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Homework Help: Projectile Motion stone dropped

  1. Oct 16, 2008 #1
    1)If a stone dropped (not thrown) from a bridge takes 4.2 s to hit the water, how high is the rock-dropper? Ignore friction.

    2)A kangaroo can jump straight up about 2.0 m. What is its takeoff speed?

    I suck at physics so yeah...
  2. jcsd
  3. Oct 16, 2008 #2
    You don't suck at physics, you just need to get in the right frame of mind:biggrin:

    Check out this thread: https://www.physicsforums.com/showthread.php?t=110015" for useful formulae.

    List all the variables that you know and the variables that you need to know., then find a formula that includes everything you know and one thing you don't. Substitute into the formula and you will get the answer you seek.

    For your first question, you know several things:
    V0 (initial velocity) = 0
    a (acceleration) = g (gravity, or 9.8 m/s2)
    t (time) = 4.2s
    x0 (starting position) = 0

    what you are looking for is
    x (finishing distance)

    Find the formula, plug the numbers in, find your answer.

    Your second question takes a little bit more thinking, but try it out and let me know how far you get.
    Last edited by a moderator: Apr 23, 2017
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