General Knowledge- Physics (Gravitation)

Gravitation

Gravitation: Every body attracts other body by a force called force of gravitation.

Newton's Law of Gravitation: The force of gravitational attraction between two point of bodies is directly proportional to their mass and inversely proportional to the squire of the distance between them.

            Consider two point bodies of masses m₁ and m₂ are placed at a distance r, The force of gravitational attraction between them,

                                            ,         

Here G is constant called universal gravitational constant.

Assuming SI units, F is measured in newtons (N), m₁ and m₂ in kilograms (kg), r in meters (m), and the constant G is approximately equal to 6.674×10⁻¹¹ N m² kg⁻²

where:

• F is the force between the masses,
• G is the gravitational constant,
• m₁ is the first mass,
• m₂ is the second mass, and
• r is the distance between the centers of the masses.

Gravity: The gravitational force of the earth is called gravity. i.e. gravity is a force by which earth pulls a body towards its center.

 The acceleration produced in a body due to force of gravity is called acceleration due to gravity (g) and its value is 9.8 m/s².

► Acceleration due to gravity is independent of shape, size and mass of the body.

Variation in g:

(i)                 Value of g decrease with height or depth from earth’s surface.

(ii)               g is maximum at poles.

(iii)             g is minimum at equator.

(iv)             g decrease as angular speed of earth increases and g increases as angular speed of earth decreases i.e. g is inversely proportional to angular speed of earth

            g α angular speed of earth.

► If the angular speed of the earth becomes 17 times its present value, the body present on the equator weightless.

Weight of a body in a lift:

(i)                 If lift is stationary or moving with uniform speed (either upward or downward), the apparent weight of body is equal to its true weight.

(ii)               If lift is going upward then apparent weight of a body is more than the true weight.

(iii)             If lift is going downward then apparent weight of a body is less than the true weight.

(iv)             If the cord of the lift is broken, it falls freely. In this situation the weight of a body in the lift becomes zero. This is the situation of weightlessness.

(v)               While going down, if the acceleration of the lift is more than the acceleration due to gravity, a body in the lift goes in the contact of the ceiling of lift.

Kepler’s Laws of planetary motion:

(i)                 All planets move around the sun in elliptical orbits, with the sun being at rest at one focus of the orbit.

(ii)               The position vector of the planet with sun at the origin sweeps out equal area in equal time i.e. the areal velocity of planet around the sun always remains constant.    

                 A consequence of this law is that the speed of planet increases when the planet is closer to the sun and decreases when the planet is far away from the sun. Speed of a planet is maximum when it is at perigee and minimum when it is at apogee.

(iii)             The squire of the period of revolution of a planet around the sun is directly proportional to the cube of mean distance of the planet from the sun.  

            If T is period of revolution and r is the mean distance of planet from the sun, then T² α r³.

            Clearly distant planets have larger period of revolution. The time period of nearest    

            planet Mercury is 88 days, where as time period of farthest planet Pluto is 247.7 years.

Satellite: Satellites are natural or artificial bodies revolving around a planet under its gravitational attraction. Moon is natural satellite where as INSAT-1B is an artificial satellite.

Orbital speed of a satellite:

(i)                 Orbital speed of a satellite is independent of its mass. Hence satellites of different masses revolving in the orbit of same radius have same orbital speed.

(ii)               Orbital speed of a satellite depends upon the radius of orbit (Height of the orbit from the surface of the earth). Greater the radius of orbit lesser will be the orbital speed.

► The orbital speed of satellite revolving near the surface of earth is 7.9 km/sec.

            Periodic revolution of a satellite: Time taken by a satellite to complete on revolution in its orbit is called its period of revolution.

i.e. period of revolution = circumference of orbit / orbital speed

(i)                 Period of revolution of a satellite depends upon the height of satellite from the surface of the earth. Greater the heights more will be the period of revolution.

(ii)               Period of revolution of a satellite is independent of its mass.

► The period of revolution of satellite near the surface of the earth is 1 hour 24 minute (84 minute).

Geo-Stationary Satellite:  If a satellite revolves in a equatorial plane in the direction of the earth rotation i.e. from west to east with a period of revolution equal to time period of rotation of earth on its own axis i.e. 24 hours, then satellite will appear stationary relative to earth. Such a satellite is called Geo-Stationary Satellite. Such a satellite revolves around the earth at a height of 36000 km. The orbit of Geo-Stationary satellite is called parking orbit. Arther C. Clarck was the first to predict that a communication satellite can be stationed in the geosynchronous orbit.

