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Homework Help: Quick question regarding photogates

  1. Feb 26, 2015 #1
    Hey everyone,

    I had a quick question that arose in a lab last week and think this is the best place to post it. Please correct me if I am wrong. It is not a "homework" question so much as a question that came to me in a lab.

    1. The problem statement, all variables and given/known data
    In my lab last week, we were using air tracks and photogates to illustrate the transfer of potential energy to kinetic energy in a cart. At the beginning of class my professor made it clear that the photogate had to be perpendicular to the track in order to measure the cart's true velocity. I haven't been able to stop thinking about this statement since the class and was hoping someone could clarify to me why the photogate must be perpendicular to the air track (and therefore direction of motion of the cart).

    So my question, I guess, is: does the angle of the photogate to the air track affect the velocity measurement of the cart?

    *The flag thickness is negligible*

    2. Relevant equations

    3. The attempt at a solution

    I tried drawing it out and breaking the length of the "flag" on the cart and the velocity into components to calculate the time the laser is obstructed. If i'm not mistaken, the angle of the track to the photogate cancels out.


    I've attached an image of my work so far. I defined "L" to be the length of the flat flag used to trigger the photogate, "v" to be the velocity vector of the cart, and "theta" to be the angle in between the track and photogate.

    Am I doing something wrong here or was my lab professor mistaken?

    Thanks in advance!
  2. jcsd
  3. Feb 26, 2015 #2


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    Staff: Mentor

    Hi josbear. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Does the photogate contain a single laser beam blocked for a duration by the flag, or does it contain two parallel beams interrupted in succession by the flag?
    Last edited by a moderator: May 7, 2017
  4. Feb 26, 2015 #3
    Hi NascentOxygen,

    The photogate has a single laser beam that is blocked by the flag.
  5. Feb 26, 2015 #4


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    Staff: Mentor

    And the geometric shape of that flag is ....?
  6. Feb 26, 2015 #5
    The flag is a rectangular piece of sheet metal with negligible thickness.
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