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Body is moving downwards in the fluid with uniform velocity.So, buoyant force = weight of the body. What will the state of the body represent:- sinking or submerged?

p.s:The doubt is regarding the definitions of 'sink' & 'submerged'states and their usage in the situation described above.

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  • $\begingroup$ Anything under the surface of water is "submerged," but only objects moving downward in water are "sinking." This seems to be a question about the English language and not a question about physics. $\endgroup$
    – rob ♦
    Commented Mar 14, 2016 at 20:34
  • $\begingroup$ I apologize if my description has not been up to the mark. What resonated the doubt is that the condition of zero net force on the body should imply that it is submerged just under the surface of the fluid.However, zero net force does not necessarily indicates that the body has zero downward velocity.Hence, the body sinks if it has uniform downward velocity.But this contradicts the fact for sinking which states that the weight of the body should be greater than the buoyant force on it. Please explain this seemingly paradoxical condition as to what state the body really is? $\endgroup$
    – katipra
    Commented Mar 14, 2016 at 21:33
  • $\begingroup$ Please note that the above dilemma arises in Archimedes' principle which does not include drag forces. $\endgroup$
    – katipra
    Commented Mar 14, 2016 at 21:37

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The body is moving downward in a fluid with uniform velocity - it means that the net force on the body must be zero.

the body is moving downward inside the fluid ,so it is "sinking " as time elapses and it can not be called submerged.

Now about forces operating on the body - the following are acting-

  1. the gravitational pull of the earth equal to (mass x acceleration due to gravity).

  2. The force of buoyancy provided by the liquid/fluid acting upwards opposite to the weight.

  3. The force of viscous drag due to relative motion between layers of fluid and the contact surface of the body and this drag is /may be proportional to the velocity of the body and acts in a direction opposed to the motion of the body-

So it acts upward .The drag is also proportional to coefficient of viscosity of the fluid.

Therefore the force equation for the moving body should be,

F(net)= mg -buoyant force - viscous drag= 0 .

A different conclusion from the question given above :

The wt. of the body must be equal to sum of the buoyant force and viscous drag acting on the body. The uniform speed is called "Terminal velocity".

This experiment is Stoke's Experiment to find the coefficient of viscosity of the fluid.

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  • $\begingroup$ I apologize if my description has not been up to the mark. What resonated the doubt is that the condition of zero net force on the body should imply that it is submerged just under the surface of the fluid.However, zero net force does not necessarily indicates that the body has zero downward velocity.Hence, the body sinks if it has uniform downward velocity.But this contradicts the fact for sinking which states that the weight of the body should be greater than the buoyant force on it. Please explain this seemingly paradoxical condition as to what state the body really is? $\endgroup$
    – katipra
    Commented Mar 14, 2016 at 22:00
  • $\begingroup$ well, in the above answer i have depicted the picture of a sinking body with uniform velocity as per your question framing.However if you wish to keep the body submerged only and just floating it can float with parts of its volume above the surface of the fluid also as does ice on water.the condition for submergence does not and can not provide a downward motion- for that to happen the wt. of the body must exceed force of buoyancy and viscous drag if taken to be acting on the body. $\endgroup$
    – drvrm
    Commented Mar 15, 2016 at 2:58

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