Volume of Solid of Revolution
Volumes of Revolution
Integral Calculus »
Centroids |
Volumes and Surface Areas »
Integration by Parts
What is a Solid of Revolution?
A solid of revolution is a three-dimensional figure obtained by rotating a two-dimensional plane region around a straight line (the axis of revolution) that lies in the same plane.
Imagine taking a flat shape, like the area under a curve, and spinning it around an axis. The path swept out by this area forms a solid object. For example:
- Rotating a rectangle around one of its sides generates a cylinder.
- Rotating a right triangle around one of its legs generates a cone.
- Rotating a semicircle around its diameter generates a sphere.
Calculus, specifically integration, provides powerful methods to calculate the volume of these solids, even when the generating region is bounded by complex curves.
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Disk Method (Rotation about the x-axis)
The Disk Method is used to find the volume of a solid of revolution when the region being revolved is bounded by a function $y=f(x)$, the x-axis (i.e., $y=0$), and vertical lines $x=a$ and $x=b$, and this region is rotated around the x-axis.
We can think of slicing the solid into infinitesimally thin circular disks perpendicular to the axis of rotation (the x-axis in this case).
- Each disk has a radius $R(x) = f(x)$ (assuming $f(x) \ge 0$).
- The area of the face of such a disk is $A(x) = \pi [R(x)]^2 = \pi [f(x)]^2$.
- The thickness of each disk is $dx$.
- The volume of an infinitesimal disk is $dV = A(x) \, dx = \pi [f(x)]^2 \, dx$.
To find the total volume $V$, we sum (integrate) the volumes of these disks from $x=a$ to $x=b$:
$$ V = \int_a^b \pi [f(x)]^2 \, dx = \pi \int_a^b [f(x)]^2 \, dx $$
This formula applies when the region is flush against the axis of rotation.
Washer Method (Rotation about the x-axis)
The Washer Method is an extension of the Disk Method. It is used when the region being revolved is bounded by two functions, an outer function $y=R(x)$ and an inner function $y=r(x)$ (where $R(x) \ge r(x) \ge 0$ on $[a,b]$), and this region is rotated around the x-axis.
When this region is revolved, it forms a solid with a hole in the middle, resembling a washer.
- The outer radius of the washer is $R(x)$.
- The inner radius of the washer (the radius of the hole) is $r(x)$.
- The area of the face of such a washer is $A(x) = \pi ([R(x)]^2 - [r(x)]^2)$.
- The thickness of each washer is $dx$.
- The volume of an infinitesimal washer is $dV = A(x) \, dx = \pi ([R(x)]^2 - [r(x)]^2) \, dx$.
To find the total volume $V$, we integrate from $x=a$ to $x=b$:
$$ V = \int_a^b \pi ([R(x)]^2 - [r(x)]^2) \, dx = \pi \int_a^b \left( [R(x)]^2 - [r(x)]^2 \right) \, dx $$
Interactive Integration Practice
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Volume Solver & Visualizer (Rotation about x-axis)
This solver calculates the volume of the solid generated by revolving the region between $R(x)$ and $r(x)$ around the x-axis. If $r(x)$ is 0 or empty, the Disk Method is used. Otherwise, the Washer Method is used. Ensure $R(x) \ge r(x) \ge 0$ over the interval $[a,b]$.
Enter function(s), limits, and click "Calculate & Plot Volume".
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Practical Applications
Calculating volumes of solids of revolution has numerous applications in various fields:
- Engineering and Manufacturing: Designing tanks, pipes, calculating material for machine parts.
- Architecture: Designing domes, columns, estimating material volumes.
- Physics: Calculating moments of inertia, fluid dynamics problems.
- Medicine: Modeling volumes of organs.
- Aerospace: Designing rocket nozzles and fuel tanks.