Physics Uncertainty Question.

Well, i have a question about uncertainty in a physics lab im working on and my physics book speaks nothing about uncertainty. any help or a point to a nice website would be fine, or an explanation

Suppose you wanted to look up the density of the block material for the purpose of verifying your results. The handbook would have a value for the density and a range of uncertainty. How would you know if your value of density and its range of uncertainty is a good result ??

once again, i have no clue how to do this.

My way to find out how dense something is is to measure its length times width times hight, I take that figure, then multiply it by the power of 2, then devide it by the ouch factor when i drop it on my foot.

Seriously, I don't know

I think what your looking for is also called percent error.

your answer minus theoretical divided by theoretical times 100

here i dug this up for ya

http://phoenix.phys.clemson.edu/tutorials/error/

sorry if this isnt what you mean

My guess would be the r^2 value that can be found on your calculator (Ti-83) by putting in info into your stat table (l1 and l2 or other ones but then you ahve to put in teh names of em') and then Stat->Calc (menu)->LinReg
Its there and the closer to 1 it is ther better your value. (I think if you put in a varible, varible those are the tables in stat it looks at.)

But that might not be right, it could be %error but I think it would say % error. It might also be somthing else. The value in the book is probably the known one for example for gravity the generally acceted value is 9.81..... to about 10 decimals or somthing crazy, and we didn't acutaly PROVE this but there have been enough documented tests to get us that far. The value it gives is how sure we are, but I COULD be wrong since I'm in Physics now so if somone more qualified tells you differnyt you should prolly listen to them (unless they say its a number green men give you).

*shudders to think of the many countless hours in Chem class spent covering Accuracy vs. Precision*

I'f I'm reading your question right, you want to know wether your result you came up with is a reasonable result based on it's accuracy when compared to a textbook value.

First off, there's %Error. That will tell you how precicely (correctly) you've made your measurement. If it's way off, then the %Error will be high and you'll know that something is up with your data. %Error is calculated like bluestreakLB says.

Secondly, there's accuracy (or how 'sure' your measurement is). This depends on how accurate your measuring tools are, and will probably be less accurate than a textbook answer. Accuracy of a measurement is (at my school anyway) determined by the number of significant digits you have. If you're measuring devices are inaccurate, your final measurment will be inaccurate as well. However, your final accuracy is only as accurate as the LEAST accurate measurement you made.

How good a measurement you made depends on both the precision and accuracy. If you can only measure density to +- 1 g/cm^3 then being off from the correct by 0.5 would not be noticed because of rounding. If however you could measure with an accuracy of +- 0.01g/cm^3, it would be considered a bad measurement to be off by so far. However, a measured value off by only 0.05g/cm^3 would be considered much more reasonable due to error sources that can creep in.

JigPu

???? I thought it was the other way round...

Precision error are errors that can be atrributed to the precision of your instruments that you used. If one pan balance measures to 0.001g but then you go and react the stuff with 20cm3 of liquid measured by a cheap and nasty measuring cylinder, then only the number of significant figures recorded on the least-precise instrument can be recorded with any certainty - not much point measuring to 0.001g if the other readings taken with other apparatus are to +-0.1g.

Accuracy is "hitting the dartboard", precision is "getting a treble 20". Precision eliminates some sources of error but not all. Most of the error in experiments/investigations are from procedural error - losing some solution on a stirring rod, leaving some in the bottom of a beaker when transferring substances etc. Only good laboratory techniques improve accuracy.

Therefore in the case of your density calculation, the sources of precision error are in measuring the mass and measuring the volume. Procedural errors are easy to avoid in this one As stated above, calculate your precision error and decide which would have the greatest impact on results, i.e. the one with the biggest percentage uncertainty. If your actual answer is different from the one in the book by a lot more than the biggest precision error then you probably have a procedural error that is affecting your accuracy.

OMG

* stoopid runs into door and knocks himself out as he escapes current thread *

stoopid said:

OMG

* stoopid runs into door and knocks himself out as he escapes current thread *

Click to expand...

Ya folks sound all edumacated and stuuuuf

bluestreakLB said:

Ya folks sound all edumacated and stuuuuf

Click to expand...

Wow... That guy makes me angry, I wanna punch him just looking there...

yeah my physics lab was yesterday but i do appreciate the responses, i figured it out now

here's a nice analogy for accuracy and precision.

My cars speedomitor is unaccurate, but precise. Whatever it reads you have to subtract 5mph then you'll have your actuall speed. But it is always 5mph off.

accuracy is the variance in the number you measure. and precision is getting the same number every time you measure.

nitewulf said:

here's a nice analogy for accuracy and precision.

My cars speedomitor is unaccurate, but precise. Whatever it reads you have to subtract 5mph then you'll have your actuall speed. But it is always 5mph off.

accuracy is the variance in the number you measure. and precision is getting the same number every time you measure.

Click to expand...

i actually had that problem and was pulled over b/c of it, i still had to pay a fine, but no points on the ol' license.