What Is Archimedes’s Principle?

Archimedes's principle states that when a body is wholly or partially immersed in a fluid, it experiences an up-thrust equal to the weight of the fluid displaced.

When an object is immersed in a fluid, two forces act on it:

1) the weight of the object acting downward, and
2) up-thrust acting upward.

It is due to up-thrust that objects apparently weight less when immersed in fluids.

An angler pulling a fish out of water experiences the sudden increase in the weight of the fish as soon as it is out of water.

It requires relatively less effort to lift a large boulder off the bottom of a river bed as long as the boulder is under water. Once the same boulder is out of the water, considerably greater effort is required to lift it.

The relative values of the weight and up-thrust determines whether an object will sink in a liquid or float in it. If the weight of the immersed object is greater than the up-thrust, the object will sink. If the weight is equal to the up-thrust, the object remains at any level like a fish. If the up-thrust is greater than the weight of the immersed object it will flow to the surface.

It can easily be shown that an object will sink in a liquid if its density is more than that of the liquid. If the density of the object is less than that of the liquid, it will float on it.

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