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Kinematics question

  1. Sep 13, 2008 #1
    A flea jmps to a maximum height of 0.390m. Find vi as it leaves the ground.

    I thought of using the equation vf^2 = vi^2 + 2ad
    but in this case, if i wanna find out vi, I get
    -vi^2 = 2ad.
    and because of the negative, it doesn't work...

    How do you solve this problem??
  2. jcsd
  3. Sep 13, 2008 #2
    Acceleration due to gravity: a = -9.8m/s2 note the minus sign!

    The reason why this is negative: the flea is jumping up (positive direction), acceleration is acting down (negative direction as it is opposite to the direction of the jump)

    This cancels out your problem of negative displacement when you realize you are working with vectors that have different directions.
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