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Homework Help: Thermodynamics First Law Problem

  1. Sep 4, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider a rock having a mass of 5 kg and a bucket containing 50 kg of liquid water. Initially, the stone is 20 m above the water, and the stone and the water are at the same temperature, T1 (state 1). The stone then falls into the water.
    For the system stone + water, determine ΔU, ΔKE, ΔPE, Q and W for the following changes of state: 1 to 2, 2 to 3, and 3 to 4, with:
    (a) State 2: the stone is just about the enter the water;
    (b) State 3: the stone has just come to rest in the bucket;
    (c) State 4: heat has been transferred to the surroundings in such an amount that the stone and water are at the same temperature T1 (temperature of state 1).

    2. Relevant equations
    ΔE = Q - W (First Law)
    ΔE = ΔE(mechanical) + ΔU
    ΔE(mechanical) = ΔKE (kinetic energy) + ΔPE (potential energy)

    3. The attempt at a solution

    I think I am messing up when establishing my system... For example, when trying to calculate the kinetic energy of the system, it should be zero since the entire rock + water are one system and the entire system is not moving. However, the rock has kinetic energy from state 1 to state 2, so should I take that into account? Same with potential energy.

    I also have no idea how to calculate Q and W other than using ΔE = Q - W. Is there another way?

    Tips are appreciated!
  2. jcsd
  3. Sep 4, 2013 #2
    My main issue is deciding what is Q and what is W. Do I include the work done by the rock in the overall work done by the system?
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