# Homework Help: Mass Moment of inertia of a cylinder with 4 holes?

1. Mar 12, 2012

### qpham26

1. The problem statement, all variables and given/known data

Hi I was reviewing for my final and I came across this problem:

Problem:
Basically there is a 6in long cylinder with dia = 24in
Given weight density: 490 lb/ft^3

Each Hole is drilled symmetrically, each has 6in dia and equally space around a 10in dia circle concentric with the cylinder.

This is the picture of the frontview of the cylinder: https://lh4.googleusercontent.com/-8m1r4npC7cg/T11OqGF6qhI/AAAAAAAAABY/4v2HCrEqhd4/s333/cyinder.png [Broken]

Thanks for your time and consideration.
Appreciate it!!!

2. Relevant equations

3. The attempt at a solution
Approach
So what I would do is:
-First get that weight density into mass density by dividing it by 32.2
-Then find the mass of the cylinder without holes.
-Find mass of each holes.
-calculate the M of I of each hole about the center axis (parallel axis theorem)
-calculate the M of I of the whole solid cylinder (no holes)
-subtract the M of I of the holes from the whole cylinder

Will I get the correct answer based on the technique above?

Last edited by a moderator: May 5, 2017
2. Mar 12, 2012

### ehild

That is a correct approach.

ehild

3. Mar 12, 2012

### qpham26

Thanks and for the mass of the cylinder, the unit is slug right?

Thanks again.

4. Mar 12, 2012

### ehild

I am sorry, I am not familiar with imperial units. But Wikipedia says it is slug.
http://en.wikipedia.org/wiki/Slug_(mass [Broken])

ehild

Last edited by a moderator: May 5, 2017
5. Apr 2, 2012

### anjuv

yah....
this only is the right way of solving it.

Last edited by a moderator: May 5, 2017