Originally Posted By: paul

we now have 1 mass and thats all.



accelerating this 1 mass results in a force of +3903.85N

500N. That's the force supplied to the mass, and equally the pipe. That force is applied for 7.8s


it goes through the first turnaround
applies a force of -3903.85 N to the pipe.

OK. Change in momentum = final momentum - initial momentum
= -39m/s*100kg - 39m/s*100kg
= -7800 kg m/s (Ns)
This means the -3900N force was applied for 2 seconds.


it then goes through the second turnaround
and applies a force of +3903.85 N to the pipe.

OK. So again the force of +3900N is applied to the pipe for a time of 2s.

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

the forces add up as follows

+500 N acceleration for 7.8s. Impulse = 500*7.8 = 3900Ns
Pipe moves forwards with p=3900 kgm/s, v=7.8m/s

-3903.85 N 1st U turn for 2s. Impulse = -3900*2 = -7800Ns
Pipe reverses direction and has v=-7.8m/s

+3903.85 N 2nd U turn for 2s. Impulse = 3900*2 = 7800Ns
Pipe reverses direction again, v=7.8m/s

I'll do the next cycle. I suppose the accelerator applies the same impulse (change in momentum) each cycle.

So we go on with ..
Impulse = +3900 kg m/s, pipe speeds up to v=15.6m/s
Impulse = -15,600 kg m/s, pipe reverses to v=-15.6m/s
Impulse = +15,600 kg m/s, pipe reverses again to v=+15.6m/s

After 2 complete cycles it's still oscillating back and forth, but going faster and faster.

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