What is the volume of:
y=square root(x), y=1, x=4
revolved about the x-axis?
Use the disc method. We create a disc by making a cross section perpendicular to the x-axis. The volume is then given by $\displaystyle \int_a^b A(x) dx$ where $\displaystyle A(x)$ is the area of our disc and $\displaystyle x= a, \ b$ are our bounds.
More here: Volume with Rings