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Homework Help: Uncertainty principle

  1. Nov 4, 2009 #1
    1. The problem statement, all variables and given/known data
    A mosquito of mass .15mg is found to be flying at a speed of 50 cm/s within an uncertainly of .5 mm/s.

    a) How precisely can it's position be known?

    2. Relevant equations

    Delta p * Delta x = h bar / 2
    p = mv

    3. The attempt at a solution

    I look at problem originally and simply solved for momentum using the given velocity 50 cm/s. Then i plugged that into the delta x * delta p = h bar / 2.
    I got the answer 7.033 X 10^-31

    Then I realized that I never used the variable describing the uncertainly of the fly's velocity. Clearly this is relevant to the solution of the problem. I think that I calculated the acutal momentum, as opposed to the uncertaintly of momentum, which the above equation calls for. How am I supposed to differentiate between the actual velocity and the uncertaintly of velocity?

    Does the problem want me to give a range of positions? I could solve for the uncertainly of momentum using 50 cm/s + or - .5 mm/s. Could I then could find a max and a min position using the uncertainly equation? What exactly should I be solving for?

    Thank you in advance!
  2. jcsd
  3. Nov 4, 2009 #2
    You found momentum p, which is not what the equation uses. It uses delta p.

    Given that [itex]p=mv[/itex] , then [itex]\Delta p=m\Delta v[/itex].

    The problem just wants to know delta x.
    Last edited: Nov 4, 2009
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