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How to extract information from question given that u=0

  1. Sep 2, 2015 #1
    1. The problem statement, all variables and given/known data
    An electron falls through a distance of 1.5 cm in a uniform electric field of magnitude 2.0 x https://www.physicsforums.com/callto:104 [Broken] N C-1. The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance . Compute the time of fall in each case

    2. Relevant equations
    s=ut +1/2 at^2

    3. The attempt at a solution
    In my book the following formula is used
    here s is distance and a is acceleration
    it is used when u is zero
    u here means initial velocity
    so my query is why should we take initial velocity to be zero as nothing such is mentioned in the question
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Sep 2, 2015 #2


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    Homework Helper

    I think this is a standard assumption when you are given no information that the electron was already moving.
    If I said, a ball falls 1.5cm under uniform gravity, how long does the ball fall for? You might intuitively assume that the initial downward velocity of the ball is 0.
  4. Sep 2, 2015 #3
    And what about velocity in other directions?
  5. Sep 2, 2015 #4
    I think answer is hidden in my question only!as nothing such is mentioned in the question, we should assume that.
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