The document provides lecture notes on physics concepts. It discusses: 1) Equilibrium, including dynamic equilibrium where the forces balance as a parachutist descends at constant velocity, and static equilibrium where the forces balance and an object does not move, like a book on a table. 2) Free fall acceleration and terminal velocity. An object in free fall accelerates at 9.8 m/s2 until air resistance equals gravity and it reaches a constant speed. 3) Other concepts covered include the variation of gravitational acceleration, acoustic echoes, properties of a second pendulum, elastic limits of materials, and monochromatic light.

1. PPSC PHYSICS FSC
PART ONE NOTES BY
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2. LECTURE NUMBER ONE
➢Equilibrium: If the sum of all the forces acting on the object is zero or it is moving with
uniform velocity,it is said to be in equilibrium.
There are two types of equilibrium:
➢Dynamic equilibrium: If body is moving with uniform velocity then it is called dynamic
equilibrium. For example paratrooper jumping out of an aeroplane, after a free fall, his
parachute opens and a little later the paratrooper starts descending with a uniform
velocity. At this instant the force of gravity acts vertically downward on the paratrooper is
balanced by parachute which acts upward.

3. LECTURE NUMBER ONE
➢ EXPLANATION: First I will the term " free fall ". Its meaning is " the sum of all
the forces acting on a body falling vertically downward ". We know that when
body falls, two forces are acting i.e upward friction force and downward gravity
force. Frictional force wants body to not move and gravity wants body to move
downward so both forces will increasing when body moves downward. Then a
time comes when both forces will become equal. Then at time body will move
with uniform speed. The body will move with uniform speed when the sum of all
forces acting on a body is zero. That's why paratrooper uses parachute to fall from
aeroplane because it will move with uniform speed which is not very fast
to damage the paratrooper. Moreover, paratrooper can change the speed by
changing the body position.

4. LECTURE NUMBER ONE
➢ Static Equilibrium: When the sum of all the forces acting on a body is zero and body will
not move then it is called static equilibrium.
For example, a book lying on the surface of table. The weight of the book acting vertically
downward is counterbalanced by upward normal reaction force of the surface of table.
➢ Explanation: Here the body is not moving instead it is placed on a surface. According to
Newton third law " every action has a reaction equal in magnitude but opposite in direction ".
So the book placed on the surface of table exert force of weight in downward direction so
the book is doing action on the surface of table. The surface of table in return exerts reaction
in upward direction. The magnitude of both forces are equal but opposite in direction
cancel effect of each other. Hence the object does not move and remains in static eqilibrium.

5. LECTURE NUMBER 2
• Free fall: The pull of Earth's gravity accelerates object falling vertically downward at the same rate 9.8 m/s2
called acceleration of of free fall.
• As an object falls faster, friction from the air increases rapidly. Eventually the air resistance becomes so great
that it balances the pull of gravity, and the object can fall no faster. It continues to fall steadily at the same speed
called terminal velocity ( When frictional force acting upward and gravitational force acting downward becomes
equall then body falls with constant speed called terminal velocity ). During freely falling the change in velocity
will remain 9.8 m/s2.
• Distance covered by the freely object is 4.9 m in 1st second
• Value of g remains constant at the surface of earth and decreases as we move upward or downward from
surface of earth and it finally becomes zero at center of earth

6. LECTURE NUMBER 2
• Value of 'g' is greater at the poles than at the equator
• The value of g on moon is 1/6th of value of g on earth
• The frequency of A.C supply is about 50 cycles per second (Hz) in Pakistan
• The reflected sound is called an echo
• Second Pendulum: A second pendulum is that pendulum which completes one vibration in two seconds. The
length of second pendulum is about one meter (0.992) and its frequency is 0.5 hertz.
• Elastic Limit: It is the limit after which a body does not obey Hook's law. After reaching the elastic limit, the
change occured in shape or size of body becomes permanent and it does not restore to original shape.
• Monochromatic Light: It is the light of single wavelength or color. Sodium light is the nearest approach to
the monochromatic light.