Using a $u$-substitution of $u=2-r^2$:
$$sqrt2piint_1^2
sqrt{u},text du=sqrt2pileft.left(frac{2usqrt u}{3}right)right|_1^2=sqrt2pileft(frac{4sqrt2-2}{3}right)$$
$$=frac{2pi}{3}left(4-sqrt2right)$$
However, the answer given in the answer key says that the volume is $frac{4pi}{3}(sqrt2-1)$, so I would like to know why my answer is incorrect, whether that be a computational mistake or faulty reasoning as to why a surface integral would work here to find the volume.



EDIT: Here is the posted solution:
Solution






calculus multivariable-calculus surface-integrals







share|cite|improve this question











$endgroup$












  • $begingroup$
    Surface integral to find a volume???
    $endgroup$
    – William Elliot
    Dec 6 '18 at 6:52










  • $begingroup$
    @WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
    $endgroup$
    – csch2
    Dec 6 '18 at 7:00










  • $begingroup$
    @WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
    $endgroup$
    – Arthur
    Dec 6 '18 at 7:05
















1












$begingroup$


Consider the cone $z^2=x^2+y^2$ between $z=0$ and $z=1$. Find the volume of the region above this cone and inside the sphere of radius $sqrt2$ centered at the origin that encloses the cone.




The straightforward approach to this problem would have been a triple integral in spherical coordinates, but for practice I tried using the cone as a surface and using a surface integral to find the volume between the cone and the sphere. I first parameterized the surface using cylindrical coordinates $x=rcostheta$, $y=rsintheta$, $z=r$ (since $z^2=x^2+y^2$), and then found the intersection points using the equations $z=r$ and $r^2+z^2=2$:
$$2z^2=2Rightarrow z=1Rightarrow r=1$$
Now that the bounds of integration have been found I used the function $z=sqrt{2-r^2}$ as the function to integrate and set up my surface integral as:
$$iint_S sqrt{2-r^2}Vertvec{r_r}timesvec{r_theta}Vert,text{d}S$$
I then computed the cross product, for $vec{r}(r,theta)=left<rcostheta,rsintheta,rright>$:
$$begin{vmatrix}
hat{imath}&hat{jmath}&hat{k}\
costheta&sintheta&1\
-rsintheta&rcostheta&0
end{vmatrix}=hat{imath}(-rcostheta)-hat{jmath}(-rsintheta)+hat{k}(rcos^2theta+rsin^2theta)$$
So the surface integral becomes
$$int_0^{2pi}int_0^1sqrt{left(2-r^2right)left(r^2cos^2theta+r^2sin^2theta+r^2right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{left(2-r^2right)r^2left(cos^2theta+sin^2theta+1right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{2r^2(2-r^2)},text drtext dtheta$$
$$=2sqrt2piint_0^1rsqrt{2-r^2},text dr$$
Using a $u$-substitution of $u=2-r^2$:
$$sqrt2piint_1^2
sqrt{u},text du=sqrt2pileft.left(frac{2usqrt u}{3}right)right|_1^2=sqrt2pileft(frac{4sqrt2-2}{3}right)$$
$$=frac{2pi}{3}left(4-sqrt2right)$$
However, the answer given in the answer key says that the volume is $frac{4pi}{3}(sqrt2-1)$, so I would like to know why my answer is incorrect, whether that be a computational mistake or faulty reasoning as to why a surface integral would work here to find the volume.



EDIT: Here is the posted solution:
Solution






calculus multivariable-calculus surface-integrals







share|cite|improve this question











$endgroup$












  • $begingroup$
    Surface integral to find a volume???
    $endgroup$
    – William Elliot
    Dec 6 '18 at 6:52










  • $begingroup$
    @WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
    $endgroup$
    – csch2
    Dec 6 '18 at 7:00










  • $begingroup$
    @WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
    $endgroup$
    – Arthur
    Dec 6 '18 at 7:05














1












1








1





$begingroup$


Consider the cone $z^2=x^2+y^2$ between $z=0$ and $z=1$. Find the volume of the region above this cone and inside the sphere of radius $sqrt2$ centered at the origin that encloses the cone.




