Suppose there is constant diameter pipe where a liquid flows through it, and if pressure of a liquid is due to external force or its weight when it is constrained or semi constrained (according to the accepted answer on this question), how can someone visualize how does pressure drop occur due to frictional losses?
Does the force on the liquid decrease somehow or does it become less constrained so it has lower pressure?
As a mental cue, it's not much different from having sliding friction between a solid block being pushed over solid ground. If you shove the block, it will slide while constantly losing energy to friction until it stops. To keep it moving at a steady speed, you must march behind it continuously pushing, so that your pushing force and the frictional force sum to zero, making the block move at constant speed.
Likewise, the water is constantly encountering friction because the pipe walls cannot move. The water directly touching the walls does not move either; going inward from the wall the water gradually increases in speed, to a maximum speed at the center (as below).
Consequently, every layer of water in the pipe is shearing against the layers adjacent to it, continuously as it flows along. Like pushing the block, a source of power is required to counteract all this friction, or else the water would stop flowing.
