Turning Work into Thermal Energy in Joule’s Experiment In Joule’s famous experiment where he determined the specific heat capacity of water, he used a paddle to increase the temperature of the water. That is, the kinetic energy of the paddle went into the thermal energy of the water. My question is exactly how did the paddle’s kinetic energy become the water’s thermal energy? Was it because


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*The paddle experienced resistive forces in the fluid that dissipated its energy, much the same way friction converts mechanical energy into thermal energy?

*The paddle exerted a force on the fluid particles that caused them to move in the same rotational motion as the paddle, thus imparting kinetic energy to them (i.e. increased internal energy)?

 A: The kinetic energy of the paddle didn't really change.  Its rotational speed was constant.  There was viscous dissipation (analogous to the action of a viscous damper) taking place within the fluid that converted the work done by the paddle into internal energy.
A: There is no either-or between your two answers. These are two aspects of the same. 
By Newtons "actio=reactio", that a force is working on the paddle is equivalent to that the paddle acts with a force on the water. The energy lost by the paddle is gained by the water. If there is a constant driving force on the paddle, then the paddle gets the same amount of energy that it lost to the water back from the driving force. 
In a multiple choice test I'd rather take your second option, though, because the first formulation is worse. I would call "dissipation" the act of converting kinetic energy into heat. And this will take place mostly after the interaction of the paddle and the water: first, the water is set in motion, then it gets slowed down by internal friction and heats up. 
