# Homework Help: Moment of inertia for a door help!

1. Jul 25, 2008

### ahello888a

1. The problem statement, all variables and given/known data
Determine the moment of inertia of a 19kg door that is 2.5m high and 1.1m wide and is hinged along long side. Ignore the thickness of the door

2. Relevant equations

I=$$\int$$ (R^2 dm)

3. The attempt at a solution
I really don't know where to start with this problem. My book only shows me how to do moments for cylindrically shaped objects and rods, so i don't know how to deal with 2d objects. I'm having particular trouble with coming up with a dm that is reliant on R.

2. Jul 25, 2008

### mgb_phys

The general definition of moment of intertia is to consider a small part of the object with mass dm at a distance r and sum the product of r*r dm.
So for the door, imagine splitting the door into thin vertical slices, then calculate the mass of the slice and the square of the distance from the hinge.
If you know calculus you can do this analytically and come up with an equation - that's what the integral you have given calculates

3. Jul 26, 2008

### ahello888a

i still have absolutely no clue how to do this. can you point to an example or something?

4. Jul 26, 2008

### ahello888a

never mind....

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