Pakistan study notes for Matric, Intermediate, B.A, B.COM BSC, M.A, BCS. Free Pakistani Urdu educational school, colleges and University notes.


Atmospheric Pressure:

We live in the bottom of deep sea of air called atmosphere. Air has mass so that it has some weight. The weight is a force thus it exerts force per unit area on us known as atmospheric pressure.

The pressure exerted by the atmosphere is called atmospheric pressure.

1 atmosphere = 1.01 * 10 5 N/m2


According to Archimedes principle:

“When a body is immersed in a liquid, it losses its weight or an upward thrust acts upon it which is equal to the weight of an equal volume of the liquid displaced.”



Proof of Archimedes Principle:

Consider a cylinder of length ‘L’ & area of cross-section ‘A’ immersed in a liquid of density


The depth at point ‘a’ = h1

The depth at point ‘b’ = h2


Pressure at point ‘a’ and ‘b’ is

Pa = pgh1


Pb = pgh


We know that

Thrust = PA

Thrust at point ‘a’ = pgh1A

Thrust at point ‘b’ = pgh2A



Net thrust = W [W=mg]

Net thrust = weight of liquid


This shows that upward thrust or the cylinder is equal to the weight of liquid displaced.

Apparent weight = actual weight – weight of the liquid displaced by


Results of Archimedes Principle:


(1)       A body will float over a liquid if displaes liquid whose weight is greater than the weight of the body.

(2)       A body will sink in a liquid if it displaces liquid whose weight is less than the weight of the body.

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