Fluid force integration problem

[tobywashere]

Homework Statement

The vertical triangular plate shown in the picture below is the end plate of a feeding trough full of hog slop, weighing 80 pounds per cubic foot. What is the force against the plate?

Homework Equations

This question is supposed to involve intregration to find volume.

The Attempt at a Solution

I rotated the triangle around the y-axis (not sure if this is what I'm supposed to do). Then I found the volume of the resulting shape (it is a cone).
V = pi * 4² * 2 / 3
V = 33.5 cubic feet
Then multiply by 80 lbs per cubic foot to get 2680.8 lbs
But this answer is much higher than the textbook's answer. I'm lost for any other way of doing this question. I know my answer can't be right because I didn't integrate anywhere.