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Free Fall (kinematics-what I do wrong?)

  1. Aug 31, 2006 #1
    A rock is tossed straight up with a velocity of +20m When it returns, it falls into a hole 10m deep.
    What is the rock's velocity as it hits the bottom of the hole?
    So basically it is asking for the impact speed right?
    So that would be
    Vf^2=Vi^2 +2adelta(s)
    It says it is wrong. I know 30.41 is the right amount of the total meters in the fall since I solve another part and got the answer right. So that means that the kinematic equation is wrong so which would it be?
  2. jcsd
  3. Aug 31, 2006 #2
    never mind it is -24.4 instead of positive.....
  4. Sep 1, 2006 #3


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    Homework Helper

    You should be careful when you type questions ,
    you had said "with a velocity of +20 m" . . .
    but +20m is a location, not a velocity.

    you also ended with vf^2 = 24 , rather than vf = 24 ...
  5. Sep 1, 2006 #4
    I always find it easy to simplify stuff in physics, like if a car going at 50m/s is chansing a car at 30m/s, it's good to think of a car going towards a stationary object at 20m/s. In this case, ignoring wind resistance, the rock will reach the point you throw at the same speed (note I don't say velocity) as you threw it up with. So it's like a rock falling down at 10m hole where initial velocity is 20m/s (if that is what you meant by 20m).
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