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Washer method rotating around vertical line (not y-axis), part 1

AP.CALC:
CHA‑5 (EU)
,
CHA‑5.C (LO)
,
CHA‑5.C.4 (EK)
Setting up the definite integral for the volume of a solid of revolution around a vertical line using the "washer" or "ring" method. Created by Sal Khan.

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  • blobby green style avatar for user Krista Noland Gollihar
    How does one determine whether it's best to integrate with respect to x vs. with respect to y? It isn't always obvious, at least not to me...
    (8 votes)
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  • blobby green style avatar for user Arne Magnus Lorentzen Ulland
    At , the outer radius is defines as 2-y^2.
    Can anybody explain why ones takes 2 - the function. My first guess was the function + 2 so that i would reach the line where x =2. This is wrong, but i can´t say i really understand why.

    And, taking 2-y^2 would that be the gap between the undefined point on the x-axes and x=2? Where is this on our shape?
    Thanks.
    (12 votes)
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    • blobby green style avatar for user Creeksider
      At this stage in the process we're trying to find the radius of the outer disk. The radius is the distance from the center of the disk to the perimeter. At the beginning of the video we're told that the figure is created by rotation around the line x=2, so we know the center of the disk is at x=2. The outer edge of the larger disk is at the point x=y^2, so the distance between the two points is 2-y^2.
      (9 votes)
  • mr pink red style avatar for user Enya Hsiao
    what if the rotating axis is not parallel to either the x axis or y axis? For example, x+y=2 or something.
    (11 votes)
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  • leaf green style avatar for user Mark Bacerra
    how did you come up with the values of the limits of integration right towards the end of the video?
    (6 votes)
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  • blobby green style avatar for user osama ahmed
    at how do you get your limits of integration? I understand that you did by inspection, but how would you do it by making the two equations equal to each other ?
    (3 votes)
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    • male robot hal style avatar for user kekecl0ndike
      In that example, Sal is integrating with respect to y. To solve for zero and one analytically, we could make the equations equivalent to each other and solve. Here's the example:
      sqrt(x) = x^2
      x = x^4

      This last equation is true only when x = 0 or x = 1.
      (4 votes)
  • purple pi purple style avatar for user Moses
    What would the solid look like of the sine function around the y-axis? A series of co-joined balls with a radius of pi/2 perhaps?
    (3 votes)
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    • duskpin ultimate style avatar for user Diego Oliveira
      Be careful: what you were visualizing is a rotation about the x-axis (not y-axis).

      In order to find out how it would look like about the y-axis, you'd have to specify how to construct the area whose rotation will produce the solid. That's because "sin(x)" by itself doesn't produce any area, it's just a continuous line. If, for example, you were to specify that it's "the area between the curve sin(x) and the x-axis", then the rotation about the y-axis would yield a "ripples on a pond" sort of figure.
      (1 vote)
  • blobby green style avatar for user Lindsey.Brodeck
    I just want to say thank you for doing this. It is helping me a ton, but I still have one question. How do you know when you don't have to integrate the outer volume but have to integrate the inner volume (and then subtract it) or vice versa? I hope that makes sense...it just seems that sometimes there's only one volume you need to integrate while the other volume is staying constant. I'm having a hard time figuring out when that is. Thank you so much!
    (2 votes)
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    • leaf grey style avatar for user dashpointdash
      I recommend to watch the exercises and think them through on your own.

      As a quick guide,
      1. Look at the rotational axis, is it parallel to the x or y-axis.
      2.Check the offset ( distance of your axis of rotation)
      3.Determine the boundaries.
      Integrate and calculate the result.
      (Practice makes perfect )
      (3 votes)
  • aqualine ultimate style avatar for user Kimilya
    So if we are rotating around the y-axis or x= a number then we make the bounds in terms of y?
    (1 vote)
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    • blobby green style avatar for user Creeksider
      Nearly all of these problems can be attacked at least two different ways. When rotating around the y-axis or other vertical line we may solve by the shell method, in which case we integrate with respect to x, or by the disk or washer method, in which case we integrate with respect to y. The reverse would be true if rotating around the x-axis or other horizontal line. Rather than try to memorize these relationships, learn to think in terms of which direction will have a small increment of thickness for the method you've chosen. If it's a horizontal thickness, it's dx, so you integrate with respect to x; if vertical, you're dealing with y.
      (3 votes)
  • blobby green style avatar for user yeboahlordina1
    Find the volume (in units3) generated when the region between the curves is rotated around the given axis.
    y=1/6−x, x=1, and x=2 rotated around the line x=6.
    (2 votes)
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  • male robot hal style avatar for user David
    If the function does not have an inverse and were asked to find the volume with respect to y, then there exists no solution, right?
    (2 votes)
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Video transcript