The straightforward approach to this problem would have been a triple integral in spherical coordinates, but for practice I tried using the cone as a surface and using a surface integral to find the volume between the cone and the sphere. I first parameterized the surface using cylindrical coordinates $x=rcostheta$, $y=rsintheta$, $z=r$ (since $z^2=x^2+y^2$), and then found the intersection points using the equations $z=r$ and $r^2+z^2=2$:
$$2z^2=2Rightarrow z=1Rightarrow r=1$$
Now that the bounds of integration have been found I used the function $z=sqrt{2-r^2}$ as the function to integrate and set up my surface integral as:
$$iint_S sqrt{2-r^2}Vertvec{r_r}timesvec{r_theta}Vert,text{d}S$$
I then computed the cross product, for $vec{r}(r,theta)=left<rcostheta,rsintheta,rright>$:
$$begin{vmatrix}
hat{imath}&hat{jmath}&hat{k}\
costheta&sintheta&1\
-rsintheta&rcostheta&0
end{vmatrix}=hat{imath}(-rcostheta)-hat{jmath}(-rsintheta)+hat{k}(rcos^2theta+rsin^2theta)$$
So the surface integral becomes
$$int_0^{2pi}int_0^1sqrt{left(2-r^2right)left(r^2cos^2theta+r^2sin^2theta+r^2right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{left(2-r^2right)r^2left(cos^2theta+sin^2theta+1right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{2r^2(2-r^2)},text drtext dtheta$$
$$=2sqrt2piint_0^1rsqrt{2-r^2},text dr$$
Using a $u$-substitution of $u=2-r^2$:
$$sqrt2piint_1^2
sqrt{u},text du=sqrt2pileft.left(frac{2usqrt u}{3}right)right|_1^2=sqrt2pileft(frac{4sqrt2-2}{3}right)$$
$$=frac{2pi}{3}left(4-sqrt2right)$$
However, the answer given in the answer key says that the volume is $frac{4pi}{3}(sqrt2-1)$, so I would like to know why my answer is incorrect, whether that be a computational mistake or faulty reasoning as to why a surface integral would work here to find the volume.



EDIT: Here is the posted solution:
Solution






calculus multivariable-calculus surface-integrals







share|cite|improve this question











$endgroup$




Consider the cone $z^2=x^2+y^2$ between $z=0$ and $z=1$. Find the volume of the region above this cone and inside the sphere of radius $sqrt2$ centered at the origin that encloses the cone.




The straightforward approach to this problem would have been a triple integral in spherical coordinates, but for practice I tried using the cone as a surface and using a surface integral to find the volume between the cone and the sphere. I first parameterized the surface using cylindrical coordinates $x=rcostheta$, $y=rsintheta$, $z=r$ (since $z^2=x^2+y^2$), and then found the intersection points using the equations $z=r$ and $r^2+z^2=2$:
$$2z^2=2Rightarrow z=1Rightarrow r=1$$
Now that the bounds of integration have been found I used the function $z=sqrt{2-r^2}$ as the function to integrate and set up my surface integral as:
$$iint_S sqrt{2-r^2}Vertvec{r_r}timesvec{r_theta}Vert,text{d}S$$
I then computed the cross product, for $vec{r}(r,theta)=left<rcostheta,rsintheta,rright>$:
$$begin{vmatrix}
hat{imath}&hat{jmath}&hat{k}\
costheta&sintheta&1\
-rsintheta&rcostheta&0
end{vmatrix}=hat{imath}(-rcostheta)-hat{jmath}(-rsintheta)+hat{k}(rcos^2theta+rsin^2theta)$$
So the surface integral becomes
$$int_0^{2pi}int_0^1sqrt{left(2-r^2right)left(r^2cos^2theta+r^2sin^2theta+r^2right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{left(2-r^2right)r^2left(cos^2theta+sin^2theta+1right)},text dr,text dtheta$$
$$=int_0^{2pi}int_0^1sqrt{2r^2(2-r^2)},text drtext dtheta$$
$$=2sqrt2piint_0^1rsqrt{2-r^2},text dr$$
Using a $u$-substitution of $u=2-r^2$:
$$sqrt2piint_1^2
sqrt{u},text du=sqrt2pileft.left(frac{2usqrt u}{3}right)right|_1^2=sqrt2pileft(frac{4sqrt2-2}{3}right)$$
$$=frac{2pi}{3}left(4-sqrt2right)$$
However, the answer given in the answer key says that the volume is $frac{4pi}{3}(sqrt2-1)$, so I would like to know why my answer is incorrect, whether that be a computational mistake or faulty reasoning as to why a surface integral would work here to find the volume.



