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Homework Help: Stuck on a problem please help

  1. Nov 7, 2007 #1
    A 0.25 kg pine cone falls from a branch 20 meters above the ground.

    a) What speed would it hit the ground if the air resistance could be ignored?

    I am not too sure how to do this but this is what I've got so far:

    g=9.8 m/s^2
    F=2.45 J
    h=20 m

    Ep=(0.25kg)(9.8m/s)(20m) Ek=1/2mv^2
    =4.9J 4.9J=1/2(0.25kg)(v^2)
    -0.25 -0.25
    4.775 = v^2
    root 4.775 = 2.2 m/s = v ?

    is this even close to correct. I think I am doing something wrong
  2. jcsd
  3. Nov 7, 2007 #2
    right method...check your calculations
  4. Nov 7, 2007 #3
    I am also not sure in my calculation of Potential Energy wether
    = 4.9 or 49
  5. Nov 7, 2007 #4
    Ep=(0.25kg)(9.8m/s)(20m) Ek=1/2mv^2
    =49J 49J=1/2(0.25kg)(v^2)
    -0.25 -0.25
    48.87 = v^2
    root 48.87 = 6.9 m/s = v ?
    Would this be correct then?
    Last edited: Nov 7, 2007
  6. Nov 7, 2007 #5
    looks good...answer makes more sense
  7. Nov 7, 2007 #6
    Thanks a lot!
  8. Nov 7, 2007 #7
    So there is a part B to the problem:

    If it actually hits the ground with a speed of 9.0 m/s, what was the average force of air resistance on it?

    Since I got a velocity of 6.9 m/s with 0 air resistance in the first part of the problem wouldn't there be some force accelerating this cone in this part? Because a falling cone with air resistance should hit the ground slower than a cone with no air resistance?
  9. Nov 7, 2007 #8
    watch your algebra in part (a)
  10. Nov 7, 2007 #9
    Is it still wrong? I cannot seem to find where I made a mistake? Which section of the calculation?
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