# Homework Help: Centroid question

1. Mar 13, 2013

### jiboom

7)
Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface.
A concrete pile for a domestic dwelling has a radius of 1000 mm at its base, and 2000 mm at the top. It is 2000 mm deep and the sides slope uniformly.
Find the volume of concrete needed to form this pile.
A plinth is formed in the shape of a hemispherical dome, radius 4 m, with its top 1 m removed. Using integration, find the position of the plinth’s centroid along its axis of symmetry.

i think i just need volume of a frustrum for first part,with base radius 2000 and top radius 1000 and height 2000??
s0
2pi[4+2+1]/3= 14pi/3

how do i go about the second part? what do i need to ingrate for this shape?

2. Mar 14, 2013

### haruspex

Suppose you set the origin at the sphere's centre and take the x axis to be the axis of symmetry of the plinth. Can you apply the 'washer' method to find the volume? Finding the moment about the y axis is very similar. Do you know a formula for the x coordinate of the mass centre?

3. Mar 14, 2013

### jiboom

not heard of the washer method. i know the formula for x bar for a hemisphere but the question wants it from integration

4. Mar 14, 2013

### haruspex

The washer method just slices a volume of revolution into discs perpendicular to the axis. Write down the expression for the area of one such disc, thickness dx, and integrate. Thst gives the volume. Repeat the process with an extra factor x to get the moment of the disc element and integrate again.