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Homework Help: Energy needed/energy lost

  1. Jan 30, 2012 #1
    1. The problem statement, all variables and given/known data
    So, I need a little help with a few problems
    So, say a 2kg pine cone drops from a tree 41 meters high. If it hits the ground at 10m/s then how much was lost to entropy?
    A 1800kg roller coaster begins at the top of 190m hill, at what height will the velocity be 24m/s and what will be its kinetic energy when it reaches a height of 10m above the ground.?
    Thanks for any or all help.

    2. Relevant equations
    Energy of work = Force times distance
    Gravitational potential = mgy
    Kinetic Energy = .5 time mass time velocity squared

    3. The attempt at a solution
    For the 2nd one, I used mgy= .5 time mass time velocity squared but submitted 24 for the velocity to find y but I don't think it's right.
    For the 1st, I used mgy+.5 time mass time velocity squared to find the energy but don't know where to go from there.
  2. jcsd
  3. Jan 30, 2012 #2
    Energy initial = energy final

    potential energy of pine cone up in tree =

    kinetic energy of pine cone before it hits + energy "lost" on the way down

    You can figure out two of those to find the third.

    For the second part,

    Energy initial = energy at some intermediate step

    potential energy up high =

    potential energy up not so high + kinetic energy at that not so high place
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