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A body if placed on a solid on earth, it gets reaction , so in the case of liquid say(water) which is very less solid the action by the body decreases as liquid molecules gets displaced , so the body gets less reaction force (less than its own weight) so

is bouyancy a reaction force only or it is reaction + pressure ???

Please dont explain in terms of pressure difference or bouyancy ..i just wanted to know where does Newtons 3rd law fit into this picture ( picture of an object immersed in liquid in presence of gravity) ???

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  • $\begingroup$ The resultant force exerted by the liquid on the body is equal and opposite to the resultant force that the body exerts on the liquid. The liquid pressure acts normal to the body, and, if you integrate the pressure distribution vectorially over the contact surface between the liquid and the solid, you get the resultant force. $\endgroup$ Dec 2, 2017 at 15:58

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Yes, buoyancy force is just another contact or "reaction" force which prevents two objects (eg a floating object and the fluid it floats in) from occupying the same space. Microscopically it has the same origin as all contact forces : the repulsion between molecules which are squashed together. The fluid reacts by getting out of the way. A solid cannot get out of the way because it does not flow. It reacts by getting compressed.

When you lower a block onto a solid surface, the surface deforms. You don't notice this with rigid materials like steel,concrete and wood, but it is still there as surely as it is with softer more elastic materials like rubber and sponge. The reaction force increases as the deformation increases, until it equals the weight of the block. The block "sinks" into the surface and the reaction force is a kind of "upthrust". The surface exerts pressure on the block which increases with depth.

When you lower a block into a liquid, the reaction force ("upthrust") increases as more liquid gets pushed out of the way. The block comes into contact with liquid at a greater depth, where the pressure is greater. Eventually it comes to rest when the upward pressure force ("upthrust") equals the weight of the block.

In both cases Newton's 3rd Law applies at all stages as the block is lowered onto or into some material, whether solid or liquid. At each stage the reaction force from the material on the block ("upthrust") equals the force which the block exerts on the material. If the "upthrust" at the current depth or deformation does not equal the weight of the block, there is an unbalanced force on the block, which moves further down.

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I believe there is maybe a misunderstanding about Newton's 3rd law of motion here.

Just to emphasise a bit more a point raised by sammy gerbil in his answer, the net force from the fluid on an object immersed in it is always equal in magnitude and opposite in direction to the force exerted by the object on the fluid itself as per Newton's 3rd law.

However, the force exerted by the object on the fluid is not necessarily equal to its own weight. In particular, as you mention, if the object is seen to sink at some constant acceleration, then the force exerted by the fluid on the object does not balance the weight of the object and we can conclude then (by Newton's 3rd law) that the force of the object on the fluid has to be less than its own weight.

If anything Newton's 3rd law may be helpful in giving you the force exerted by the bottom slab of a fluid onto a fluid slab carrying both its own weight and the pressure owing to the above fluid slab within the hydrostatic equilibrium condition.

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