Research: Determining a unknown objects mass

[Franster]

I was givin a research projects Determine the masses of two unknown objects without weighing them or using density

I have tried a couple of methods but they all failed

[deleted-71552]

I was givin a research projects Determine the masses of two unknown objects without weighing them or using density

I have tried a couple of methods but they all failed

Hello, Franster! Can you tell us a little about what you've looked at so far and what didn't work for you? Who gave you the project, and can you tell us a little more about the objects themselves?

[Franster]

It is a school project.
We need to find a method that we can use to determine the masses of two objects and we aren't allowed to weigh them or use their density to determine the mass.

Our teacher will give us two objects in class. Then we should use the method we created to determine the masses of the two objects.

I am currently using a pulley system. I used a known weight say 1kg an then counterbalance it with the two objects. I have made a sketch.



I will determine the size of the angle and use a forces diagram to determine the tension in the ropes and maybe from their try to determine the masses of the two unkown objects. But I am not sure how to determine the mass.

[deleted-71447]

I like your idea and your sketch. What is your highest level of course work in math and physics? That info will help us to give you advice that is appropriate and not overly complicated or simple.

Can you solve for the mass of each object, one at a time, or do you have to solve for the mass of both, simultaneously?

[deleted-71552]

Hello, Franster!

Thank you for posting that sketch. It really helps us to understand what you are thinking about.

Is there a requirement to deterine the mass of the two objects simultaneously, as you have shown in your sketch? Or will you be examining the objects one at a time?

If the method described in your sketch is acceptable, it may also be acceptable to measure the mass by the length of a spring. If you hang objects of known mass from a spring and record the length of the spring for each, you can build a table of legths and masses. Then when you hang the unknowns from the spring, measure, and use the table to guess the mass. This is essentially using a scale, so I'm not sure that it is an acceptable solution.

I've been doing some reading about mass on the Internet. I did a Google search on "mass of an object" and found some interesting sites. I think the key to solving this problem is to note the relationship between mass and inertia.

I found this NASA site particularly interesting:

http://www-spof.gsfc.nasa.gov/stargaze/Smass.htm

and

http://www-spof.gsfc.nasa.gov/stargaze/Sskylab.htm

These sites talk about a method for determining body mass of astronauts in the zero-gravity environment of the SkyLab space station. Now, I know that you probably can't build a sophisticated oscillating chair the way NASA did, but their method suggests some interesting concepts.

The oscillating chair works because it takes more force to start and stop an object that has greater mass. Such an object requires more force to overcome its greater inertia. Therefore, objects with greater mass will take longer to decelerate, stop and then accelerate back at each end of the oscillation. The key for you might be to use the force required to accelerate an object at rest to determine its mass.

Here's an idea: Place an object on a nearly frictionless surface. Apply a nearly constant sideways force to the object and measure the amount of time it takes to move a fixed distance. The higher the mass, the longer it will take to cross. Like the spring example, above, you could test a number of known masses, build a table, and then test the unknowns.

For the nearly frictionless surface, you could use smooth plastic disks on a mirror or glass surface that has been waxed with car wax. (Think about air hockey!) For the nearly constant force, you could use a long thin string of rubberbands. (The force actually woudn't be constant, and you should probably talk about that in your report, but it doesn't substantially affect the results of this experiment...)

Make two lines on the glass surface - start and finish. Beyond the finish end, attach the rubberband to a piece of wood. Attach the other end of the rubberband to the plastic disk. You then pull back on the disk to stretch the rubberband to the start line, load the mass onto the disk, start the timer and release. Record the times for known masses to move from start to finish. Build your table of times for mass, run your unknowns, and you have it!

You must be careful to select a rubberband that isn't so strong that it just pulls the disk way too quickly. You also need to make sure the distance the disk travels is sufficient to give you measurements of say 5 seconds or more. You'll have to experiment with different lengths and bands to get good data.

Your report must talk about the relationship between mass and inertia and explain any potential sources of error, friction, etc.

I hope this helps.