# Rotational motion

1. Jun 20, 2005

### devious_

Can anyone help me with this question?

A uniform circular disc has mass 4m and radius r. A particle of mass m is attached to the end of the disc at point A of its circumference. The loaded disc is free to rotate about a horizontal axis which is tangential to the disc at the point B, where AB is a diameter. The disc is released from rest with AB at an angle of 60 degrees with the upward vertical. Find the magnitude of the force exerted by the disc on the axis when AB makes an angle of 60 degrees with the downward vertical.

The answer is supposed to be $mg \sqrt{111}$, but I can't seem to get it.

I considered the disc and the particle to be one rigid body. I found the position of the body's center of mass and its moment of inertia about B. Then I began setting up the necessasry equations, working with the body as if it was a particle of mass 4m+m moving with circle motion with radius 6r/5, which is the distance of the center of mass from B. Did I do something wrong?

2. Jun 20, 2005

### OlderDan

What did you do with the moment of inertia? This system does not bahave like a particle of mass 4m+m moving in a circle. It behaves like a physical pendulum. What did you get for I?

3. Jun 21, 2005

### devious_

I spotted my mistake! Thanks anyway.

Last edited: Jun 21, 2005