Originally Posted By: paul
they seem to work well for me , whats the problem that you have with them.


They work well because you invent suitable units and meanings that lead to the 4000N force you somehow decided you should be getting. Doing it correctly, you have no choice about units, you have no choice what the answer's going to be, it just comes out one way, like it or not.


Quote:

how do you determine acceleration if you dont use
distance and time , where do you start?
suppose you dont know what the force is , or you dont know what the mass is?


If you know enough other variables, you don't need to know distance. In this case we know the time it takes, we know the velocity change it experiences, we know it's mass, so we can calculate the force as below.



Quote:

could you please put your equation up again , I cant find it.



The mass spends 1s turning around.
It enters with a _velocity_ of -40m/s
It leaves with a velocity of +40m/s
It's velocity changes by (+40m/s) - (-40m/s) = 80m/s
It does that change in 1s.
Acceleration = (change in velocity) / (time taken)
Acceleration = 80m/s / 1s = 80m/s^2
The _mass_ must experience a +ve force to turn around. That force is:
F=ma
F=100kg * 80m/s^2
F=8000N
Furthermore, the pipe must experience the same force that the mass does, but in the opposite direction. That force is
F_pipe = -8000N



Quote:

and Im not playing a fool , Im genuinely concerned with how
you get 8000N from 4000N


Maybe your stumbling block is assuming it somehow 'started' as 4000N then increased to 8000N. There was never any 4000N to begin with.