Calculus 3 : Cylindrical Coordinates

Study concepts, example questions & explanations for Calculus 3

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Example Questions

Example Question #1 : Cylindrical Coordinates

Convert the following into Cylindrical coordinates.

\(\displaystyle x=\sin(t)\)

\(\displaystyle y=\cos(t)\)

\(\displaystyle z=\tan(t)\)

Possible Answers:

\(\displaystyle r=1\)

\(\displaystyle \theta=\tan^{-1}(\tan(t))\)

\(\displaystyle z=\tan(t)\)

\(\displaystyle r=t\)

\(\displaystyle \theta=\tan(\cot(t))\)

\(\displaystyle z=\tan(t)\)

\(\displaystyle r=t\)

\(\displaystyle \theta=\tan^{-1}(\tan(t))\)

\(\displaystyle z=1\)

\(\displaystyle r=1\)

\(\displaystyle \theta=\tan^{-1}(\cot(t))\)

\(\displaystyle z=\tan(t)\)

\(\displaystyle r=1\)

\(\displaystyle \theta=\tan^{-1}(\cot(t))\)

\(\displaystyle z=t\)

Correct answer:

\(\displaystyle r=1\)

\(\displaystyle \theta=\tan^{-1}(\cot(t))\)

\(\displaystyle z=\tan(t)\)

Explanation:

In order to convert to cylindrical coordinates, we need to recall the conversion equations.

 

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle \theta=\tan^{-1}(\frac{y}{x})\)

\(\displaystyle z=z\)

 

Now lets apply this to our problem.

\(\displaystyle r=\sqrt{(\sin(t))^2+(\cos(t))^2}=\sqrt{1}=1\)

\(\displaystyle \theta=\tan^{-1}\Big(\frac{\cos(t)}{\sin(t)}\Big)=\tan^{-1}(\cot(t))\)

\(\displaystyle z=\tan(t)\)

 

Example Question #2 : Cylindrical Coordinates

When converting rectangular coordinates to cylindrical coordinates, which variable remains fixed? 

Possible Answers:

\(\displaystyle y\)

\(\displaystyle x\)

\(\displaystyle x,y\)

\(\displaystyle z\)

None of them are fixed.

Correct answer:

\(\displaystyle z\)

Explanation:

To convert a point \(\displaystyle (x,y,z)\) into cylindrical corrdinates, the transformation equations are

\(\displaystyle x = r\cos\theta\)

\(\displaystyle y = r\sin\theta\)

\(\displaystyle z = z\).

Choices for \(\displaystyle r, \theta\) may vary depending on the situation, but the \(\displaystyle z\) coordinate remains the same.

Example Question #1 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (5,12,7)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (13,67.38^{\circ},7)\)

\(\displaystyle (17,67.38^{\circ},7)\)

\(\displaystyle (17,112.62^{\circ},7)\)

\(\displaystyle (17,67.38^{\circ},13)\)

\(\displaystyle (13,112.62^{\circ},7)\)

Correct answer:

\(\displaystyle (13,67.38^{\circ},7)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (5,12,7)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow13\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow67.38^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=7\)

 

Example Question #2 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (4,-4,8)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (5.66,-45^{\circ},8)\)

\(\displaystyle (5.66,45^{\circ},8)\)

\(\displaystyle (5.66,135^{\circ},8)\)

\(\displaystyle (5.66,-135^{\circ},8)\)

Correct answer:

\(\displaystyle (5.66,-45^{\circ},8)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (4,-4,8)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow5.66\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow -45^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=8\)

Example Question #2 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (1,\sqrt{3},4)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (3,120^{\circ},4)\)

\(\displaystyle (4,-120^{\circ},2)\)

\(\displaystyle (2,120^{\circ},4)\)

\(\displaystyle (3,60^{\circ},4)\)

\(\displaystyle (2,60^{\circ},4)\)

Correct answer:

\(\displaystyle (2,60^{\circ},4)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (1,\sqrt{3},4)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow 2\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow 60^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=4\)

