tbr test 3 physical sciences q 8

[2010premed]

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When the flow directions of both one-way valves are reversed while the press chamber is full and the press plate is raised, the press plate:
A. descends at a gradually decreasing rate.
B. descends at a gradually increasing rate.
C. descends at a constant rate.

Ans: A

I thought it was C b/c in the passage it states that "The press plate is sufficiently heavy so that when fluid flow is back toward the reservoir, the plate descends completely by gravity and empties the chamber." So I thought that since it's completely due to gravity, then it must be constant...
The explanation says that somehow as fluid flows from press chamber to reservoir the pressure changes, I don't get how the pressure is decreasing... and then it says that since the change in pressure is decreasing then flow rate also decreases (Pois. formula).

Exp: When the press plate is raised (at its highest point), fluid in the press chamber is held there against natural, gravitationally induced flow by Valve 2. As soon as the valve is opened to allow backflow, assuming that the press plate may move freely, the fluid returns to the state of lowest potential energy. Because of the potential energy difference between the reservoir and press plate, the fluid in the press plate chamber is at a higher pressure than the fluid in the reservoir. As the fluid drains from the press chamber and fills the reservoir, the pressure difference decreases. According to Poiseuille's law, as the pressure differential across a pipe decreases, so does the volume flow rate. As the fluid drains, the press plate lowers, and the potential energy of the fluid gradually decreases. The flow rate and the rate at which the press plate descends in turn also decrease.

 

[PiBond]

less fluid is leaving the hydraulic press chamber as time goes on because after a while there starts to be water pressing back on the valve 2 which reduces the pressure difference. this reduces Q (flow rate). while the force due to the plate is constant (force of gravity), there is a reduced pressure difference as a result of the amount of water building up in the reservoir. therefore, the plate should descend at a decreasing rate since there's a decreasing pressure difference as time goes on because the reservoir is filling

 

[2010premed]

Ah, ok I see it now. One things that I still am not seeing though is how to reconcile the constant force with the plate descending at a decreasing rate, I see now that the rate that the plate descends relies on the flow rate and not the force of gravity, but how can the constant force of gravity still be there?

 

[PiBond]

Ah, ok I see it now. One things that I still am not seeing though is how to reconcile the constant force with the plate descending at a decreasing rate, I see now that the rate that the plate descends relies on the flow rate and not the force of gravity, but how can the constant force of gravity still be there?

the force of gravity on the plate is just "mg". it's not going anywhere and is pointing in the same direction the entire time.