- #1

Physics Person

- 7

- 0

## Homework Statement

There is a diagram of a rod with its center of mass labeled and a ball hanging down from it, attached by a string. Two torques are applied on either side. You are asked to find the mass of the ball and the mass of the rod.

## Homework Equations

Torque = Force * Lever Arm * sine of angle between them

## The Attempt at a Solution

I designated the center of mass of the rod as the axis, and then calculated the torque applied by each force by multiplying the magnitude of the force by the distance between where the force was applied and the center of mass of the rod. There is an angle of 60 degrees on the other side of where one force is being applied, so I calculated that the angle between the force and r (the lever arm, or distance from where that force is applied to the center of mass of the rod) is 120 degrees by subtracting 60 degrees from 180 degrees. What confuses me, though, is that, when I did my calculations, I got a negative value for the torque resulting from the ball's weight. I know torque can be negative, since it is a vector, but what confuses me about this result is that there is another torque that appears to be acting in the same direction as the one caused by the weight of the ball, yet it is positive. So I must have done something wrong. What is it?

Here are my calculations:

9N * 0.09m = 0.81 Nm

5.02N * sin120 degrees * 0.06m = 0.260846852 Nm

0.81 + Tball = 0.260846852

This is where I got the negative result.