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Homework Help: I only have three variables

  1. Sep 2, 2011 #1
    1. The problem statement, all variables and given/known data

    Physics: Only three variables and I don't know how to solve?
    here is one problem: Heather and Jerry are standing on a bridge 50 m above a river. Heather throws a rock straight down with a speed of 20 m/s . Jerry, at exactly the same instant of time, throws a rock straight up with the same speed. Ignore air resistance. --- I need to find the time elapsed between the splashes.

    A rock is tossed straight up with a velocity of +20 m/s. When it returns, it falls into a hole 10m deep. ----I need to find velocity and time for the rock at the moment it reaches the bottom of the hole .

    For both of these I only have three variables, Velocity initial, change in X, and A. Should I assume that the V final is 0 in these?? otherwise I am confused. Thanks!!

    2. Relevant equations

    all 5 standard kinematics

    3. The attempt at a solution

    The questions is conceptual...currently i cannot attempt an answer as far as I know.
  2. jcsd
  3. Sep 2, 2011 #2
    You have 3 variables, not including one variable that you want to find (work out what this one is first!) Then go through your 5 equations and use the one that includes all 4. You might have to do some rearranging!
  4. Sep 2, 2011 #3


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    Homework Helper

    The first of your questions is easier than it looks, if you see the trick...
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