CHAPTER # 13 RELATIVITY
(MCQs)
1. C 2. D 3.C 4.C 5.D
SHORT QUESTION AND ANSWERS
13.1. What is the difference between inertial and non-inertial frame of reference?
ANSWER: Frame of reference is defined as the coordinate system that can be used to determine positions and velocities
of objects in that frame. The difference between the two types; inertial and non-inertial frames are as follows:
INERTIAL FRAME NON-INERTIAL FRAME
A frame of reference which is at rest or which is moving An accelerating frame of reference is called non-inertial
with uniform velocity along a straight line. frame of reference.
Objects are said to have no acceleration at all (a= 0; i.e. Objects have acceleration even in absence of external forces
object continues its state of rest or uniform motion along because frame itself is accelerating with reference to an
straight line unless acted upon by external force.) inertial frame.
Newtonian laws are valid. Newtonian laws are not valid.
Practically, no frame is perfectly inertial Non-inertial frames exist in real life.
Studied under Classical Physics (concerned with Studied under Quantum/ Modern Physics (concerned with
macroscopic sized objects and high speed related problems) microscopic sized objects ,high speed and energetic
objects)
Example: Earth is an inertial frame of reference (neglecting Example: an elevator falling freely under action of gravity.
its centripetal acceleration which is negligible)
13.2. What are the postulates of special theory of relativity?
ANSWER: Special theory of relativity proposed by Albert Einstein in 1905 was based upon following two postulates:
PRINCIPLE OF RELATIVITY: The laws of physics are valid in any inertial frame of reference.
Example: Whether experiment is performed in stationary or uniformly moving inertial frame, all laws of physics are valid.
For example, one observer in a stationary bus and other in a constantly moving bus have same observations of ball falling
vertically downwards.
SPEED OF LIGHT: The speed of light in vacuum (c=2.99×108 m/s) is always constant for all observers, regardless of
relative velocity between source of light and observer. It
greatly contradicts Newtonian relative motion.
For example, if you are moving with 0.5c in
direction of light, Newtonian physics claims your speed
to be 0.5c but Special theory claims the speed to be
still at c.
13.3. Explain why it is impossible for a particle with mass to move faster than speed of light?
ANSWER: By Einstein’s Special Theory of Relativity, relativistic mass expression is given by
m = mₒ / √1 – (v²/c²)
If speed of object is equal to speed of light i.e, v = c, then the expression becomes √1- c²/c² = √ 1 – 1 = √0 = 0
A non-zero expression (mₒ) divided by zero is actually undefined. It means if object moves with speed equal to speed of
light or greater than speed of light, mass of object becomes infinite which is impossible. In other words, we can say that
c= 2.99×108 m/s is the speed limit, no object in inertial frame can move greater or equal to this speed. On the other hand,
we have photon which moves with speed of light as it is a massless particle.
13.4. Imagine a train travelling at speed of light. A person on this train looks into the mirror placed in front of him.
Will the person see his reflection of themselves in the mirror?
ANSWER: According to first postulate of Special Theory of Relativity, all laws of physics are valid in all inertial frames.
The train is moving with speed of light (c= 3×108 m/s) i.e. with a constant speed, it is in inertial frame. Hence, the
physical phenomenon of reflection of light will be observed in by the person. Light reflects from the mirror, reaches the
observer’s eye and form an image.
(MCQs)
1. C 2. D 3.C 4.C 5.D
SHORT QUESTION AND ANSWERS
13.1. What is the difference between inertial and non-inertial frame of reference?
ANSWER: Frame of reference is defined as the coordinate system that can be used to determine positions and velocities
of objects in that frame. The difference between the two types; inertial and non-inertial frames are as follows:
INERTIAL FRAME NON-INERTIAL FRAME
A frame of reference which is at rest or which is moving An accelerating frame of reference is called non-inertial
with uniform velocity along a straight line. frame of reference.
Objects are said to have no acceleration at all (a= 0; i.e. Objects have acceleration even in absence of external forces
object continues its state of rest or uniform motion along because frame itself is accelerating with reference to an
straight line unless acted upon by external force.) inertial frame.
Newtonian laws are valid. Newtonian laws are not valid.
Practically, no frame is perfectly inertial Non-inertial frames exist in real life.
Studied under Classical Physics (concerned with Studied under Quantum/ Modern Physics (concerned with
macroscopic sized objects and high speed related problems) microscopic sized objects ,high speed and energetic
objects)
Example: Earth is an inertial frame of reference (neglecting Example: an elevator falling freely under action of gravity.
its centripetal acceleration which is negligible)
13.2. What are the postulates of special theory of relativity?
ANSWER: Special theory of relativity proposed by Albert Einstein in 1905 was based upon following two postulates:
PRINCIPLE OF RELATIVITY: The laws of physics are valid in any inertial frame of reference.
Example: Whether experiment is performed in stationary or uniformly moving inertial frame, all laws of physics are valid.
For example, one observer in a stationary bus and other in a constantly moving bus have same observations of ball falling
vertically downwards.
SPEED OF LIGHT: The speed of light in vacuum (c=2.99×108 m/s) is always constant for all observers, regardless of
relative velocity between source of light and observer. It
greatly contradicts Newtonian relative motion.
For example, if you are moving with 0.5c in
direction of light, Newtonian physics claims your speed
to be 0.5c but Special theory claims the speed to be
still at c.
13.3. Explain why it is impossible for a particle with mass to move faster than speed of light?
ANSWER: By Einstein’s Special Theory of Relativity, relativistic mass expression is given by
m = mₒ / √1 – (v²/c²)
If speed of object is equal to speed of light i.e, v = c, then the expression becomes √1- c²/c² = √ 1 – 1 = √0 = 0
A non-zero expression (mₒ) divided by zero is actually undefined. It means if object moves with speed equal to speed of
light or greater than speed of light, mass of object becomes infinite which is impossible. In other words, we can say that
c= 2.99×108 m/s is the speed limit, no object in inertial frame can move greater or equal to this speed. On the other hand,
we have photon which moves with speed of light as it is a massless particle.
13.4. Imagine a train travelling at speed of light. A person on this train looks into the mirror placed in front of him.
Will the person see his reflection of themselves in the mirror?
ANSWER: According to first postulate of Special Theory of Relativity, all laws of physics are valid in all inertial frames.
The train is moving with speed of light (c= 3×108 m/s) i.e. with a constant speed, it is in inertial frame. Hence, the
physical phenomenon of reflection of light will be observed in by the person. Light reflects from the mirror, reaches the
observer’s eye and form an image.
