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### What are black holes

According to special theory of relativity, the maximum velocity that can exist is the velocity of light
c=3X108 m/s
Now we also Know that escape velocity which is the velocity needed for an object to become essentially free of the gravitational effect of another object is given
$v=\sqrt{\frac{2\mathrm{GM}}{R}}$
or
${v}^{2}=\frac{2\mathrm{GM}}{R}$
We may speculate on the mass and radius of a steller body that has an escape velocity of c.
Then
${c}^{2}=\frac{2\mathrm{GM}}{R}$
or
$R=\frac{2\mathrm{GM}}{{c}^{2}}$
This quantity R is called the Schwarzschild Radius and is usually designated by RS
Substituting the values of G and c,we have
RS=1.485 X10-2 M
The above equation gives us the relation between M and RS. It states that A body of Mass M in kg and radius RS in m or smaller produces such a strong gravtitional at its surface that no particle on its surface can escape.This even applied to electromagnetic radiation ( photons) including light.
So Even light cannot escape from such planet or body. That is the reason such body bodies are termed as Black holes
The most common way for a black hole to form is probably in a supernova, an exploding star. When a star with about 25 times the mass of the Sun ends its life, it explodes. The outer part of the star screams outward at high speed, but the inner part of the star, its core, collapses down. If there is enough mass, the gravity of the collapsing core will compress it so much that it can become a black hole. When it’s all over, the black hole will have a few times the mass of the Sun. This is called a “stellar-mass black hole”, what many astronomers think of as a “regular” black hole.

### Fictitous Force and Free falling Elevator

A non inertial Frame of Refrence is the frame of refrence which is having acceleration(a0) Example Free Falling elevator
Newtons law are valid in inertial frame of refrence only. We cannot apply them as it is in non inertial frame of refrence as it left out the acceleration of the object because of the frame of refrence.
Let me provide simple example to illustrate this.
You are moving in car which is accelerating. From your frame of refrence,a block on the road would be accelerating in opposite direction.
Now if we apply newton's law, we will find acceleration to be zero as no net force is acting. But the block has acceleration from car frame of refrence.
So to fix this issue,we will add one fictitous quantity in the Newton law equation like
F+F0=Ma
Where F0 is the fictitous force and it is in opposite direction of accleration frame
F0=-Ma0
Where ais the acceleration of Non inertial frame of refrence
Now let me further illustrate this with example of free falling elevator
A free falling elevator is an non inertial frame of refrence.The acceleration of the elevator as defined from the observer on the ground will be
g=-gk
Where k is the unit vector along vertical axis in the upward direction
Let us take an unattached object of mass M in the elevator.
From the frame of refrence of free falling elevator,Two forces are acting on the mass
Fictitous Force due to non inertail frame of refrence
Ffic=MgIt is opposite to the acceleration of the elevator
Gravitional Force on the Mass
FG=-Mgk
So the total Force on the body from elevator frame of refrence
FT=Ffic+FG=0
Which mean body is unacclerated in the elevator frame of refrence. The body will appear to be suspended in the air ,if it has no initial relative velocity to the elevator
Few more important things about Fictitous force
• Fictitous force are called pseudo force.
• It simple represent the effect of the acceleration of the non inertial frame of refrence
• They dont have any physical significance
• Whenever we work the problem in non inertial frame of refrence,always make sure to include fictitous force in the newton law

### Different type of Force and their origin

A) Normal Force: When we put a book on a table ,the molecules of the book exert downward forces on the molecules of the table.The molecules composing the upper layer of the tabletop move downward until the repulsion of the molecules below balances the forces applied by the book.
This compression always occur but it is too slight to notice
The following things are good to notice about normal force
a) For a body resting on a surface,Normal force is equal and opposite to the resultant of all the other forces which acts on the body in the direction perpendicular to surface
b) Weight and Normal force are not action reaction pair as they act on same body whereas action -reaction acts on different body

B) Tension: Tension usually comes under picture for string and rope. The string can be think of small short sections interacting by contact forces
Each section pulls the section to either side of it and by Newton third law,it is pulled by the adjacent section. The magnitude of the forces acting between the section is called Tension
The Following things are good to notice about tension
a) If the tension is uniform,the net string force on each small section is zero and section remains at rest
b) if the string is accelerating ,the tension generally varies along the string
c) If there is external force on the string then also tension varies along the string

C) Friction Force: Friction force generally comes into picture when one body moves or tries to move along the surface of the second body
Friction always opposes the motion which would occur in its absence. Friction occurs becuase of interatomic forces at the actual area of contact on atomic scale.
a) When we try to push a box on the table, if we push the box gently ,the box remains at rest. Force of friction assumes a value equal to the force applied by us. The force of friction cannot increase indefinately. If we push the box hard,the box start to slide

D) Viscous Forces: A body moving through liquid or gas is retarted by the force of viscosity exerted on it by fluid. It arised because a body moving through the medium exerts forces which set the nearby fluid into motion.By newton third law,the fluid exerts a reaction force on the body
The Following things are good to notice about viscosity
a)Viscous forces are proportional to velocity
b) It always retard the motion