EDIT: Here is the posted solution:
Solution






calculus multivariable-calculus surface-integrals




calculus multivariable-calculus surface-integrals






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 6 '18 at 20:20







csch2

















asked Dec 6 '18 at 6:21









csch2csch2

237311




237311












  • $begingroup$
    Surface integral to find a volume???
    $endgroup$
    – William Elliot
    Dec 6 '18 at 6:52










  • $begingroup$
    @WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
    $endgroup$
    – csch2
    Dec 6 '18 at 7:00










  • $begingroup$
    @WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
    $endgroup$
    – Arthur
    Dec 6 '18 at 7:05


















  • $begingroup$
    Surface integral to find a volume???
    $endgroup$
    – William Elliot
    Dec 6 '18 at 6:52










  • $begingroup$
    @WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
    $endgroup$
    – csch2
    Dec 6 '18 at 7:00










  • $begingroup$
    @WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
    $endgroup$
    – Arthur
    Dec 6 '18 at 7:05
















$begingroup$
Surface integral to find a volume???
$endgroup$
– William Elliot
Dec 6 '18 at 6:52




$begingroup$
Surface integral to find a volume???
$endgroup$
– William Elliot
Dec 6 '18 at 6:52












$begingroup$
@WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
$endgroup$
– csch2
Dec 6 '18 at 7:00




$begingroup$
@WilliamElliot it was obviously not the most practical way to evaluate it but if my understanding is correct it should work, since the volume is contained between the cone (used as the surface) and the sphere (above the surface)
$endgroup$
– csch2
Dec 6 '18 at 7:00












$begingroup$
@WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
$endgroup$
– Arthur
Dec 6 '18 at 7:05




$begingroup$
@WilliamElliot We use line integrals to find areas all the time. It's how most of us learn about integrals in the first place. And if you use triple integrals to find a volume, then once you've done the innermost integral, what you're left with is a surface integral to find a volume. So it's not strange at all.
$endgroup$
– Arthur
Dec 6 '18 at 7:05










1 Answer
1






active

oldest

votes


















0












$begingroup$

We can also calculate by cap area and its solid angle.



Denoting radius by $ R={sqrt 2}$



Cap Area $A= 2 pi R. R (1-1/sqrt 2 ) $



Solid Angle of Cap = $dfrac{A}{ R^2} = 2 pi (1-1/sqrt 2 )$



Remaining Solid Angle $V_1 = 4 pi -dfrac{A}{ R^2} = 2 pi (1+1/sqrt 2 )$



Remaining Volume $V= dfrac{R^3}{3}. V_1 = dfrac{4 pi }{3}(sqrt 2 +1)$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
    $endgroup$
    – csch2
    Dec 6 '18 at 7:02










  • $begingroup$
    My answer still does not tally with the key !
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:07










  • $begingroup$
    I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
    $endgroup$
    – csch2
    Dec 6 '18 at 7:08










  • $begingroup$
    Maybe. Btw, you should also have indicated your preferred method at the start.
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:11










  • $begingroup$
    I have edited the title to include the preferred method
    $endgroup$
    – csch2
    Dec 6 '18 at 7:12











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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









0












$begingroup$

We can also calculate by cap area and its solid angle.



Denoting radius by $ R={sqrt 2}$



Cap Area $A= 2 pi R. R (1-1/sqrt 2 ) $



Solid Angle of Cap = $dfrac{A}{ R^2} = 2 pi (1-1/sqrt 2 )$



Remaining Solid Angle $V_1 = 4 pi -dfrac{A}{ R^2} = 2 pi (1+1/sqrt 2 )$



Remaining Volume $V= dfrac{R^3}{3}. V_1 = dfrac{4 pi }{3}(sqrt 2 +1)$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
    $endgroup$
    – csch2
    Dec 6 '18 at 7:02










  • $begingroup$
    My answer still does not tally with the key !
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:07










  • $begingroup$
    I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
    $endgroup$
    – csch2
    Dec 6 '18 at 7:08










  • $begingroup$
    Maybe. Btw, you should also have indicated your preferred method at the start.
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:11










  • $begingroup$
    I have edited the title to include the preferred method
    $endgroup$
    – csch2
    Dec 6 '18 at 7:12
















0












$begingroup$

We can also calculate by cap area and its solid angle.



