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Newtonian Physics Lab- Uncertainty and Angular Momentum

  1. Oct 28, 2008 #1
    Before I begin, let me say this is my first time on this forum, and am terribly sorry if I screw something up here. Let me also say that my physics lab is ahead of my physics class, so my knowledge on angular momentum is lacking.

    Also, this lab is due tomorrow. I know it's short notice, but I had assumed that the TA would have responded by now, and had only just thought of asking here.

    1. The problem statement, all variables and given/known data
    The purpose of the lab is to essentially prove that angular momentum is conserved. I am not quite aware of all the mechanics, but it does concern Kepler's Second law that if the area of an object oribiting an object is the same over two time intervals, then the two intervals must equal each other.

    In the lab, we have a air hockey disc on a mini air hocky table orbiting a peg. Using a movie recording program and ImageJ, I am able to determine the position of the center peg and the position of the puck's center of mass in each frame. The actual data is accurate enough, and I understand that. The area the puck is traveld is measured by a triangle, who's points are known.

    What I don't understand, is how to calculate the uncertainty for this lab. I am very well aware that the partial derivative of the formula is needed to find uncertainty, but the problem is this- I have no idea what the actual standard deviation of the data is. Without this standard deviation, the method I know for determining uncertainty. Please see my "attmpts for solutions" as to what I have done to rectify the matter.

    2. Relevant equations

    The only equations given are the ones that follow (please pardon the lack of professional formulas, as I do not know this forum's code for said forumlas):

    s= (a+b+c)/2


    a= Sqrt((X1-X2)^2 + (Y1-Y2)^2)

    b= Sqrt((X1-Xcenter)^2 + (Y1-Ycenter)^2)

    c= Sqrt((X2-Xcenter)^2 + (Y2-Ycenter)^2)

    3. The attempt at a solution
    As stated earlier, I have no idea how to find uncertainty here. I have emailed the TA and all my class mates, and they seem to be in the same boat as me, or haven't responded, and the TA has of yet to respond (yes, I have emailed her several times).

    The lab manual itself is very vauge on the matter. It merely says to "guess the area" uncertainty, and then explain how I got the uncertainty in my lab report (summed up version).

    I thank y'all for helping me in advance
  2. jcsd
  3. Oct 28, 2008 #2


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    Wouldn't there be uncertainty in these readings since you are looking from one image to another?

    Welcome to PF
  4. Oct 28, 2008 #3
    You'd think so, wouldn't you? Problem is, this was a new program, and the way the book showed us how to use it, all I got were the "average" values. And sadly the program is only avaliable at my college, which is a good hour commute there and whatnot, and even then I'd have to go into the lab classroom, which has a stupid timer lock that only unlocks when a class is going to start. >.>

    So, yes, there would be uncertainty. Problem is, I have no idea how to calculate it, nor do I have access to the images due to the above reasons.

    And thanks, I've used the search feature here a few times for problems, but my first time posting here. :D
  5. Oct 28, 2008 #4


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    If you were using a scale with division marks, I would use 1/2 a division as my uncertainty for each reading. i.e. since the eye can usually only resolve half a division. At least that's what I was taught.
  6. Oct 28, 2008 #5
    I guess that's as good as I can get it. >.>

    Thanks for your help, would like to know if anyone else has another approach.
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