Escape Velocity:  Escape velocity is that minimum velocity with which a body should be projected from the surface of the earth so as it goes out of the gravitational field of the earth and never returns to the earth.

► Escape velocity is independent of mass, shape and size of the body and its direction of projection.

► Escape velocity is also called second cosmic velocity.

► For earth escape velocity = 11.2 km/sec.

     For moon escape velocity = 2.4 km/sec.

► Orbital speed of satellite V₀ = √gR and escape velocity V_e= √2gR

Where R is the radius of earth i.e. V_e= √2 V₀ i.e escape velocity is √2 times the orbital velocity.

Therefore if orbital velocity of a satellite is increased √2 times (increased by 41%), then the satellite will leave the orbit and escape

***

General Knowledge- Physics ( Work, Energy and Power)

Work, Energy and Power

Work: If a body gets displacement when a force acts on it, work is said to be done. Work is defined as the product of force and displacement of the body along the direction of the force.

If a body gets displaced by S when force F acts on it.

Then the work W =FS cos θ

Where θ = angle between displacement and force.

If both force and the displacement is in the same direction, then W =FS

Work is scalar quantity and its SI unit is Joule.

Energy: Capacity of doing work by a body is called energy.

► Energy is a scalar quantity and its SI unit is joule.

► Energy developed in a body due to work done on it is called mechanical energy. Mechanical Energy is of two types:

(i)                 Potential Energy

(ii)               Kinetic Energy

Potential Energy: The capacity of doing work developed in a body due to its position or configuration is called its potential energy.

            Example:

(i)                 Energy of stretched or compressed spring

(ii)               Energy of water collected at a height

(iii)             Energy of spring in a watch.

Potential energy of a body in the gravitational field of the earth is mgh.

Where m = mass, g = acceleration due to gravity and h = height of the body from the surface of the earth.

Kinetic Energy: Energy posses by a body due to its motion are called Kinetic Energy of the body.

If a body of mass m is moving with a speed of v, the kinetic energy of the body is ½ mv²

i.e. Kinetic Energy = ½ mv²

Principal of Conservation of Energy:  Energy can neither be created nor can be destroyed. Only energy can be transformed from one form to another form. Whenever energy is utilized in one form, equal amount of energy is produced in other form. Hence total energy of the universe remains the same. This is called the principal of conservation of energy.

Some Equipment used to transform Energy

S.No.	Equipment	Energy Transformed
1.	Dynamo	Mechanical energy into electrical energy
2.	Candle	Chemical Energy into light and heat energy
3.	Microphone	Sound energy into electrical energy
4.	Loud Speaker	Electrical energy into sound energy
5.	Solar Cell	Solar energy in to electrical energy
6.	Tube light	Electrical energy in to light energy
7.	Electric Bulb	Electrical energy in to light energy
8.	Battery	Chemical energy into electrical energy
9.	Electric Motor	Electrical energy in to mechanical energy
10.	Sitar	Mechanical energy in sound energy

Relation between Momentum and Kinetic Energy

                        K.E = P²/2m where p= momentum = mv

Clearly when momentum is doubled Kinetic energy becomes four times.

Power: Rate of doing work is called power.

 If an agent does w work in t time, then power of agent =W/t

SI unit of power is watt named as a respect to the scientist James Watt.

                        Watt= joule/sec.

                        1 KW = 10³ watt

                        1MW = 10⁶ watt

Horse power is a practical unit of power. 1H.P. = 746 watt.

                        1 watt second = 1 watt* 1 Second = 1joule.

                        1 watt hour (Wh) =3600 joule

                        1 kilowatt hour (kwh) = 3.6*10⁶ joule.

W, kw, MW & H.P. are units of power.

Ws. Wh, kwh are units of work and energy.

*****

General Knowledge- Physics (Motion)

Motion

Scalar Quantities: Physical quantities which have magnitude only and no direction are called scalar quantities.

Example: Mass, Speed, work, volume, time, power, energy etc.

Vector Quantities: Physical quantities which have magnitude and direction both and which obey the law of triangle are called vector quantities.

Example: Displacement, velocity, acceleration, force, momentum, torque etc.

 

Electric current though have direction, is a scalar quantity because it does not obey the triangle law.

Moment of inertia, pressure, reflective index, and stress are tensor quantities.

