A model of a thermodynamic system is described in which particles (representing atoms) interact with one another, the surroundings, and the earth's gravitational field according to the principles of classical mechanics. The system's energy "E" and internal energy "U" are defined. The importance is emphasized of the dependence of energy and work on a frame of reference. The changes delta"E" and delta"U" during a process are shown to depend on displacements of the particles at the system boundary and on the contact forces exerted on these particles by the surroundings. Formulas in terms of these displacements and forces are derived for the work and heat appearing in the first law of thermodynamics. The work and heat described by these formulas are shown to be quantitative transfers of energy across the system boundary. (Contains 7 notes and 3 figures.)