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Physics 20-1: Energy

  1. Jan 8, 2009 #1
    1. The problem statement, all variables and given/known data
    a) A 0.25 kg pine cone falls from a branch 20 m above the ground. if it actually hits the ground with a speed of 9.0m/s, what was the average force of air resistance exerted on it?

    b) For the pulley system illustrated to the right, when the masses are released, what is the final speed of the 12kg mass just before it hits the floor?

    * the diagram is basically a 4.0 kg box at rest on the floor with a rope attached to it, the rope is thrown over a pulley, and attached to the hanging 12 kg box. the total distance that the 12 kg box travels is 5.0m.

    2. Relevant equations
    i have for a) W = F d and for b) Ep = mgh, Ek = 1/2mv^2


    3. The attempt at a solution
    i have no idea how to get started.
     
  2. jcsd
  3. Jan 8, 2009 #2
    For the first you have to use that the air resistance force, is proportional to the square of the velocity in this case.
    For the second one, just use conservation of energy
     
  4. Jan 8, 2009 #3
    For the first you have to use that the air resistance force, is proportional to the square of the velocity in this case.
    For the second one, just use conservation of energy
     
  5. Jan 8, 2009 #4
    For a)
    consider other definitions of mechanical work. e.g. the change in kinetic energy.

    A starting point for b) should be to consider the forces acting on each box
     
  6. Jan 8, 2009 #5
    thx guys! i figured them both out. now i just need to be able to apply them in my exam today xD
     
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