rotational inertia for rotation about an axis?

Question

Four point masses of 3.6 kg each are arranged in a square on massless rods. The length of a side of the square is d = 0.60 m.


(a) What is the rotational inertia for rotation about an axis passing through masses B and C?
kg·m2

(b) What is the rotational inertia for rotation about an axis passing through masses A and C?
kg·m2

(c) What is the rotational inertia for rotation about an axis passing through the center of the square and perpendicular to the plane of the square?


Point A is the top left corner of the square, point B is the top right corner of the square, point C is the bottom right corner of the square and point D is the bottom left corner of the square.

Answer 1

I=mr^2

A:
r=0.60^2, m= 3.6kg*2
I=mr^2
I=3.6kg*2(0.60^2)
I=2.592 kg*m^2

Answer 2

The procedure is the same for each problem: Take each mass, multiply it by the square of its distance from the axis, and add it to the total. What's the problem?

Shortcut: Masses on the axis have zero rotational inertia, so they can be ignored.

Shortcut 2: The diagonal of a square is sqrt(2) times the length of a side.