Distance: Distance is length of actual path covered by a moving object in a given time interval.

Displacement: Shortest distance covered by a body in a definite direction is called displacement.

►Distance is a scalar quantity whereas displacement is a vector quantities both having   

     the same unit (meter).

►Displacement may be positive, negative or zero but distance is always positive.

►In general magnitude of displacement ≤ distance.

Speed: Distance travelled by a moving object in unit interval of time is called speed. i.e.

         

    Speed =  Distance/ Time

It is a scalar quantity and its SI unit is meter/ second (m/s)

Acceleration of the object is defined as the rate of change of velocity of the object i.e.

Acceleration = Change in velocity / Time

 It is a vector quantity and its unit is meter/second² (m/s²)

If velocity decreases with time then acceleration becomes negative and is called retardation.

Circular Motion: If an object describes a circular path (circle) then its motion called circular motion. If object moves with a uniform circular speed, then its motion called uniform circular motion.

            Uniform circular motion is an accelerated motion because then direction of velocity changes continuously.

Angular velocity: The angle subtended by the line joining the object from the origin of circle in unit time interval is called angular velocity.

            It is generally denoted by w and w = θ/t

If T= time period = time taken by the object to complete on revolution, n= no. of revolution in one second

            Then nT =1 & w=2π / T = 2πn.

► In one revolution, the object travels 2πr distance.

So, Linear speed is =wr = angular speed * radius.

Newton’s Laws of motion:  Newton, the father of physics has established the laws of motion in his book “principia” in 1667.

Newton’s first law of motion: Every body maintains its initial position of rest or motion with uniform speed in on a straight line unless an external force acts on it.

► First law also called the law of Galilieo or law of inertia.

► Inertia: Inertia is the property of a body by virtue of which the body opposes change in   

     initial state of rest or motion with uniform speed on a straight line.

     Inertia is or two types namely (i) Inertia of rest (ii) Inertia of motion

     Some examples of inertia:

(i)                 When a car or a train starts suddenly, the passengers bends backward.

(ii)               When a running horse stops suddenly, the rider bends forward.

(iii)             When a coat/ blanket beaten by a stick, the dust particles are removed.

► First law gives the definition of force.

► Force: Force is that external cause which when acts on a body changes or tries to  

     change the initial state of the body.

     Momentum: Momentum is the property of a moving body and is defined as the 

     product of mass and velocity of the body i.e.

           

                        Momentum = mass*velocity

            It is a vector quantity having SI unit kilogram /s.

Newton’s second law of motion: The rate of change in momentum of a body is directly proportional to applied force on the body and it takes place in the direction of the force.

            If F= applied force, a= acceleration produced and m= mass of the body

            Then, nF =ma.

► Newton’s second law gives the magnitude of force.

► Newton’s first law is contained in the second law.

Newton’s third law of motion: To every action, there is an opposite and equal reaction.

            Examples of third law:

(i)                 Recoil of a gun

(ii)               Motion of a rocket

(iii)             Swimming

(iv)              While drawing water from the well, if the string breaks up then man drawing water falls back.

Principal of conservation of linear momentum: If no external force acts on a system of bodies, the total linear momentum of the system of bodies remains constant.

            As a consequence, the total momentum of bodies after and before collision remains the same.

Impulse: When a large force acts on a body for a very small time, then force is called impulsive force. Impulse is defined as product of force and time.

            Impulse = force* time = change in momentum.

► It is a vector quantity and its direction is direction of force. Its SI unit is Newton  

     second(Ns).

Centripetal Force: When a body travels along a circular path (circle), its velocity changes continuously, naturally an external force always acts on the body towards the center of the path.

            The external force required to maintain the circular motion of the body is called centripetal force.

            If a body of mass m moving on a circular path of radius R with uniform speed v, then the required centripetal force, F =mv²/R

Centrifugal Force: In applying Newton’s laws of motion, we have to consider some forces which can not be assigned to any object in the surroundings. These forces are called pseudo force or inertial force.

Centrifugal force is such a pseudo force. It is equal and opposite to centripetal force.

► Cream separator, centrifugal driver works on the principal of centrifugal force.

► Centrifugal force should not be confused as the reaction to the centripetal force because forces of action and reaction act on different bodies.

Moment of Force: The rotational effect of a force on a body about an axis of rotation is described in terms of moment of force.

