# Homework Help: How to find mass of a meterstick from torque

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1. Jan 20, 2016

### ZurasE

1. The problem statement, all variables and given/known data
Here is my assignment:
For this inquiry, you will be using your meter stick, some string, some weights of known mass, and the spring force gauge from your kits. Here's the question to answer:

• what is the mass of the meter stick?
You can't measure the mass directly! That's forbidden, but you can use what you know about torques to answer the question.

Some hints:

• hang the meter stick from some point that is not the center.
• use the spring force gauge to impart a torque
In a document called "meter stick mass", clearly explain your method and include a picture of your experimental setup. After you have done the experiment, compare your mass gained by the torque method to the actual mass of the meter stick.

2. Relevant equations
So I know I am supposed to use torque to figure this out.
Torque=distance from center of mass times force
Also
F=ma

3. The attempt at a solution
So if I were to hang the meter stick at the 25 cm mark, that would make the mass of the ruler three times heavier on one side than the other. So if I were to hang a weight on one end, and a weight on the other, I could take the difference, and that would be the difference in the weights of the two parts of the meterstick.
Example:
Lets say I hung a weight of four on the long end, and it took a weight of six on the short end to balance. I subtract these and get 2. So if the difference is two in the weights of the weights that I hung, wouldn't that be the difference in the weights of the two sections of the meterstick? So in this case it would be 3 N on the long end and 1N on the short end. Is that right?

2. Jan 20, 2016

### haruspex

Your statement about torque is not correct. The torque generated by a force about an axis is the force multiplied by the distance from the line of action of the force to the axis. The axis need not be the mass centre.
Will a 4N mass on the long arm get the system to balance? Analyse the moments about the pivot.

3. Jan 20, 2016

### ZurasE

I changed it, could you look over it again?

4. Jan 20, 2016

### haruspex

No. Think about torques. The two weights are at different distances from the pivot. What torque does each exert about the pivot?