There are difficulties with the concept of work (1-9), particularly for irreversible processes. A thoughtful writer on that topic has said (9) "For mechanically irreversible volume changes, we usually cannot calculate the work from thermodynamic considerations." We believe that conclusion is incorrect. In this paper we give definitions of work and heat that overcome earlier difficulties. Both quantities are defined in terms of experimental quantities. The definitions are simple and general. They apply equally well to reversible and irreversible processes. Using these definitions, we are able to resolve several important problems as well as to obtain fresh insights into the First Law. Some of the results are surprising. They force a reevaluation of one's thinking about work and heat.

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The adiabatic piston problem involves two ideal gases contained in a horizontal cylinder and separated by an insulated piston that moves without friction. Initially, the piston is held in place, and the pressures of the two gases are different. When the piston is released, it undergoes oscillations that are damped due to frictional effects caused by the gases, and its motion finally ceases. The final pressures of the two gases are equal, but the final temperatures are typically different. Approximate solutions are developed for the time dependence of the piston motion, the temperatures of the gases, and the entropy changes of the gases. These results are compared with exact solutions of the coupled differential equations derived by Crosignani et al. ͓"Approach to thermal equilibrium in a system with adiabatic constraints," Am. J. Phys. 64, 610-613 ͑1996͔͒.