Tag: physics

Questions Related to physics

Which among the following is the major cause of nutation in Earth's axis:

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. Earth's Oblate spheroid shape

  2. The difference between Land and Sea hemispheres

  3. Oceanic Currents

  4. TIdal Forces

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Correct Option: D
Explanation:
In case of earth the principal sources of tidal forces are the sun and moon while continuously change location relative to each other and cause nutation in earth's axis, So option (D) correct.

The mean radius of the earth is $R$, its angular speed about its own axis $\omega$ and the acceleration due to gravity at the earth surface is $g$. The cube of radius of orbit of 'geostationary satellite' will be

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. $(R^{2}g/ \omega)$

  2. $(R^{2}\omega/ g)$

  3. $(Rg/ \omega^{2})$

  4. $(R^{2}g/ \omega^{2})$

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Correct Option: D
Explanation:

$mr\omega^{2} = \dfrac {GMm}{r^{2}}$
$\Rightarrow r\omega^{2} = \dfrac {GM}{r^{2}}$
$\Rightarrow r^{3} = \dfrac {GM}{\omega^{2}} = \dfrac {GM}{R^{2}} . \dfrac {R^{2}}{\omega^{2}}$
$\Rightarrow r^{3} = g \dfrac {R^{2}}{\omega^{2}}$
Hence (D) is correct.

Instrument used for determining specific gravity is

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. Hydrometer

  2. Barometer

  3. Calorimeter

  4. None of these

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Correct Option: A

For a ball falling in a iiquid with constant velocity, ratio of resistance force due to the liquid to that due to gravity is 

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. $\frac{{2{a^2}\rho g}}{{9xv}}$

  2. $\dfrac { 2 a ^ { 2 } \rho g } { 9 \eta ^ { 2 } }$

  3. $\dfrac { 2 a ^ { 2 } ( \rho - \sigma ) g } { 9 \eta }$

  4. none of these

Show answer
Correct Option: A
Explanation:

Force due to liquid (stokes law):

$f = 6\pi xrv$
Force due to gravity:
$\begin{array}{l} f=mg \ =\rho .\dfrac { { 4\pi { r^{ 3 } } } }{ 3 } .g \end{array}$
Ratio =
$\ \begin{array}{l} \dfrac { { 4\rho .\pi { r^{ 2 } }.g } }{ { 3.6\pi xr.v } }  \ =\dfrac { { 2{ a^{ 2 } }\rho g } }{ { 9xv } }  \end{array}$
$\therefore \,\,\,\,\,\dfrac{{2{a^2}\rho g}}{{9xv}}$
so,
Option $A$ is correct answer.

If a charge particle projected in a gravity-free room it does not deflect, 

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. electric field and magnetic field must be zero

  2. both electric field and magnetic field may be present

  3. electric field will be zero and magnetic field may be zero

  4. electric field may be zero and magnetic field may be zero

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Correct Option: A

If the earth shrinks such that its mass does not change but radius decreases to one quarter of its original value then one complete day will take:

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. 96 h

  2. 48 h

  3. 6 h

  4. 1.5 h

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Correct Option: D
Explanation:

We know that angular momentum of spin $\displaystyle =I\omega $
Bythe conservation of angular momentum
$\displaystyle \frac { 2 }{ 5 } M{ R }^{ 2 }.\frac { 2\pi  }{ T } =\frac { 2 }{ 5 } M{ \left( \frac { R }{ 4 }  \right)  }^{ 2 }.\frac { 2\pi  }{ T' } $
$\displaystyle T'=\frac { T }{ 16 } =\frac { 24 }{ 16 } =1.5h$

Weight $W _ { m }$ of the body can be given as 

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. $m g - m \frac { \left( v _ { e } + v \right) ^ { 2 } } { R }$

  2. $m g - m \frac { \left( v _ { e } - v \right) ^ { 2 } } { R }$

  3. $\frac { m } { R } \left[ v _ { e } ^ { 2 } - \left( v _ { e } + v \right) ^ { 2 } \right]$

  4. $m g + m \frac { \left( v _ { e } + v \right) ^ { 2 } } { R }$

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Correct Option: A

Why are the passengers in the upper deck of a double-decker bus not allowed to stand?

physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

  1. This ensures that the centre of gravity of the system may not rise up and the bus may not be toppled due to unstable equilibrium

  2. This ensures smaller centripetal force, thus helping the driver to negotiate the roundabouts properly

  3. If the passengers are in standing position, they may start oscillating due to jerks and there is a possibility of resonance, causing the bus to be toppled

  4. This is just for the safety reason

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Correct Option: A
Explanation:

When the passengers stand, the center of gravity rises. If it rises much it becomes unstable and may topple down.

Two thin lenses of focal length $f _1$ and $f _2$ are in contact and coaxial. The power of the combination is

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. $\sqrt{\frac{f _1}{f _2}}$

  2. $\sqrt{\frac{f _2}{f _1}}$

  3. $\frac{f _1+f _2}{f _1f _2}$

  4. $\frac{f _1-f _2}{f _1f _2}$

Show answer
Correct Option: C
Explanation:

For the lenses placed coaxially, 

We have the relationship $P=P _1+P _2+P _3+......$ 
So,for given case P=$P _1+P _2$ or $P $= $\dfrac{1} {f _1} $+ $\dfrac{1} {f _2} $

So,C is the correct answer. 

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with each other. The combination has a focal length equal to

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. Infinite

  2. $50 cm$

  3. $60 cm$

  4. $10 cm$

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Correct Option: A