Paul's Online Math Notes
[Notes]
Notice

On August 21 I am planning to perform a major update to the site. I can't give a specific time in which the update will happen other than probably sometime between 6:30 a.m. and 8:00 a.m. (Central Time, USA). There is a very small chance that a prior commitment will interfere with this and if so the update will be rescheduled for a later date.

I have spent the better part of the last year or so rebuilding the site from the ground up and the result should (hopefully) lead to quicker load times for the pages and for a better experience on mobile platforms. For the most part the update should be seamless for you with a couple of potential exceptions. I have tried to set things up so that there should be next to no down time on the site. However, if you are the site right as the update happens there is a small possibility that you will get a "server not found" type of error for a few seconds before the new site starts being served. In addition, the first couple of pages will take some time to load as the site comes online. Page load time should decrease significantly once things get up and running however.

Paul
August 7, 2018

Calculus II - Notes
 Applications of Integrals Previous Chapter Next Chapter Series & Sequences Arc Length with Parametric Equations Previous Section Next Section Polar Coordinates

## Surface Area with Parametric Equations

In this final section of looking at calculus applications with parametric equations we will take a look at determining the surface area of a region obtained by rotating a parametric curve about the x or y-axis.

We will rotate the parametric curve given by,

about the x or y-axis.  We are going to assume that the curve is traced out exactly once as t increases from α to β.  At this point there actually isn’t all that much to do.  We know that the surface area can be found by using one of the following two formulas depending on the axis of rotation (recall the Surface Area section of the Applications of Integrals chapter).

All that we need is a formula for ds to use and from the previous section we have,

which is exactly what we need.

We will need to be careful with the x or y that is in the original surface area formula.  Back when we first looked at surface area we saw that sometimes we had to substitute for the variable in the integral and at other times we didn’t.  This was dependent upon the ds that we used.  In this case however, we will always have to substitute for the variable.  The ds that we use for parametric equations introduces a dt into the integral and that means that everything needs to be in terms of t.  Therefore, we will need to substitute the appropriate parametric equation for x or y depending on the axis of rotation.

Let’s take a quick look at an example.

 Example 1  Determine the surface area of the solid obtained by rotating the following parametric curve about the x-axis.                                           Solution We’ll first need the derivatives of the parametric equations.                                       Before plugging into the surface area formula let’s get the ds out of the way.                                              Notice that we could drop the absolute value bars since both sine and cosine are positive in this range of θ given.   Now let’s get the surface area and don’t forget to also plug in for the y.
 Arc Length with Parametric Equations Previous Section Next Section Polar Coordinates Applications of Integrals Previous Chapter Next Chapter Series & Sequences

[Notes]

 © 2003 - 2018 Paul Dawkins