Inertia: blocks attached to rod of negligible mass

[cantgetno]

Homework Statement

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

Homework Equations

I=MR^2

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?

 

[tiny-tim]

Hi cantgetno!

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
…
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?

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?

 

[cantgetno]

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

metres? m=mass

 

[tiny-tim]

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

metres? m=mass

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

In that case, everything is fine.

 

[cantgetno]

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

thanks anyway

 

[tiny-tim]

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

hee hee

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

for some reason, you shifted them from the left to the right …

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

… which makes it much easier to make a mistake, and much less easy to spot a mistake.