Originally Posted By: paul

if the spring stops the mass in 0.5 seconds it requires 8000N
to do that , it then has 8000N stored force to push the mass
away again accelerating the pipe in the opposite direction.

Indeed. If 'opposite direction' means opposite to the direction the accelerator started moving the pipe in

Quote:

so if the turn is increased in lenght to 80 meters the
pipe would stop , and the mass would still have a speed of 40 m/s when it exits the turn.


Careful, all these variables aren't independent of each other. Since it's twice as long, the mass now spends 2s in the turnaround, so the overall effect is half the force applied for twice as long.

When I earlier worked out that the pipe would reverse direction, I was using a 1s turnaround. With a 2s turnaround it only needs half the force to achieve the same result.