Suppose we have two syringes -- both in the halfway position -- filled with water and connected by a tube (there is no air in the system, only water).

If we push one of the syringe plungers in, then the other syringe's plunger will move outward. I'm content with this

What happens if I pull out one of the syringe plungers though? Would the other plunger move in?

  • If the extra volume I have introduced was filled with air, then I think maybe nothing would happen, since the air would maybe keep the pressure the same?
  • But since there is no air in the system, I think this would introduce a vacuum, at which point I don't know what happens. One thought is that pressure in the syringe whose plunger I pulled out would have to decrease (since the volume has increased), but I don't know whether this would move in the other plunger.

So to state the question once more: Suppose two plungers are connected as described in the first paragraph. If we pull one of the plungers out, what happens to the plunger of the other syringe, and why does this happen? (i.e. if a force is being exerted what is causing this force?

  • 1
    $\begingroup$ Sounds like you are assuming that the tube that connects them will not collapse. I probably can find samples of flexible tubing in the shop at work that will not collapse before the plunger of the second syringe moves, and I probably can find other samples that will collapse. $\endgroup$ Commented May 29, 2020 at 19:40
  • $\begingroup$ @SolomonSlow Yes, I was implicitly assuming that $\endgroup$
    – user106860
    Commented May 29, 2020 at 19:52

1 Answer 1


If you pull one plunger out by some small amount (delta L) this asserts a low pressure on the fluid throughout the system. Atmospheric pressure then pushes on the outside of the piston in the other syringe, and the net effect is the transfer of a volume of liquid in the amount (piston area x delta L) from the one syringe into the other that you are pulling on.

  • $\begingroup$ Let me see if I understand correctly: initially (before pulling), the pressure on the fluid must be = to the pressure of the fluid in the other syringe, which both equal atmospheric pressure (assuming everything is in equilibrium). What I pull on the plunger (by delta L), the pressure in that syringe decreases, and then the atmospheric pressure pushes on the outside of the other piston to equalize everything again? $\endgroup$
    – user106860
    Commented May 29, 2020 at 19:51
  • $\begingroup$ yes that's it exactly $\endgroup$ Commented May 29, 2020 at 20:32

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