Now upthrust is completely related to water.
The definition is,
Upthrust: The upward force exerted by water on any object is known as upthrust or buoyant force.
The explanation is,
When we release a object in the water there are two forces acting on it, that are Gravitational force and the upthrust.
You see that the lighter objects would float on the water, this flotation is due to the upthrust which keeps the object floating
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