If you have a structure like a large bike air pump (i.e cylinder, air hose at bottom, plunger etc) and you make the following adjustments: 1. insert the hose back at the top of the pumping cylinder (making it a cyclical system) 2. fill the cylinder with water so that there is no air in cylinder and hose....the whole system is filled with water 3. completely seal/enclose the unit so that it is enclosed fluid Questions: 1. if the plunger is near the bottom of the cylinder, do you need a force large enough to move the equivalent mass of water in the cylinder above? I.e if the mass of water above the plunger in the cylinder is 10kg (it's a large bike pump) and the plunger is 0.5 kg, then the force needed to move the water even a little is over (10.5kg * 9.81ms) = 103N (ignore friction)?? Or is it the fact that there is a slight pressure differential coming from the full column of water in the tube that will have a piston effect? So what I am thinking is that even the slightest force that is just bigger than the force of the plunger (0.5kg*9.81) will result in the plunger moving up which results in the water at the top forced down the tube and back up below the plunger (large piston) 2. Would it be any different (answer) if the pump was not enclosed and large cylinder is open at the top (provided we did it slowly so as to not spill over top)?