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Help! Total mechanical energy question

  1. Apr 20, 2008 #1

    adv

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    [SOLVED] Help! Total mechanical energy question:)

    Q. Suppose you are standing on a 30 m building and throw a 1 kg rock upwards at 20 m/s. Knowing that total mechanical energy is conserved, calculate the speed of the rock when it hits the ground.


    Total Mechanical Energy equation:

    Etotal = Egravitational + Ekinetic
    Et = Eg + Ek
    = mgh + ½mv2


    What is the speed of the rock when it hits the ground? Well, the gravitational potential energy would be 0, if the reference point is the ground, yes? The upwards motion of the rock has thrown me off:confused:
     
  2. jcsd
  3. Apr 20, 2008 #2

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    Correct me if I'm wrong, but when the rock hits the ground, wouldn't the speed be zero? It's not moving.
     
  4. Apr 20, 2008 #3

    PhanthomJay

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    The problem is asking you to determine the rock's speed at the instant just before it hits the ground.
    Since total mechanical energy is conserved, the mechanical energy that the rock has at the instant it leaves the thrower's hand must be equal to its total mechanical energy at the instant just before it hits the ground.
     
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