What we're going to do in this video is take the region between the two curves, y is equal to square root of x on top and y is equal to x squared on the bottom and rotate it around a vertical line that is not the y-axis. So we're going to rotate it around the vertical line x is equal to 2. We're going to rotate it right around like that. And if we rotate it like that, we will get this shape, so this strange-looking shape. It's hollowed out in the middle. You can kind of use the y equals x squared part kind of hollows out the middle. And it's really the stuff in between that forms the wall of this rotated shape. So let's think about how we can figure out the volume. And we're going to do it using the disk method, sometimes called the ring method-- actually, it's going to be more of the washer method-- to do this one right over here. So we're rotating around a vertical line. We want to use our disk or ring or washer method. And so it'll be really helpful to have a bunch of rings stacked up in the y direction. So we're probably going to want to integrate with respect to y. Let me make it clear what I'm talking about. Each of our rings are going to look something like this. So let me do my best attempt to draw. So that's the inner radius of the ring defined by y is equal to x squared. And then the outside radius of the ring might look something like this. My best attempt to draw it reasonably. The outside radius of the ring might look something like that. And the depth of the ring would be a little dy just like that. So let me draw the depth. So let me see, right over here. You could kind of view it as the height of the ring, because we are thinking in a vertical direction. And that would be a ring for a given y for, say, this y right over here. It's essentially the ring generated if you were to take this rectangle of height dy and rotate it around the line x equals 2. And we're going to construct a ring like that for each of the y's in our interval. So you could imagine stacking up a whole set of rings just like this and then taking the sum of the volumes of all of those rings and the limit of that side as you have an infinite number of rings with-- or close to infinite-- or really, infinite number of rings with infinitesmally small height or depth or dy. So let's figure out how we would do this. Well, we know that all we have to do is figure out what the volume of one of these rings are for given y as a function of y and then integrate along all of them, sum them all up. So let's figure out what the volume of one of these rings are. And to do it, we're going to express these functions as functions of y. So our purple function, y is equal to square root of x, if we square both sides, we would get y squared is equal to x. And let me swap the sides so it makes it clear that now x is a function of y. x is equal to y squared. That's our top function the way we've drawn a kind of outer shell for our figure. And then y is equal to x squared. If you take the principal root of both sides of that and it all works out because we're operating in the first quadrant here. That's the part of it that we care about. So you're going to get x is equal to the square root of y. That is our yellow function right over there. Now, how do we figure out the area of the surface of one of these rings or one of these washers? Well, the area-- let me do this in orange because I drew that ring in orange. So the area of the surface right over here in orange as a function of y is going to be equal to the area of the circle if I just consider the outer radius, and then I subtract out the area of the circle constructed by the inner radius, just kind of subtract it out. So the outer-circle radius, so it's going to be pi times outer radius squared minus pi-- let me write it this way-- outer-- well, I'll just write it-- outer radius-- this is going to be as a function of y-- outer radius squared minus pi times inner radius squared. And we want this all to be a function of y. So the outer radius as a function of y is going to be what? Well, it might be easier to visualize-- actually, I'll try it both places. So it's this entire distance right over here, essentially, the distance between the vertical line, the horizontal distance between our vertical line, and our outer function. So if you think about it in terms of x, It's going to be 2 minus whatever the x value is right over here. So the x value right over here is going to be y squared. Remember, we want this is a function of y. So our outer radius, this whole distance, is going to be 2 minus the x value here as a function of y. That x value is y squared. So the outer radius is 2 minus x-- sorry. 2 minus y squared. We want it as a function of y. And then our inner radius is going to be equal to what? Well, that's going to be the difference, the horizontal distance, between this vertical line and our inner function, our inner boundary. So it's going to be the horizontal distance between two and whatever x value this is. But this x value as a function of y is just square root of y. So it's going to be 2 minus square root of y. And so now, we can come up with an expression for area. It's going to be-- and I'll just factor out-- actually, I'll leave the pi there. So it's going to be pi-- right over here-- it's going to be pi times outer radius squared. Well, the outer radius is 2 minus y squared-- and let me just-- well, I'll just write it. 2 minus y squared-- and we're going to square that-- squared, minus pi times the inner radius squared. Well, we already figured that out. The inner radius is 2 minus square root of y. And we're going to square that one, too. So this gives us the area of one of our rings as a function of y, the top of the ring, where I shaded in orange. And now, if we want the volume of one of those rings, we have to multiply it by its depth or its height the way we've drawn it right over here. And its height-- we've done this multiple times already right over here-- is an infinitesimal change in y. So we're going to multiply all that business times dy. This is the volume of one of our rings. And then we want to sum up all of the rings over our interval. So we're going to sum up all of the rings over the interval. And when you take the integral sign, it's a sum where you're taking the limit as you have an infinite number of rings that become infinitesimally small in height or depth, depending on how you view it. And what's our interval? So we've looked at this multiple times. These two graphs-- you could do it by inspection. You could try to solve it in some way, but it's pretty obvious that they intersect at-- remember we care about our y interval. They intersect at y is equal to 0 and y is equal to 1. And there you have it. We've set up our integral for the volume of this shape right over here. I'll leave you there, and in this next video, we will just evaluate this integral.