# Homework Help: Help with lab question(percent error)

1. Dec 9, 2008

### packerfan128

1. The problem statement, all variables and given/known data
Lab on circular motion. The class swung a mass in a circular motion by a string.
--Find percent error of the trail.
Total mass=0.25kg
period=.427 seconds
2. Relevant equations

I am not sure

3. The attempt at a solution

This is one of those problems that drew me a blank... I have no idea where to start. It would be great if someone could just start me off with what i am trying to take the % error of. I know that % error is actual/theoretical X 100%. I just don't know what I am trying to solve. Thanks.

2. Dec 9, 2008

### fluidistic

I call "uncertainty" what you call "error".
What is the uncertainty of the Total mass? Of the radius and the period?
Once you know the total mass, radius and period, how did you calculated the trail? (I mean, what formula did you use?).

3. Dec 9, 2008

### packerfan128

the total mass is certain... we mass the iron on the end of the string. The radius is also certain... I measured it with a ruler. I guess the only thing that could really be uncertain is the period (we measured it with a top watch). The formula is T=2(pi)*(SquROOT(length/9.8)) So would i calculate what the theoretical period would be, then divide it by the period i got?

4. Dec 10, 2008

### fluidistic

I don't think there's something certain. You said the mass is certain but I guess you measured it with a balance and my guess is that it's accuracy is about 0.01g.
So you'd have $$(0.25 \pm 0.01 ) g$$. You realize the uncertainty is 4% which is not 0% as you suggested.

By the way, I don't understand what you had to find. Why did you measured a period?
Another thing :
I don't want to scare you but how did you do such a precise measurement which was done handily?
Last thing :
is of no help here. For example what would you take as a theoretical length, mass, etc.?