Denoting radius by $ R={sqrt 2}$



Cap Area $A= 2 pi R. R (1-1/sqrt 2 ) $



Solid Angle of Cap = $dfrac{A}{ R^2} = 2 pi (1-1/sqrt 2 )$



Remaining Solid Angle $V_1 = 4 pi -dfrac{A}{ R^2} = 2 pi (1+1/sqrt 2 )$



Remaining Volume $V= dfrac{R^3}{3}. V_1 = dfrac{4 pi }{3}(sqrt 2 +1)$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
    $endgroup$
    – csch2
    Dec 6 '18 at 7:02










  • $begingroup$
    My answer still does not tally with the key !
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:07










  • $begingroup$
    I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
    $endgroup$
    – csch2
    Dec 6 '18 at 7:08










  • $begingroup$
    Maybe. Btw, you should also have indicated your preferred method at the start.
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:11










  • $begingroup$
    I have edited the title to include the preferred method
    $endgroup$
    – csch2
    Dec 6 '18 at 7:12














0












0








0





$begingroup$

We can also calculate by cap area and its solid angle.



Denoting radius by $ R={sqrt 2}$



Cap Area $A= 2 pi R. R (1-1/sqrt 2 ) $



Solid Angle of Cap = $dfrac{A}{ R^2} = 2 pi (1-1/sqrt 2 )$



Remaining Solid Angle $V_1 = 4 pi -dfrac{A}{ R^2} = 2 pi (1+1/sqrt 2 )$



Remaining Volume $V= dfrac{R^3}{3}. V_1 = dfrac{4 pi }{3}(sqrt 2 +1)$






share|cite|improve this answer









$endgroup$



We can also calculate by cap area and its solid angle.



Denoting radius by $ R={sqrt 2}$



Cap Area $A= 2 pi R. R (1-1/sqrt 2 ) $



Solid Angle of Cap = $dfrac{A}{ R^2} = 2 pi (1-1/sqrt 2 )$



Remaining Solid Angle $V_1 = 4 pi -dfrac{A}{ R^2} = 2 pi (1+1/sqrt 2 )$



Remaining Volume $V= dfrac{R^3}{3}. V_1 = dfrac{4 pi }{3}(sqrt 2 +1)$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 6 '18 at 7:00









NarasimhamNarasimham

20.7k52158




20.7k52158












  • $begingroup$
    Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
    $endgroup$
    – csch2
    Dec 6 '18 at 7:02










  • $begingroup$
    My answer still does not tally with the key !
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:07










  • $begingroup$
    I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
    $endgroup$
    – csch2
    Dec 6 '18 at 7:08










  • $begingroup$
    Maybe. Btw, you should also have indicated your preferred method at the start.
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:11










  • $begingroup$
    I have edited the title to include the preferred method
    $endgroup$
    – csch2
    Dec 6 '18 at 7:12


















  • $begingroup$
    Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
    $endgroup$
    – csch2
    Dec 6 '18 at 7:02










  • $begingroup$
    My answer still does not tally with the key !
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:07










  • $begingroup$
    I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
    $endgroup$
    – csch2
    Dec 6 '18 at 7:08










  • $begingroup$
    Maybe. Btw, you should also have indicated your preferred method at the start.
    $endgroup$
    – Narasimham
    Dec 6 '18 at 7:11










  • $begingroup$
    I have edited the title to include the preferred method
    $endgroup$
    – csch2
    Dec 6 '18 at 7:12
















$begingroup$
Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
$endgroup$
– csch2
Dec 6 '18 at 7:02




$begingroup$
Thank you for your answer. I thought of using this method as well, and it confirms that the answer given in the answer key is correct. However I am not so much looking for the correct answer as I am why the method I used is incorrect
$endgroup$
– csch2
Dec 6 '18 at 7:02












$begingroup$
My answer still does not tally with the key !
$endgroup$
– Narasimham
Dec 6 '18 at 7:07




$begingroup$
My answer still does not tally with the key !
$endgroup$
– Narasimham
Dec 6 '18 at 7:07












$begingroup$
I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
$endgroup$
– csch2
Dec 6 '18 at 7:08




$begingroup$
I did not notice the different sign! That is strange; maybe I should post the given solution for comparison
$endgroup$
– csch2
Dec 6 '18 at 7:08












$begingroup$
Maybe. Btw, you should also have indicated your preferred method at the start.
$endgroup$
– Narasimham
Dec 6 '18 at 7:11




$begingroup$
Maybe. Btw, you should also have indicated your preferred method at the start.
$endgroup$
– Narasimham
Dec 6 '18 at 7:11












$begingroup$
I have edited the title to include the preferred method
$endgroup$
– csch2
Dec 6 '18 at 7:12




$begingroup$
I have edited the title to include the preferred method
$endgroup$
– csch2
Dec 6 '18 at 7:12


















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