1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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:

    m=0.25
    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)
    2(4.9J=1/2(0.25kg)(v^2))
    9.8J=(0.25kg)(2v^2)
    -0.25 -0.25
    9.55J=2v^2
    ------------
    2
    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
    Ep=(0.25kg)(9.8m/s^2)(20m)
    = 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)
    2(4.9J=1/2(0.25kg)(v^2))
    98J=(0.25kg)(2v^2)
    -0.25 -0.25
    97.75J=2v^2
    ------------
    2
    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?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?