# Rotational inertias

1. Oct 13, 2006

### kiwinosa87

I absolutely suck at physics. Here is the question I'm stuck on:
The masses and coordinates of four particles are as follows: 69 g, x = 2.0 cm, y = 2.0 cm; 38 g, x = 0, y = 4.0 cm; 21 g, x = -3.0 cm, y = -3.0 cm; 24 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?

Any help would be appreciated!!!

2. Oct 13, 2006

### Staff: Mentor

How do you find the rotational inertia of a particle about an axis? If the particle has mass "m" and is a distance "R" from the axis, what is its rotational inertia about that axis?

3. Oct 13, 2006

### kiwinosa87

would you use the same equation for a particle as you would for an sphere??? I know that the rotational inertia is equal to mR^2. Is that what you mean?

4. Oct 13, 2006

### Staff: Mentor

Not sure what you mean. (My immediate answer would be no.)
Yes. To find the rotational inertia of several particles, just find it for each particle and add them up. The only tricky part is making sure you are using the correct "R", since it depends on what axis you are using.

5. Oct 13, 2006

### kiwinosa87

Okie, I understand that that's the correct equation, but how do I distinguish between the different axes??

6. Oct 13, 2006

### Staff: Mentor

Not sure where the problem is. The axes you need to consider are just the usual x, y, & z axes.

To test your understanding, what would "R" be for the first particle about the x-axis? Draw yourself a picture.

7. Oct 13, 2006

### kiwinosa87

okie, so it's either 2, or the square root of 8, or maybe 7...???? Im sorry, I'm seriously physics retarded!

8. Oct 13, 2006

### kiwinosa87

OKie, so I guess you gave up on me...any other takers...?

9. Oct 13, 2006

### OlderDan

Even the helpers have to go do other things sometimes.

From reading earlier posts I see you have the fundamental equation needed to find the rotational inertia of each particle. The total rotational inertia is the sum over the individual particles. What you need to know is the distance R for each particle from the axis of rotation. Those distances might be different for each axis.