P: 12 Finding COM, and inertia please help my answers don't make sense? Ok first you need to understand the COM formula and see how it works. Basically you need to specify a coordinate to tell where the COM is. That means you need to find the center of mass in the x direction, y direction, and z direction of the system or object. So we start by separating everything in components( x, y and z). Lets do the x direction first. We let $d_{nx}$ represent the distance in the x direction for each particle (n=1,2,3,4) Then $X_{com} = \frac{m_2 d_{2x}+m_3 d_{3x} + m_1 d_{1x}+m_4 d_{4x}}{M}$ where M is the total mass. Note that $d_{2x}=d_{1x}=0$ and $d_{3x}=d_{4x}=2 m$ Similarly for y: $Y_{com} = \frac{m_2 d_{2y}+m_3 d_{3y} + m_1 d_{1y}+m_4 d_{4y}}{M}$ Note that $d_{2y}=d_{3y}=0$ $d_{1y}=d_{4y} = 2 m$ Similarly for z: $Z_{com} = \frac{m_2 d_{2z}+m_3 d_{3z} + m_1 d_{1z}+m_4 d_{4z}}{M}$ But Note that $d_{1z} = d_{2z} = d_{3z} = d_{4z} = 0$ then $Z_{com} = 0$ Then at the end you get the COM to be at $(X_{com},Y_{com},Z_{com})$ Now try to plug in the numbers and see what you get. (Easy!!! =)) Do the same thing for the next part (moment of inertia), first understand the formula/equation and just follow it slowly.