# Rotational inertias problem

## Homework Statement

The masses and coordinates of four particles are as follows: 27 g, x = 2.0 cm, y = 2.0 cm; 16 g, x = 0, y = 4.0 cm; 43 g, x = -3.0 cm, y = -3.0 cm; 61 g, x = -2.0 cm, y = 4.0 cm. What are the rotational inertias of this collection about the (a) x, (b) y, and (c) z axes?

## Homework Equations

I'm not sure.. would I use parallel axis theorem?

## The Attempt at a Solution

I don't know how to solve this problem.. help please =)

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tiny-tim
Homework Helper
I'm not sure.. would I use parallel axis theorem?
Hi Shatzkinator! You only need the parallel axis theorem for a solid body, for which you already know the moment of inertia about its c.o.m.

These are just point masses, so just use the definition of moment of inertia (rotational inertia) Two thin rods (each of mass 0.50 kg) are joined together to form a rigid body as shown in Fig. 10-66. One of the rods has length L1 = 0.30 m, and the other has length L2 = 0.60 m. What is the rotational inertia of this rigid body about (a) an axis that is perpendicular to the plane of the paper and passes through the center of the shorter rod and (b) an axis that is perpendicular to the plane of the paper and passes through the center of the longer rod?

Fig. 10-66
Problem 114.

also this question i can't get part B