When it comes to thermal/statistical mechanics, I'm a little retarded... so bear with me please. I've used the helmholtz free energy (F) dozens of times, almost as many times as I've heard and read that F is a measure of the 'useful work attainable from a closed thermodynamic system,' at constant temperature and volume. I don't really understand what that means though. How does a closed system even do work when its temperature and volume are fixed (therefore it can't lose heat and can't exert p dV work right?) Again, I've seen the equations, and heard the lines; but I'm hoping some enlightened and compassionate soul can share their wisdom and philosophical insight to what it all really means. Thanks!