# Finding weight of an unknown mass

1. Dec 21, 2008

### gallimaufry

The goal is to find the weight of an unknown mass by building some sort of device. The mass given can be anywhere between 10g and 2000g. Once the mass is given to you, the only thing you can do is place it on your device and that's it. You are not allowed to touch the device afterwords. The device must work on its own and do what it does, but the person is allowed to look at the device and do any calculations needed. For example, if you need to know a distance, then a meter stick would be mounted on the device so that you do not have to touch the device. The idea i came up with, is a simple old style balance with a fulcrum in the center and a known weight on one side and an unknown weight on the other side. The known weight will be moved by motor until the angle the balance is at is less then 30 degrees. There will be a sensor in the middle where the fulcrum is that will tell me the angle it is at on an lcd. I need to know how to be able to calculate the unknown mass when the balance is not balanced and the things i know are the known mass, the angle the balance is at, the distance from the fulcrum of the unknown mass, and the distance of the known mass. If you can explain how to solve for the mass, that would be great help. Thanks for the help.

2. Dec 21, 2008

### Staff: Mentor

Why not just use a kitchen scale?

3. Dec 21, 2008

### gallimaufry

You cannot use any pre-made scales and the scale must also be able to run on batteries no more than 12 volts. Thanks for the reply!

4. Dec 21, 2008

### Staff: Mentor

OK, then I would recommend making your own kitchen scale using a strain gauge. That will be a lot easier than setting up a motor with a feedback loop and figuring out some way to measure the torque.

5. Dec 21, 2008

### gallimaufry

How do strain gauges work, this might be a good idea except i left out the trick to the challenge. When you show your device you will be given a mystery material that you will have to incorporate into the scale. So the more simple, the harder it is to incorporate a random material. I will look into the strain gauge and maybe even try it out. Thanks

6. Dec 21, 2008

### Staff: Mentor

A strain gauge is made of a material whose resistance changes as a function of its length. You attach the strain gauge to a cantilever beam, build it into a simple Wheatstone bridge, and amplify it. Most kitchen scales work this way. You can make it more accurate by periodically reversing the excitation voltage on the Wheatstone bridge and taking the difference between the forward and reverse voltages.