Upthrust is the upward force exerted on an object immersed in a fluid (liquid or gas). It acts because the pressure on the bottom surface of the object is greater than the pressure on the top surface.
Why upthrust exists
Pressure in a fluid increases with depth (P = density x g x h). The bottom of a submerged object is deeper than the top, so the fluid pushes harder on the bottom than on the top. The net upward force is the upthrust.
Floating and sinking
An object floats when the upthrust equals its weight. An object sinks when its weight is greater than the maximum upthrust the fluid can provide.
For a floating object: upthrust = weight of the object.
The upthrust on a floating object equals the weight of fluid displaced by the submerged part of the object.
Density and floating
An object floats in a fluid if its average density is less than the density of the fluid. It sinks if its average density is greater.
A steel ship floats because the hull encloses a large volume of air, making the average density of the ship (steel plus air) less than the density of water. A solid steel block sinks because its density (about 7800 kg/m cubed) is much greater than water (1000 kg/m cubed).
Apparent weight
When you hold an object in water, it feels lighter than in air. The apparent weight is the true weight minus the upthrust:
apparent weight = weight minus upthrust
This can be used to measure the upthrust on an object experimentally: weigh it in air, then weigh it submerged in water. The difference is the upthrust.
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