Example Question #91 : 3 Dimensional Space

A point in space is located, in Cartesian coordinates, at \(\displaystyle (-9,40,11)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (41,-77.32^{\circ},11)\)

\(\displaystyle (41,105.61^{\circ},11)\)

\(\displaystyle (41,102.68^{\circ},11)\)

\(\displaystyle (41,74.39^{\circ},11)\)

\(\displaystyle (41,77.32^{\circ},11)\)

Correct answer:

\(\displaystyle (41,102.68^{\circ},11)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (-9,40,11)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow41\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow 102.68^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=11\)

Example Question #2 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (-3,-4,10)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (5,53.13^{\circ},10)\)

\(\displaystyle (10,53.13^{\circ},5)\)

\(\displaystyle (5,-126.87^{\circ},10)\)

\(\displaystyle (5,126.87^{\circ},10)\)

\(\displaystyle (10,126.87^{\circ},5)\)

Correct answer:

\(\displaystyle (5,-126.87^{\circ},10)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (-3,-4,10)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow5\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow -126.87^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=10\)

Example Question #8 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (-4,8,10)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (8.94,41.62^{\circ},10)\)

\(\displaystyle (8.94,116.57^{\circ},10)\)

\(\displaystyle (8.94,-41.62^{\circ},10)\)

\(\displaystyle (8.94,-63.44^{\circ},10)\)

\(\displaystyle (-8.94,-63.44^{\circ},10)\)

Correct answer:

\(\displaystyle (8.94,116.57^{\circ},10)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (-4,8,10)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow8.94\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow 116.57^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=10\)

Example Question #9 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (6,8,10)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (10,108.81^{\circ},10)\)

\(\displaystyle (10,-71.19^{\circ},10)\)

\(\displaystyle (10,53.13^{\circ},20)\)

\(\displaystyle (10,-126.87^{\circ},10)\)

\(\displaystyle (10,53.13^{\circ},10)\)

Correct answer:

\(\displaystyle (10,53.13^{\circ},10)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (6,8,10)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow10\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow 53.13^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=10\)

Example Question #10 : Cylindrical Coordinates

A point in space is located, in Cartesian coordinates, at \(\displaystyle (-5,-5,-5)\). What is the position of this point in cylindrical coordinates?

Possible Answers:

\(\displaystyle (7.07,70^{\circ},-5)\)

\(\displaystyle (7.07,110^{\circ},-5)\)

\(\displaystyle (7.07,-135^{\circ},-5)\)

\(\displaystyle (7.07,-120^{\circ},-5)\)

\(\displaystyle (7.07,45^{\circ},-5)\)

Correct answer:

\(\displaystyle (7.07,-135^{\circ},-5)\)

Explanation:

When given Cartesian coordinates of the form \(\displaystyle (x,y,z)\) to cylindrical coordinates of the form \(\displaystyle (r,\theta,z)\), the first and third terms are the most straightforward.

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle z=z\)

Care should be taken, however, when calculating \(\displaystyle \theta\). The formula for it is as follows: 

\(\displaystyle \theta=arctan(\frac{y}{x})\)

However, it is important to be mindful of the signs of both \(\displaystyle y\) and \(\displaystyle x\), bearing in mind which quadrant the point lies; this will determine the value of \(\displaystyle \theta\):

Quadrants

It is something to bear in mind when making a calculation using a calculator; negative \(\displaystyle y\) values by convention create a negative \(\displaystyle \theta\), while negative \(\displaystyle x\) values lead to \(\displaystyle |\theta|>90^{\circ};\frac{\pi}{2}\)

For our coordinates \(\displaystyle (-5,-5,-5)\)

\(\displaystyle r=\sqrt{x^2+y^2}\rightarrow7.07\)

\(\displaystyle \theta=arctan(\frac{y}{x})\rightarrow -135^{\circ}\) (Bearing in mind sign convention)

\(\displaystyle z=-5\)

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