Thermodynamics First Law Problem

  • #1

Homework Statement

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).

Homework Equations

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

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
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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