How to calculate buoyant force

Written by anthony smith
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How to calculate buoyant force

Buoyancy, or buoyant force, is based on Archimedes' Principle. This principle states, "Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object." Archimides' Principle is important in hydro-engineering applications, such as shipbuilding. The steps below detail how to calculate buoyant force.

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Things you need

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    The Steps

  1. 1

    Obtain the volume of the object on which you wish to calculate the bouyant force. We will call this value for the volume "V."

  2. 2

    Determine what percentage (based on volume) of the object will be submerged in the water.

  3. 3

    Convert this percentage to a decimal number. We will call this value "v." For example; if 100 per cent of the object is to be submerged, v= 1.0 . If 50 per cent of the object is to be submersed, v= 0.50 .

  4. 4

    Substitute this value into the equation for buoyant force: FB = (V)(v) x (SPH2O) Where FB = buoyant force and SPH2O = Specific gravity of water (considered constant at 28.3kg per cubic foot).

  5. 5

    Multiply V by v, and then multiply by 62.4 in order to get the value for the buoyant force, expressed in pounds. Consider the following example of a 2-foot by 2-foot by 2 foot cube to be submersed 25 per cent of the way in water. V = 222 = 8 cubic feet v = 25% = 0.25 SPH2O = 28.3 Kilogram per cubic foot FB = 8 *0.25 * 62.4 = 566 Kilogram

    That's how you calculate buoyant force, and it helps illustrate how those large ships made of steel can float. That's quite a bit of force needed to push that 2 foot cube just one fourth of the way in the water!

Tips and warnings

  • For laymen's calculations, the specific weight of water can be assumed to be 283 Kilogram per cubic foot, as this value does not vary significantly in normal temperature ranges.

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