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Archimedes Principle

Archimedes’ Principle states that when a body is immersed in a fluid it experiences an upthrust (or buoyancy force) which is equal to the magnitude of the weight of the fluid displaced.

In this experiment we investigate that principle. In our everyday life we would say a weighing scakes tells us the weight of an object in grams. However in Physics this is incorrect. A weighing scales measures mass which is measured in grams or kilograms. The weight of an object described the force exerted on an object by gravity. 

In the below video we see when a weighing scales is put on an incline the mass measured by the weighing scales reduces as the incline becomes greater. This is because gravity always acts downwards. The weighing scales measures the force perpendicular to itself. As such when the weighing scale is on an incline it is only getting a portion of the full force of gravity. That portion decreases as the incline increases.  For a demonstration of the force of gravity on a mass on an incline try this Visualizing Forces on an Incline simulation. Click "Show Gravity Components". 

[GIF - Weighing scales on an incline]

Knowing this we can use a weighing scales to emaure the upthrust force of a fluid. In this experiment a beaker is filled with water and placed on a weighing scale and the weighing scale tared so it shows 0g. The mass is suspended from air (so its weight does not affect the measurements) and placed in the water. The mass measured by the weighing scale is the upthrust force. It measures as a negative measurement as it is acting upwards in the opposite direction of gravity. Using F (force) = m (mass) * a (accleration, in this case 9.81 m/s^2) we can calculate the upthrust force in Newtons. 

[GIF - Make gif of lowering and raising mass]

If you want to explore the concepts of density and buoyancy try you hand with these online labs / simulations:

Verification of Archimedes' Principle

Determination of Density of Solid

Force Buoyancy Lab


School of Physics

Scoil na Fisice

Room 213 (Physics Office), 2nd floor, Kane Science Building, University College Cork, Ireland.,