Rotating a Curve & Line Around the X Axis: A Math Problem

In summary, the problem involves rotating the curve x=y^(2) and the line x=4 about the x-axis. The formula for finding the volume is pi* integral from 0 to 4 of (square root of x)^(2) dx. While the teacher's answer of 8pi is correct, some may argue that it should be 16pi due to symmetry. The shell method can also be used, with the correct formula being 2pi* integral from 0 to 2 of y[4-y^(2)] dy. However, the limits would vary if only the area above the x-axis is being revolved.
  • #1
hvidales
29
0

Homework Statement



The curve x=y^(2) and the line x=4 is rotated about the x axis.

Homework Equations



pi* integral from a to b of Radius^(2)

The Attempt at a Solution



pi* integral from 0 to 4 of (square root of x)^(2) dx.

My teacher has this answer as 8pi but I think that that is wrong. Shouldn't it be 16 pi because you have to account for symmetry?
 
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  • #2
hvidales said:

Homework Statement



The curve x=y^(2) and the line x=4 is rotated about the x axis.

Homework Equations



pi* integral from a to b of Radius^(2)

The Attempt at a Solution



pi* integral from 0 to 4 of (square root of x)^(2) dx.

My teacher has this answer as 8pi but I think that that is wrong. Shouldn't it be 16 pi because you have to account for symmetry?

Your formula for the volume of the little disk is for the whole disk: ##\pi r^2 dx##. That's the area of the circle times the thickness. Your teacher is correct.
 
  • #3
I see! How would you do this if you are doing the shell method? I am stuck at this part:

2pi* integral from -2 to 2 of y[4-y^(2)] dy. I am stuck there lol.
 
  • #4
hvidales said:
I see! How would you do this if you are doing the shell method? I am stuck at this part:

2pi* integral from -2 to 2 of y[4-y^(2)] dy. I am stuck there lol.

You almost have it correct. But remember that the ##2\pi y## accounts for revolving the area. You only revolve the area above the ##x## axis. Your original statement of the problem is not well worded in that regard. So ##y## would only vary from 0 to 2. Otherwise the volume is given twice.
 
  • #5
I see and thanks. However, how come for this problem: y=x^(2), y=2-x^(2), about x=1 the limits are from -1 to 1.?
 
  • #6
Remember, the limits are always for the area that is being revolved. Think about what limits you would use if you were just calculating the area.
 
  • #7
Thank you!
 

1. What is the purpose of rotating a curve and line around the x-axis?

The purpose of rotating a curve and line around the x-axis is to create a three-dimensional representation of the two-dimensional curve and line. This allows for a better understanding of the shape and its relationship to other objects in space.

2. What are the steps involved in rotating a curve and line around the x-axis?

The steps involved in rotating a curve and line around the x-axis include finding the coordinates of the points on the curve and line, determining the angle of rotation, using trigonometric functions to calculate the new coordinates after rotation, and plotting the new points to create the rotated shape.

3. How does the angle of rotation affect the resulting shape?

The angle of rotation determines the amount of rotation that will occur. A larger angle will result in a more significant change in the shape, while a smaller angle will result in a smaller change. The direction of rotation (clockwise or counterclockwise) is also determined by the angle of rotation.

4. What is the difference between rotating a curve and rotating a line around the x-axis?

Rotating a curve around the x-axis involves rotating each individual point on the curve, while rotating a line involves rotating the entire line as a single object. Additionally, the resulting shape of a rotated curve will generally be curved, while a rotated line will remain straight.

5. Are there any real-world applications of rotating a curve and line around the x-axis?

Yes, there are many real-world applications of rotating a curve and line around the x-axis. For example, it is used in computer graphics to create three-dimensional images, in engineering to design complex structures, and in physics to study the motion of objects in space.

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