Tag: physics

Quizzes (598) Questions (13441)

Questions Related to physics

Multiple choice relativistic mechanics option a: relativity physics

When a rod moves at a relativistic speed v, its mass

  1. must increase by a factor of $\gamma$

  2. may remain unchanged

  3. may increase by a factor other than $\gamma$

  4. may decrease
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Mass of moving object    $m  = \dfrac{m _o}{\sqrt{1 - \dfrac{v^2}{c^2}}}   = m _o \gamma$      where $m _o$ is the rest mass and  $\gamma  = \dfrac{1}{\sqrt{1- v^2/c^2}}$

Thus mass of the moving rod must increases by a factor of  $\gamma$.

Multiple choice relativistic mechanics option a: relativity physics

By what fraction does the mass of a boy increase when he starts running at a speed of 12 km h$^{-1}$?

  1. $11\times {10}^{-27}$

  2. <span>$20\times {10}^{-27}$</span>

  3. <span>$6.17\times {10}^{-27}$</span>

  4. <span>$25.89\times {10}^{-27}$</span>
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given :   $v = 12$ km/h $ = 12\times \dfrac{5}{18}  =3.33$  m/s

Mass of the boy while running       $m  =\dfrac{m _o}{\sqrt{1 - (\frac{v^2}{c^2})^2}}$
$\therefore$  Fractional increase in the mass         $\dfrac{\Delta m}{m _o} = \dfrac{\dfrac{m _o}{\sqrt{1- (v/c)^2}} - m _o}{m _o}  = [1-(v/c)^2]^{-1/2} - 1$
$\implies $     $\dfrac{\Delta m}{m _o} = 1+ \dfrac{v^2}{2c^2}  -1  = \dfrac{v^2}{2c^2}$               (for $ v<< c$)

$\therefore$    $\dfrac{\Delta m}{m _o}  = \dfrac{3.33^2}{2 (3\times 10^8)^2}   =6.17\times 10^{-17}$  kg

Multiple choice relativistic mechanics option a: relativity physics

A girl runs a $100 m$ dash. Her dad cheers for her as he watches.
According to the special theory of relativity (although the effects would be very slight) which of the following statements could be accurately stated (because of the effects of special relativity)?

  1. After the race, the dad could say to his daughter, "You were less massive during the race than you are now"

  2. After the race, the girl could say, "My time was a little better than I expected, but the race actually was not quite $100 m$ long

  3. During the race, the girl could yell to here dad, "You are less massive now than you were before the race!"

  4. After the race, the dad could say to his daughter, "During the race, you were taller than you are now"

  5. During the race, the girl could yell to her dad, "You are shorter than you were before the race"
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
After the race, the girl could say, "My time was a little better than I expected, but the race actually was not quite 100m long, as Einstein's theory of special relativity states that the length of objects moving at relativistic speeds undergoes a contraction along the dimension of motion.
Multiple choice relativistic mechanics option a: relativity physics

The dynamic mass of electron having rest mass $m _0 $ and moving with speed $0.8 c $ is :

  1. $0.6 m _0 $

  2. $0.8 m _0 $

  3. $ \dfrac{5}{3} m _{o}$

  4. $ 1.25 m _0 $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Mass of the moving electron is,

$m = \dfrac{{{m _0}}}{{\sqrt {\left( {1 - \frac{{{v^2}}}{{{c^2}}}} \right)} }}$

$ = \dfrac{{{m _0}}}{{\sqrt {\left( {1 - \frac{{{{\left( {0.8c} \right)}^2}}}{{{c^2}}}} \right)} }}$

$ = \dfrac{{{m _0}}}{{\sqrt {\left( {0.36} \right)} }}$

$ = \dfrac{{{m _0}}}{{0.6}}$

$ = \dfrac{{5{m _0}}}{3}$

Thus mass of the moving electron is $\dfrac{{5{m _0}}}{3}$.

Multiple choice relativistic mechanics option a: relativity physics

When a material particle of rest mass $'m _0'$ attains the velocity of light, its mass becomes.

  1. $0$

  2. $2m _0$

  3. $4m _0$

  4. Infinity
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Rest mass of an object is the inertial mass of that object when it is at rest. The relation between object's mass and rest mass is given by:  


$m = \dfrac{m _0}{\sqrt{1- \dfrac{v^2}{c^2}}}$

where $m _0$ is rest mass, $v$ is velocity of object and $c$ is speed of light. 
Now, if some particle attains the velocity of light then according to the equation mentioned above it's mass becomes infinity. 

Multiple choice relativistic mechanics option a: relativity physics

Statement saying that energy is radiated or absorbed in discrete packets is given by ____________.

  1. Planck

  2. Einstein

  3. Stephen

  4. Wien
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
According to Planck’s quantum theory, Different atoms and molecules can emit or absorb energy in discreet quantities only. 
Multiple choice relativistic mechanics option a: relativity physics

Quantum effects are important only when observing _____________.

  1. small objects

  2. very large objects

  3. atomic size objects

  4. all of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Quantum theory is the theoretical basis of modern physics that explains the nature and behaviour of matter and energy on the atomic and subatomic level. So quantum effect is important only when observing atomic size objects.
Multiple choice relativistic mechanics option a: relativity physics

The velocity of a particle is dependent on the times as $\upsilon =k(t-1)$ where $k=2\ m/s^{2}$. The distance covered in first three seconds will be

  1. $18\ m$

  2. $5\ m$

  3. $3\ m$

  4. $6\ m$
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice relativistic mechanics option a: relativity physics

Special theory of relativity treats problems involving :

  1. inertial frame of reference

  2. non-inertial frame of reference

  3. non-accelerated frame of reference

  4. accelerated frame of reference
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Einstein’s special theory of relativity is also based on two postulates, the constancy of the speed of electromagnetic radiation and the validity of physical laws in all inertial frame of reference
Multiple choice relativistic mechanics option a: relativity physics

Behavior of matter on atomic level cannot be explained with ____________.

  1. modern physics

  2. classical physics

  3. relativistic mechanics

  4. atomic spectra
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
As Classical physics is the study of motion, projectiles, pulleys, and the planets. It mainly deals with the movement of large objects through space at a relatively low slow speed.