This document discusses the key concepts of relativistic mechanics and the advent of modern physics, including: - Relativistic mechanics deals with the relative motion of observers and objects. Special relativity applies to inertial frames of reference, while general relativity applies to non-inertial frames. - The basic assumptions of special relativity are that the laws of motion are the same in all inertial frames, and the speed of light is constant in all frames. - Special relativity results in mass variation, length contraction, and time dilation between frames in relative motion. - Mass and energy are also related by Einstein's famous equation, E=mc2.

1. ADVENT OF MODERN
PHYSICS
CHAPTER # 17
2ND YEAR PHYSICS
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2. RELATIVISTIC MECHANICS
 The branch of physics which deals with the relative motion of
observer and object is called Relative Mechanics.
 In physics, relativistic mechanics refers to mechanics compatible with special
relativity and general relativity.
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3. FRAME OF REFERENCE
FRAME OF
REFERENCE
Frame of reference is a co-
ordinate system in which
measurement is carried out.
In order to know the change in
position of an object a
reference point is required.
Point O in the figure is
the reference point or Origin
and together with three axes,
this system is called the
coordinate system.
INERTIAL FRAME OF
REFERENCE
The frame of reference moving
with constant velocity i.e. zero
acceleration is called inertial
frame of reference
Newton laws of mechanics are
applicable in inertial frame of
reference
NON INERTIAL FRAME
OF REFERENCE
The frame of references having
acceleratory motions are called
non-inertial frame of references
Newton laws of mechanics are
not applicable in non-inertial
frame of references.
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4. THEORY OF RELATIVITY
SPECIAL THEORY OF RELATIVITY
The part of relative mechanics that deals with
the inertial frame of references is called
special theory of relativity.
GENERAL THEORY OF RELATIVITY
The part of relative mechanics that deals with
non inertial frame of references is called
general theory of relativity.
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6. BASIC ASSUMPTIONS OF SPECIAL
THEORY OF RELATIVITY
 The motion of a body can be represented by same equation of motion for all the
inertial frame of reference whatever is their velocity.
 For all the inertial frame of reference the velocity of light remains constant. i.e. c =
3 x 108 m/s.
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7. RESULTS OF SPECIAL THEORY OF
RELATIVITY
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8. MASS VARIATION
According to the special theory of relativity, the mass of
an object in a frame of reference at rest is called its rest mass mo. if this mass is
measured by an observer moving with a constant speed v relative to the object,
then it will not remain constant if the speed v is comparable to c. The mass m in
the moving frame will vary according to the mass variation
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10. LENGTH CONTRACTION
In the theory of special relativity it has been found that the measurement of
length of a rod in a stationary frame of reference is not the same when the rod
is measured by the observer in the moving frame of reference with the velocity
relative to the rod, provided that the measurement is made along the direction
of motion.
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13. TIME DILATION
Time is regarded as an absolute quantity in classical mechanics whereas in the
special theory of relativity it is considered to be a relative entity based on the
measurement of time in frame of references in relative motion.
The time interval between two events taking place at the same point in space as
timed with a clock at rest with respect to that point is called the proper time
interval and is denoted by To. Time measured with a clock in motion with
respect to the events is known as relativistic time it is represented by T. Both of
the time intervals To & T refer to the time elapsed between the same pair of
events occurring in the two frames moving with a relative speed v.
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15. MASS ENERGY RELATION
We know that the speed of light is a universal constant. We cannot reach speeds
greater than the speed of light by the relativistic addition of velocities. The question is
how to reconcile with this result of special relativity with Newton's second law, F=ma. It
would be seen that any constant force, no matter how small, applied for a considerably
very long time, should continuously accelerate any mass 'm' at a rate a=f/m until the
speed was arbitrarily very large. Einstein, concluded that energy has inertia i.e. the
more energy a body possess, the more inertia that body will display. Since, inertia is a
property of matter, which is associated with mass. Thus from Einstein's argument mass is
simply a property attributed to the total energy of the body and only the total energy is
required, to know the total mass of the body. Thus, in special theory of relativity total
energy and mass are related by the famous Einstein's equation.
E=mc2
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