// So I was given this physics project to do as a final ISU project. Engineers only!! :) //

[simms]

Well, I'm in a senior level physics class, and our final ISU is to find the mass of an unknown object. That easy. We can employ any factors known to physics, including but not limited to Hooke's law, deflection of objects, compression/expansion, ballastic pendulums and such. The thing is, there's a number of ways mass can be solved using physics, but we need the most accurate one available.

Our mark depends on how accurate our value is compared to the real one based on a digital scale. We CANNOT read the value off the system we create, nor can we purchase a system - we have to fabricate it from our own materials. I realize error is based upon systematic and random error, but WHICH system is most accurate?

Our physics teacher has stated a few examples:
[*]Hooke's Law (using period vs mass, or extension vs mass) - Basically a slinky on a piece of plywood with the deflection of the object measured.
[*]Compound system - two spring attached to one central balance - Haven't done this one, don't know how it works.
[*]Cantilever system - measuring the deflection when the object placed at the end of a ruler, persay with the other end flat on a table.
[*]Initeral (sp?) system - same as cantilever, but you measure the deflection sideways (so it swings back and forth)
[*]Ballastic pendulum - A period with 10 oscillations with the mass on it, period is given and mass is solved using a pendulum.
[*]Compression - Not sure, basically mass is put on a spring and deflection is measured how much the spring 'sank' down.
[*]Something about Young's modulus might work too, but I have no idea how that works.

I forgot to state that the unknown mass is within a given range of 100g-2000g.

So there are 5 methods. Which would you recommend is the most precise, or if you know of another system, please mention it.
-Simon

 

[MichaelD]

No, we will not do your homework for you.

Not that I could even figure out how OPEN you physics textbook....

But I'll speak for the masses. Good luck with this.

 

[dopcombo]

why not try to do most of the methods and then compare it with the final result?
do statistical analysis with like 10 different weights and that should bulk up your paper significantly

 

[simms]

Originally posted by: MichaelD
No, we will not do your homework for you.

Not that I could even figure out how OPEN you physics textbook....

But I'll speak for the masses. Good luck with this.

Thanks for your response MichaelD, but it's not a question of doing my homework. I'm not asking you to SOLVE anything or whatnot. I'm simply asking from your experience (if you have any) which system you've dealt with that is most precise. Notice how I don't ask a question, I ask you to recommend a solution. What you will tell me may be wrong but that will be my discretion, and my experience. I simply ask the elders on this forum for their input on guidance and the wisdom of their years above mine.

Originally posted by: dopcombo
why not try to do most of the methods and then compare it with the final result?
do statistical analysis with like 10 different weights and that should bulk up your paper significantly

I actually think I would do that, but we can only use one method for the test. As well, if there are any ones that I've MISSED that would be helpful!

 

[Koing]

Originally posted by: dopcombo
why not try to do most of the methods and then compare it with the final result?
do statistical analysis with like 10 different weights and that should bulk up your paper significantly

Yeah I would do that.

Why not try 3-4 of them for *known* weights that you have

Then use the most accurate one on the unknown weight?

I said pendulum as it would be easier to measure and I wouldn't fancy working out the others......

 

[SuperTool]

Can you calibrate whatever system with known masses?

 

[simms]

Originally posted by: SuperTool
Can you calibrate whatever system with known masses?

Yes, you can caliberate the system however you want to find constant values of a spring constant k, for example. Obviously I plan on viguorously testing the system I create with several different masses. But you only get ONE attempt at measuring the mass, so if you're off with your k you're SOL. That's why I'd like to find the best system that minimizes error so that the error in k can be minimized as well.

 

[simms]

Originally posted by: Koing

Originally posted by: dopcombo
why not try to do most of the methods and then compare it with the final result?
do statistical analysis with like 10 different weights and that should bulk up your paper significantly

Yeah I would do that.

Why not try 3-4 of them for *known* weights that you have

Then use the most accurate one on the unknown weight?

I said pendulum as it would be easier to measure and I wouldn't fancy working out the others......

You'd be surprised when you take a stopwatch and calculate the period, and see how much they vary in between trials. If you use the extension system, you eliminate time, so you just deal with extension, right? No oscillations to worry about like in period.

 

[Yossarian]

send the unknown mass to the moon then measure how much the earth's orbit is affected by its gravity.

 

[MichaelD]

Hey simms, I wish you luck. This stuff is way over my head.

 

[simms]

Originally posted by: PipBoy
send the unknown mass to the moon then measure how much the earth's orbit is affected by its gravity.

lol

Originally posted by: MichaelD
Hey simms, I wish you luck. This stuff is way over my head.

Thanks! I'm sure if you ever take physics you'll have no problem because of this thread I created years ago

 

[YellowRose]

not an eng. but sounds quite simple. I would chose the compression of a spring and Hooke's Law.

 

[Sukhoi]

I'd say the pendulum should be quite accurate if you can get very good bearings on the moving joint, and some method better than a stopwatch to measure the period.

 

[Sukhoi]

Oh, IMHO (I chould be way wrong here) I would avoid springs and other deformable objects. Who knows how much your spring is going to change properties from when you're calibrating it to when you do the test.

 

[CTho9305]

Just speculating... but if you put it on a styrofoam boat and measured how far down it floated.... you could find the weight of the displaced water pretty accurately.

 

[Sukhoi]

Um, is the period of a pendulum dependant on mass?

 

[Sukhoi]

D00d, just do water displacement!

 

[Yossarian]

these water displacement suggestions are no good, that only gives you volume

 

[CTho9305]

Um, you do know that the density of water is a very convenient number? (Assuming you are within a reasonable temperature range)

 

[Turin39789]

why not just weigh it

 

[Martin]

stay away from Young's modulus, it'd be almost impossible for you to make a good aparatus that uses it. and even ify ou do that, its still not very good.

about water displacement, how would you figure out the weight from that? They probably won't give him the density of the object...



And ISU...I haven't heard that word in almost a year. For my OAC physics ISU, we had to make a projectile launcher.

I am guessing you're going to UofT eng next year?

 

[Yossarian]

I will pose this question tomorrow to some of my fellow super smart mechanical engineers. I would probably go with spring extension myself.

Or wait! Just drop it and time it, heavier objects fall faster!!!! j/k

 

[brxndxn]

these water displacement suggestions are no good, that only gives you volume

hahaha... what happens if you measure the mass of the water displaced? ... or, if you find the volume and asume that the mass of the water is 1gm/cm^3

 

[Yossarian]

Originally posted by: brxndxn

these water displacement suggestions are no good, that only gives you volume

hahaha... what happens if you measure the mass of the water displaced? ... or, if you find the volume and asume that the mass of the water is 1gm/cm^3

then you would know what the object would weigh if it were made of water

 

[brxndxn]

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Originally posted by: brxndxn

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these water displacement suggestions are no good, that only gives you volume
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hahaha... what happens if you measure the mass of the water displaced? ... or, if you find the volume and asume that the mass of the water is 1gm/cm^3
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then you would know what the object would weigh if it were made of water

If the object weighs less than water, it will float. If it floats, you measure the volume of the water displaced. Mass of displaced water will equal mass of object.