# Inertia: blocks attached to rod of negligible mass

1. Dec 9, 2008

### cantgetno

1. The problem statement, all variables and given/known data
Small blocks, each with mass m, are clamped at the ends and at the center of a rod of length L and negligible mass.
Find the formula for I

2. Relevant equations
I=MR^2

3. The attempt at a solution

I=m(0.5 L)^2 + m(0.5 L)^2
I=2(0.5L)^2 m
I=0.5L^2 m

am i right?

2. Dec 9, 2008

### tiny-tim

Hi cantgetno!
mmm …

i] moment of inertia isn't measured in metres

ii] about which point is I being measured?

iii] what about the mass in the middle?

3. Dec 9, 2008

### cantgetno

Re: Inertia

oh its around the middle
so that makes the mass in the middle mass x 0

metres? m=mass

4. Dec 9, 2008

### tiny-tim

D'oh! m is mass … should have spotted that!

In that case, everything is fine.

5. Dec 9, 2008

### cantgetno

Re: Inertia

oh crap
i put 1/2 l m^2 ... im an idiot

thanks anyway

6. Dec 9, 2008

### tiny-tim

hee hee

Useful tip: keep your m's in the same place

for some reason, you shifted them from the left to the right …
… which makes it much easier to make a mistake, and much less easy to spot a mistake.