Answer :
When fluid is flowing through the pipe.
Why?
No fluid results in the following:
Sum forces in y-direction: sum F = ma, W_pipe + W_fluid = ma, however if there was no fluid it be W_pipe = ma.
Why?
No fluid results in the following:
Sum forces in y-direction: sum F = ma, W_pipe + W_fluid = ma, however if there was no fluid it be W_pipe = ma.