            Moment of force about an axis of rotation is measured as the product of magnitude of force and the perpendicular distance of direction of force from the axis of rotation.

i.e. Moment of force =Force*moment arm.

► It is a vector quantity.

► Its unit is Newton meter (Nm).

Center of Gravity: The center of gravity of the body is that point through which entire weight of body acts. The center of gravity of a body does not change with the change in the orientation of the body in the space.

    The weight of a body acts through center of gravity in the downward direction. Hence a body can be brought to equilibrium by applying a force equal to its weight in the vertically upward direction through center of gravity.

Equilibrium: If the resultant of all the forces acting on a body is zero then the body is said to be in equilibrium.

    If a body is in equilibrium, then it will be either in the state of rest or in uniform motion. If it is at rest, the equilibrium is called static otherwise dynamic.

            Static equilibrium is of following three types:

(i)                 Stable Equilibrium: If on slight displacement from equilibrium position, a body regains its original position, it is said to be in stable equilibrium.

(ii)               Unstable equilibrium: If on slight displacement from equilibrium position,  a body moves in the direction of displacement and does not returns to its original position, the equilibrium is said to be unstable equilibrium. In this equilibrium the center of gravity of the body is at the highest position.

(iii)             Neutral Equilibrium: If on slight displacement from equilibrium position a body has no tendency to come back to the original position or to move in the direction of displacement, it is said to be in neutral equilibrium, the center of gravity always remains at the same height.

Condition for stable Equilibrium: For stable equilibrium of a body, the following two conditions should be fulfilled.

(i)                 The center of gravity of a body should be at the minimum height.

(ii)               The vertical line passing through the center of gravity of the body should pass through the base of the body.

*****

General Knowledge- Physics (Units)

Unit :- The chosen standard used for measuring a physical quantities is called unit.
Unit should be:-
(i)  Well defined                                       (ii) Easy to reproduce
(iii) Easy to compare                               (iv) Internationally accepted
(v)Independent of change in physical conditions

Units are of two types:- (i) Fundamental Unit (ii) Derived Unit

System of Units:- Units depend on choice.each choice of units leads to a new system(set) of units. The Internationally accepted unit systems are:-

(i) CGS System (ii) MKS System (iii) FPS System (iv) SI Units

In SI units there are seven fundamental units as given in the table below:-

Physical Quantity	SI Unit	Symbol
Length	Meter	m
Mass	Length	kg
Time	Second	s
Electric Current	Ampere	A
Temperature	Kelvin	K
Luminous Intensity	Candela	Cd
Amount of substance	Mol	mol

          Besides these two fundamental units, two supplementary units are also defined, viz., radian[rad] for plane angle and steradian(sr) for solid angle.


► All the units which are defined/ expressed in the terms of fundamental units are called derived units.

Some important derived units are:- 

Physical Quantity	CGS units	SI unit	Relation
Force	Dyne	Newton	1 Newton=105dyne
Work	erg	Joule	1 Joule= 107erg

                      
Some practical units of length mass and time:-

Length	Mass	Time
Light year= distance traveled by light in one year in vaccum . 1 Ly = 9.46*1015m 1 Parsec= 3.26ly =2.06*1016m 1 Nautical mile or sea mile                          = 6020 ft. 1 micron = 1µm =10-6m. 1 Angestron (A0) =10-15m.	1 Quintol = 102kg 1 Metric ton =103kg 1 Atomic mass unit(amu) or     dalton =1.66*10-27kg 1Slug =14.59 kg 1 Pound = 0.4537 kg 1 Chandrashekhar limit = 1.4 times the mass of the Sun = 2.8*1030kg.	1 Solar year = 86400 seconds. 1 year 365/2 solar days. 1 Lunar month =27.3 solar days Tropical year = It is the year in which total solar eclipse occurs. Leap year = It is the year in which the month of February is of 29 days.

Prefixes used in metric system:

Prefix Name	Symbol	Power-of-Ten
yocto	y	10-24
zepto	z	10-21
atto	a	10-18
femto	f	10-15
pico	p	10-12
nano	n	10-9
micro	µ	10-6
milli	m	10-3
centi	c	10-2
deci	d	10-1
[unity]	[none]	100
deka	da	10+1
hecto	h	10+2
kilo	k	10+3
mega	M	10+6
giga	G	10+9
tera	T	10+12
peta	P	10+15
exa	E	10+18
zetta	Z	10+21
yotta	Y	10+24