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Old Jul 12th 2017, 03:12 AM   #1
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Rock falls in water

A rock of mass $\displaystyle m$ is at a high $\displaystyle h$ at rest. Under this height there is a container of water of mass $\displaystyle M$, deep $\displaystyle d$. The rock, the water, the air have the same temperature (condition 1). Find the variations of internal energy, the heats and the works transferred in the system rock+water from the condition 1 to the other conditions:
Condition 2: the rocks is going in the water (shortly before)
Condition 3: the rock has just stopped at the bottom of the container
Condition 4: the rock, the water, the air returned to the initial temperature.
The friction between rock and air is negligible.
How can I deal with this problem?
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Old Jul 12th 2017, 05:28 AM   #2
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Originally Posted by Berker View Post
A rock of mass $\displaystyle m$ is at a high $\displaystyle h$ at rest. Under this height there is a container of water of mass $\displaystyle M$, deep $\displaystyle d$. The rock, the water, the air have the same temperature (condition 1). Find the variations of internal energy, the heats and the works transferred in the system rock+water from the condition 1 to the other conditions:
Condition 2: the rocks is going in the water (shortly before)
Condition 3: the rock has just stopped at the bottom of the container
Condition 4: the rock, the water, the air returned to the initial temperature.
The friction between rock and air is negligible.
How can I deal with this problem?
Well, what do you know? Before the rock is dropped, condtion 1, it has potential energy relative to the ground. What is that potential energy? In condition 2, shortly before the rock hits the water, the rock has less potential energy. What is its potential energy now? Of course, that lost potential energy has converted to kinetic energy. What is its kinetic energy? When the rock has stopped at the bottom of the container, it has 0 kinetic and potential energy. By the 'conservation of energy' that must have gone somewhere. Where did it go? Find the temperature of rock and water from that.

Last edited by HallsofIvy; Jul 15th 2017 at 09:24 AM.
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