Joule's Apparatus and Kinetic Energy

[Bennigan88]

In my textbook, Joule's apparatus (two blocks falling from height h turn a paddle that stirs and increases the internal energy of water) is claimed to convert the potential energy of the two blocks into internal energy that can be measured by the increase in temperature of the water... but what happens to the kinetic energy of the blocks as they are falling? They have velocity and mass so there must be kinetic energy, how is that accounted for in the setup of the apparatus?

 

[Simon Bridge]

That has to be factored into the experimental design and the final calculation of course. If your book claimed that all the falling energy becomes heat then your book is in error - but I suspect they do not make such a claim.

The kinetic energy of the blocks is less than if there was no water... they fall slower.
The water also gains rotational kinetic energy, and there are issues with calorimetry.
All these can be taken into account in a careful setup. Joule worked out the mechanical energy deficit and demonstrated that this was consistent with the heat increase in the water. The experiment more easily disproves the, then accepted, calorimic theory of heat that treated heat as a substance in it's own right.
See also:
http://www.eoearth.org/article/On_the_Mechanical_Equivalent_of_Heat_(historical)