Physics · Mechanical Properties Of Fluids · NEET
Two forces act on it. Its real weight W pulls it down. The fluid pushes it up with the buoyant force F_b. Your hand or the spring balance only has to support the difference. So apparent weight = W minus F_b. The body did not lose mass, it just gets an upward help from the fluid, so it feels lighter.
It does not depend on the object's own weight or what the object is made of. From F_b = rho_fluid x V_displaced x g, it depends only on the fluid's density (rho_fluid), the submerged volume (V_displaced), and g. A steel block and a wooden block of the same submerged volume in the same water feel the same upthrust, even though their weights are very different.
It is the volume of the object that is actually inside the fluid. If the object is fully submerged, V_displaced equals the object's full volume. If it floats, V_displaced is only the part under the surface, not the whole object. This is why a floating ship displaces water equal to its own weight, not equal to its full volume.
Compare densities. If the object's density is less than the fluid's density, it floats (like ice in water). If it is more, it sinks (like a coin). If they are equal, it stays wherever you put it. For a floating body the rule is: weight of object = weight of fluid displaced, so rho_object x V_object = rho_fluid x V_submerged.
Yes. Upthrust, buoyancy, and buoyant force all mean the same upward push a fluid gives to any object placed in it. It comes from pressure being larger at the bottom of the object than at the top, because pressure increases with depth (P = P0 + rho g h).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
When a body is fully or partly dipped in a fluid, the fluid pushes it up with a force equal to the weight of the fluid it displaces.
F_buoyant = rho_fluid x V_displaced x g, where rho_fluid is the fluid density, V_displaced is the submerged volume, and g is the acceleration due to gravity.
Apparent weight is what a spring balance reads when the object hangs in a fluid. Apparent weight = real weight minus buoyant force = W minus rho_fluid x V x g.
Pressure in a fluid increases with depth. The bottom face of the object is deeper, so it feels more pressure than the top face. This extra upward push at the bottom is the net buoyant force, and it always acts upward.
A body floats when its weight equals the buoyant force at the surface, which means rho_object x V_object = rho_fluid x V_submerged. In short, its average density must be less than or equal to the fluid density.
No. Once the object is fully submerged, its displaced volume stays the same, so F_b stays the same at any depth (assuming the fluid density does not change). It does not keep increasing as you push it deeper.
