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Homework Help: Velocity with changing direction

  1. Feb 1, 2008 #1
    A particle moving at 5m/s reverses its direction in 1s to move at 5m/s in the opposite direction. If its acceleration is constant, what is its displacement from its original position at 1s?

    I know how to solve the equation:
    1) x = x0 + v0t + ½ at2
    2) a is unknown therefore solve for a

    My question is how do you know the the initial velocity is negative? I would say the initial velocity is 5m/s and after it changes direction velocity is then -5m/s. The solution says the opposite, i.e. initial velocity is -5m/s and final velocity is 5m/s. How do you know? Is there some hard and fast rule I am unaware of?

    Please help.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 1, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    It doesn't make any difference which direction you choose as plus or minus, as long as you are consistent in your sign convention. Also, you're going to need another equation in addition to (or in substitution of) the one you have shown to get the answer. No matter what, the plus and minus sign will wear down the best of us.
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