Re: can a liquid provide an upward and downward force at the same time?

Date: Sat Jul 19 11:35:52 2003
Posted By: Gareth Evans, Industrial R&D practitioner and manager ( retired )
Area of science: Physics
ID: 1058475503.Ph
Message:

Question 1058475503.Ph: Can a liquid provide an upward and downward force at the same time? I'm having some difficulty fully understanding buoyancy. If I take a plunger of the type used to unblock drains and then I attach it to the bottom of a swimming pool using its "suction". I think water pressure will be holding it at the bottom but also a buoyancy force will be trying to raise it to the surface of the pool.

I think you understand buoyancy, that is, when an object is less dense than the surrounding fluid, the lowest potential energy of the combined system is when the more dense component is lower. Assuming your plunger is “lighter” ( less dense ) than water it will float.

Often, conflicting influences occur, such as opposing forces so it is not unreasonable to expect that is the case in your example. Let’s look at how the plunger works in this case. When you attach the plunger to the bottom of the pool you push it towards bottom and expel some of the water past the plunger rim. The rim forms a seal between the volume of water in the plunger cup and the rest of the water in the pool. When you release the plunger it attempts to assume its original shape due to the elastic nature of the cup’s rubbery material.

It’s a little easier to see what is happening when the operation is carried out in air. You may need a little water to help form the seal even on quite smooth surfaces. After pushing the plunger, when it is released it will move away from the surface as it attempts to return to its original shape. This expands the volume inside the plunger thus lowering the pressure. There is now a difference in pressure between inside and outside the cup and the net pressure from outside provides the force which prevents the cup expanding further. In other words a balance of forces is established.

Returning to the swimming pool, the same effects occur but because water is effectively incompressible by these relatively small forces, the volume change is negligible even though a pressure difference is established. If you try to pull the plunger away, the pressure difference will increase to frustrate your attempts, until, that is the seal is broken and the pressure difference is eliminated. The plunger stuck to the bottom of the pool will have a small force, due to its buoyancy, attempting to pull it away but all this will do will be to produce a very small deformation of the cup and a consequent small pressure reduction inside the cup relative to the case where the plunger is not buoyant.

After all this, the simple answer to your question is …….”YES !”

PS. Thanks for a well articulated question.

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