Two pistons are inserted into a horizontal pipe with cross-sectional area A and length 2l. The left piston is connected to a wall by a spring with a force constant k. Initially, the left piston is at the end of the pipe, and the right piston is in the middle of the pipe as shown. The spring is relaxed, and the pressure P between the pistons equals the atmospheric pressure.

Then the right piston is slowly pulled to the right until it reaches the end of the pipe. What force F must be applied to the piston to keep it in that position? The pipe remains at rest. Neglect friction and assume constant temperature.

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