Buoyant Force and Flotation

What Causes Buoyant Force?

The difference between the forces acting on the UPPER surface and the LOWER surface is the net force acting UPWARDS.

This net force is known as the BUOYANT FORCE. (Remember! Pressure = Force / Area). If you rearrange the Force to become the subject F = P x A.

Pressure, P = hpg (h = height, p = density, g = gravitational force)

Say, 

Force acting on the upper surface, F1 = P1 A
                                                        = h1pg A

Force Acting Underside, F2 = P2 A
                                          = h2pg A

Net Force Acting Upwards = F2 - F1

                                         = Buoyant Force
                                         = Weight of Liquid Displaced = mg

h1 is the distance from the TOP area of the object to the surface

P1 is pressure 1

p is density of liquid

g is gravity value

A is the total surface area.


h2 is distance from surface to BOTTOM part of the object

P2 is pressure exerted on the lower side

Therefore, the buoyant force is equal to the weight of liquid displaced, which is in accordance with Archimedes' Principle as I will explain here.

In the above figure, the object is lowered into the water, the following observations are made.

i) The object experiences a reduction in weight. The object of the weight in water is less than its weight in air. The apparent loss in weight of the obejct is caused by the buoyant force of the surrounding water 
on the object.

Apparent loss in weight of object
=Weight of object - weight of object in water.

ii) The object displaces a vlume of water.
Volume of water displaced
= volume of the submerged part of the stone

iii) From the figure, the apparent loss in weight is due to the buoyant force.

Therefore :
Bouyant Force = Actual weight - weight in water
= (Say) 70N - 40 N
= 30 N  

Reference:

http://physics.weber.edu/carroll/Archimedes/principle.htm