Archimedes Principle

Archimedes’ Principle state that “ When an object is immersed in a fluid (a liquid or a gas) ,the buoyant force  
( upthrust force) on the object  is equal  to the weight of fluid displaced by the object.

From  Archimedes’s Principle :

Buoyant Force, Fb  = Weight of fluid displace
            =  mg                 (note : F = ma)
            =  ρVg               (note : ρ = m/V  )
                                             
Thus     Fb     =   ρ V g

Where  Fb   =  Bouyant Force or Upthrust
             ρ     =  Density of fluid
            V     =  Volume of fluid displaced or the volume of the object that immersed in the fluid.


Equation for buoyant force

1. Buoyant force = Weight of fluid displaced
2. Buoyant force = ρVg
3. Buoyant force = Weight object in air – weight in water

Actual weight and apparent weight

Buoyant force makes thing seem to be lighter.

The weight of an object in air is its actual weight
The weight measured when the object is immersed in a fluid is its apparent weight.
The buoyant force is equal to the apparent loss in weight.
The object displaces a volume of water.
The bigger the volume of the object immersed, the bigger the volume of the liquid displaced.

•  Volume of liquid displaced  = volume of the submerged part of the  object.

Work example :


An object weights 2.5 N.  When it is fully submerged in a liquid, its apparent weight is 2.2 N.

(a) What is the buoyant  force on the object in  water?
 Tips : Fb= Wc- Wa

(b) What is the weight of water displaced by the object?
 Tips : Weight , W = mass, m x gravitational force, g
In SPM take g = 10 Nkg^-1

(c) Determine the volume  of water displaced by  the object.
[density of water = 1000 kgm^-3]
 Tips : Density = mass / volume 

(d) What is the volume of the object?
 Tips : Volume of liquid displaced  = volume of the submerged part of the  object.  

Buoyant Force

An object immersed in water is subjected to two forces:
a. the weight of the object which acts downwards, (It pushes the object down)
b. an up-thrust or buoyant force which acts upwards.(it pushes the object up)


If the force from the weight of the object is greater than the force from buoyancy, the object sinks. 
Weight , W > Buoyant force, Fb ; object sink


If the buoyant force is greater than the force from the weight of the object, the object floats.

Weight , W < Buoyant force , Fb ; boject float