Upthrust:

             Upthrust is a force that pushes an object up and makes it seem to lose weight in a fluid. (Remember, a fluid means a liquid or a gas).

            The upthrust, or buoyancy, keeps ships afloat. The upthrust, or buoyancy, keeps swimmers on top of the water.

Principle of floatation:

              An object sinks if its weight is greater than the upthrust acting on it. An object floats if its weight is equal to or less than the upthrust. When an object floats in a fluid, the upthrust acting on it is equal to the weight of the object.

In the case of a floating object, the object may be partially immersed. The upthrust is always equal to the weight of the fluid displaced by the object. This is the principle of floatation. It states that:

        A floating object displaces a fluid having a weight equal to the weight of the object. Archimedes' principle applies to liquids as well as gases.