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Homework Help: Velocity, Force of Friction, and Total Energy in a System Question

  1. Apr 19, 2012 #1
    1. The problem statement, all variables and given/known data
    A 200 kg boulder falls from rest off a 120.0 m high cliff. If the boulder experiences a frictional force of 280 N as it falls, what would be the velocity just before it hits the ground?

    2. Relevant equations

    kinetic energy = (1/2)mv2
    gravitational potential energy = mgh
    work = (net force)(Δd)

    3. The attempt at a solution
    Total force = mgh = 200kg*9.8N/kg*120m = 235200 J

    Wfriction = FΔd
    Wfriction = 280 N*120 m
    Wfriction = 33600 J

    total energy = kinetic energy + work done by friction
    kinetic energy = 235200 J - 33600 J
    kinetic energy = 201600 J
    201600 J = (1/2)mv2
    v = 45 m/s

    However, according to my teacher, this answer is incorrect.
  2. jcsd
  3. Apr 19, 2012 #2
    What does your notation "N" represent? If, as I suspect, it is saying that the friction force is proportional to velocity (this is typical for air drag), then your treatment of the friction as constant is not appropriate here.
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