We can safely assume friction is zero. In reality we can get arbitrarily close to zero and add a bit of energy every now and then to compensate. The mass could even be an electron travelling through a superconductor, where there's exactly zero friction.
the 100 kg mass never presses against the pipe
in the (+) direction as it passes through the 1st turn.
so the pipe never has a (+) force placed on it , that
would cause the pipe to move in the (+) direction other
than the friction between the mass and the 1st turn.
Yes I agree. The _pipe_ never has a + force on it in the 1st turn. But the mass does. That's from Newton's 3rd law: action and reaction are equal and opposite. You used this law to find that both the pipe and mass experience a force in the accelerator.
When I say 'stop' I mean stop in the direction we're considering. Sure it keeps going at a speed of 40m/s, but as its velocity changes from backwards to forwards, the longitudinal component of it must obviously pass through 0.
It's the same with a pendulum. The _vertical_ component of velocity reaches 0 at the bottom of the swing. Sure it keeps moving, but it's stopped in the vertical direction. If you remove all forces at that moment, then it'll remain stopped in the vertical direction - and fly off in a straight line sideways.
If you're saying the mass doesn't experience a force in the turnaround, then you're saying Newton's 1st, 2nd and 3rd laws are all wrong.
The 1st law says it should keep going in a straight line, since there's no force acting on it. But it's turning.
The 2nd law says F=ma: zero force = 100kg * zero acceleration. But it's clearly accelerating because its direction is changing.
The 3rd law says if it applies -4000N to the pipe, then the pipe applies +4000N to the mass.
