This means the -3900N force was applied for 2 seconds.

NO !!!!

the mass has a final velocity of 39.038 m/s when it enters the turnaround after being accelerated.

the turnaround would have to have a lenght of
39.038 meters * 2 = 78.076 meters
for the mass to consume 2 entire seconds while passing through it.

so your wrong.

It's meaningless to add up forces while ignoring the times they're applied for. I'll include time below...

and its even less meaningless to include erroneous times , Im not sure why your so concerned with time as the time is given in the acceleration of the mass.

ie...5 meters / SECOND / SECOND...

meaning each second..

Maybe it's travelling a longer and longer distance each cycle too, or maybe it isn't. Havn't worked that out.

maybe that is where your insertion of 2 seconds will come in !!!

3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.