# 4 Point masses form a body - Inertia & Rotational Kinetics

1. Nov 7, 2012

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

Four point masses form a rigid body (they are connected by massless rigid rods) given the positions M1= 3kg (2m, 4m, 0m) M2= 2kg (1m, -4m, 0m) M3= 1kg (10m, 2m, 0m) M4= 5kg (-5m, 2m, 0m)
Find:
A) Moment of inertia of this system when it rotates about x axis
B) Moment of interia of this system when it rotates about y axis
C) Total rotational kinetic energy in the (A) case when ω= 4 rad/s
D) Total rotational kinetic energy in the (B) case when ω= 4 rad/s

2. Relevant equations

Rotational K = 1/2Iω2

3. The attempt at a solution

I'm really terrible at things with center of mass but I believe we'd have to somehow find the total systems Center of Mass in order to find it's moment(s) of Inertia. However I don't know how to do this with the different axis' (x & y). And as I know from the formula, I can't solve for Rotational Kinetic energy without them

Help?

2. Nov 7, 2012

### SteamKing

Staff Emeritus
Do you know how to calculate the center of mass of a body knowing the components of the body and the c.m. of each component? Do you know the formula for calculating center of mass? In cases such as this, try drawing a picture showing each component mass and its location from the origin.

You will have to deal with this before proceeding to calculate moment of inertia for the composite body.

3. Nov 7, 2012