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Homework Help: 2D Kinematics - 2 rocks off a bridge

  1. Feb 5, 2014 #1
    2D Kinematics -- 2 rocks off a bridge

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

    Heather and Jerry are standing on a bridge 60m above a river. Heather throws a rock straight down with a speed of 14m/s . Jerry, at exactly the same instant of time, throws a rock straight up with the same speed. Ignore air resistance.

    a. How much time elapses between the first splash and the second splash?
    b. Which rock has the faster speed as it hits the water?

    2. Relevant equations

    3. The attempt at a solution

    H and J are 60 m above a river.
    Heather throws the rock down with a speed, v=14 m/s
    Jerry throws it up, v = 14 m/s

    a. I'm assuming that I have to use some vertical/horizontal motion formula, but I don't know...
    b. Don't they both have the same acceleration 9/8 m/s^2 so the same speed?
  2. jcsd
  3. Feb 5, 2014 #2
    You will not need to worry about horizontal motion.
    You are right that they will both have the same acceleration, but they both have different initial velocities.

    Look for a kinematics equation that has everything you need. It should have...
    v0, g, t, y0, y

    You have already identified v0, g. What are t, y0, and y?
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