# Moment of Inertia of spherical masses

## Homework Statement

Three small spherical masses are located in a plane at the positions shown below.
The masses are Q=0.700 kg, R=0.400 kg, and S=0.800 kg. Calculate the moment of inertia (of the 3 masses) with respect to an axis perpendicular to the xy plane and passing through x=0 and y=-3. [Since the masses are of small size, you can neglect the contribution due to moments of inertia about their centers of mass.]

I=∑m*r^2,

## The Attempt at a Solution

I tried finding the Center of mass with turned out to be (-0.42, -1.15) and then tried to find the radius to plug into the equation, but that gave me the wrong answer and I am not sure where to even start with this problem.

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SteamKing
Staff Emeritus
Homework Helper

## Homework Statement

Three small spherical masses are located in a plane at the positions shown below.
The masses are Q=0.700 kg, R=0.400 kg, and S=0.800 kg. Calculate the moment of inertia (of the 3 masses) with respect to an axis perpendicular to the xy plane and passing through x=0 and y=-3. [Since the masses are of small size, you can neglect the contribution due to moments of inertia about their centers of mass.]

I=∑m*r^2,

## The Attempt at a Solution

I tried finding the Center of mass with turned out to be (-0.42, -1.15) and then tried to find the radius to plug into the equation, but that gave me the wrong answer and I am not sure where to even start with this problem.

Go back to the definition of 'r' in the equation for calculating moment of inertia. It's nice you found the c.o.m. for the system, but it's irrelevant for solving this problem.

1 person
Thank you for your help. I got it by using the equation I posted, I was just was confusing r as the radius not as the distance from the axis.