Although we can't calculate the force on each particle as it moves, nor can we calculate the force on the center of mass of a moving system, we can use the force applied to a piston to calculate the work done on a gas. The pressure times the change in the volume equals minus the work done on a gas.
Playlist: • An Introduction to Thermal Physics by...
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The textbook I am using is: Schroeder, Daniel V. "An Introduction to Thermal Physics" 1st ed., Addison Wesley Longman, 2005.