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Work-Kinetic Energy Problem

  1. Mar 17, 2012 #1
    1. The problem statement, all variables and given/known data

    A 0.16 kg pinecone falls 18 m to the ground where it lands with a speed of 11 m/s.

    a)How much work was done on the pinecone by air resistance?

    b)What was the average for of air resistance exerted on the pine cone?

    2. Relevant equations

    ƩF=ma

    Vf^2=Vi^2 + 2aΔx

    W=FΔx=maΔx=1/2(mVf^2)-1/2(mVi^2)

    3. The attempt at a solution

    I tried using the kinematic equation to find initial velocity while setting the acceleration as gravity. I got 18.78 m/s but that doesn't seem right. I tried using that initial velocity in the work-kinetic energy theorem and it came out to -28.275. However that was the wrong answer. I don't understand where to start with this problem let alone part B. Someone help me please.
     
  2. jcsd
  3. Mar 17, 2012 #2
    The 18.78 m/sec represents the speed if there were no wind resistance. Note that it is larger than what was presented in the problem statement.

    Write an energy balance such as:

    PE(1) + KE(1) = PE(2) + KE(2) + Work
     
  4. Mar 17, 2012 #3
    The net work done is equivalent to the work by gravity minus the work by friction.

    EDIT: My way is essentially a rearrangement of LawrenceC's formula above.
     
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