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System of unit - class-XII

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If 1 kg wt = 10 N, the value of gravitational intensity will be 

  1. $\displaystyle 10{ m }/{ { s }^{ 2 } }$

  2. $\displaystyle \frac { 1 }{ 10 } { m }/{ { s }^{ 2 } }$

  3. $\displaystyle 1{ m }/{ { s }^{ 2 } }$

  4. $\displaystyle \frac { 1 }{ 100 } { m }/{ { s }^{ 2 } }$


Correct Option: A
Explanation:

If $1\,kg\,wt=10\,N=F$

Then gravitational intensity is given by,

$g=\dfrac Fm$

$=\dfrac{10 }{1}=10 m/s^2$

The magnetic field flux is expressed in

  1. Dynes

  2. Oersted

  3. Gauss

  4. Weber


Correct Option: D
Explanation:

Magnetic Flux is defined as the number of magnetic field lines passing through a given closed surface. It gives the measurement of the total magnetic field that passes through a given surface area. S.I. unit of magnetic flux is Weber $(Wb)$.

Which of the following class of forces is different from others?

  1. Pulling of a cart

  2. Stretching of a coiled spring

  3. Kicking of a football

  4. Electrical force


Correct Option: D
Explanation:

Pulling of a cart, stretching of a coiled spring and kicking of a football take place due to contact forces. contact force is any force that requires contact to occur. But electrical force is a non contact force which acts on an object without coming physically in contact.

Which kind of waves are produced by supersonic jet planes?

  1. Shock waves

  2. Seismic waves

  3. Infrasonic waves

  4. Infrared waves


Correct Option: A
Explanation:

The waves produces by the supersonic jet planes are shock waves. When a wave moves faster than the local speed of sound in a fluid, it is a shock wave.

100 square millimetres ($mm^2$ )= ________ square centimetre($cm^2$).

  1. $100$

  2. $10$

  3. $1$

  4. $0.1$


Correct Option: C
Explanation:
1 Sq. Centimeter = 100 Sq. Millimeters
1 Sq. Millimeter = 0.01 Sq. Centimeter

_______ square centimetres = $1$ square decimetre ($dm^2$ )

  1. $100$

  2. $10$

  3. $1000$

  4. $0.1$


Correct Option: A
Explanation:

$1\ cm=0.1\ dm$ or $10\ cm=1\ dm$

Thus, $1\ dm\times  1\ dm=10\ cm\times 10\ cm$
So, $1\ dm^2=100\ cm^2$

Which of the following is used as SI prefixes:

  1. Micro

  2. Mica

  3. Mikro

  4. Maca


Correct Option: A
Explanation:

Micro is a prefix used in defining some units.  Whenever the word micro is attached as a prefix to an SI unit,  it means it is one million times smaller than that unit. For example, 1 micrometre is one millionth part of a metre. Rest all other options are not   prefixes 

Which of the following is not used as SI prefixes:

  1. Deca

  2. Kilo

  3. Femto

  4. Mica


Correct Option: D
Explanation:

When 'Deca' is attached as a prefix to a unit, it means it is 10 times of that unit.  Kilo means thousand times of a unit.  Femto means of $ {10}^{-15} $ of a unit. Mica is not a prefix but a name of a specific material.

Which of the following is used as SI prefixes:

  1. Femto

  2. Atto

  3. Zepto

  4. All of the above


Correct Option: D
Explanation:

Here femto, atto and zepto, all are used as a prefix in the SI system.

In metric systme, $1 $ femtometer $=10^{-15}$ meter,  $1 $ attometer $=10^{-18}$ meter and $1 $ zeptometer $=10^{-21}$ meter

Which of the following is used as SI prefix:

  1. Mega

  2. Hecto

  3. Tera

  4. Mara


Correct Option: A,B,C
Explanation:

Mega, Hecto and Tera are used as SI prefix. Mega is used for $10^{6}$, Hecto for $10^2$ and Tera for $10^{12}$.

If the values of force and length are increased four times, then the unit of energy will be increase by:

  1. 4 times

  2. 2 times

  3. 8 times

  4. 16 times


Correct Option: D
Explanation:
The correct option is D
We have, $f=4[MLT^{-2}]$ and $d=4[L]$

$E=f\times d$

We know that Energy is equal to the product of force and distance.

Thus,
$=4[MLT^{-2}]\times4[L]$

$=16[ML^2T^{-2}]$

If both increased by 4times then the energy increased by $16\ times$

If unit of mass, length and time are tripled, then unit of energy becomes:

  1. $3$

  2. $\dfrac{1}{3}$

  3. $9$

  4. $\dfrac{1}{9}$


Correct Option: A
Explanation:
Unit of Energy $kg\,m^2/s^2$

Unit of length $= m$ (metre)

Unit of mass $= kg$ (kilogram)

Unit of time $= s$ (second)


Thus unit of energy in terms of unit of mass, length and time is given by,

$Unit \,of\, Energy =\dfrac{(Unit\,of\,mass)\times(Unit\,of\,length)^2}{(Unit\,of\,time)^2}$

If the unit of mass, length and time are tripled the unit of energy will be,

$Unit \,of\, Energy =\dfrac{(3\times Unit\,of\,mass)\times(3\times Unit\,of\,length)^2}{(3\times Unit\,of\,time)^2}$

$Unit \,of\, Energy =3\times \dfrac{(Unit\,of\,mass)\times(Unit\,of\,length)^2}{(Unit\,of\,time)^2}$

$Unit\,of\,energy=3\times Unit\,of\,energy$

Therefore, if the unit of mass, length and time are tripled, the unit of energy will be, 3 times more than the original unit of energy.

If a thermometer reads freezing point of water at 20 degree Celsius and boiling point as 150 degree Celsius, how much thermometer can read when the actual temperature is 60 degree Celsius?

  1. 98 degrees celsius

  2. 10 degree Celsius

  3. 40 degree Celsius

  4. 60 degree Celsius


Correct Option: A

The unit of length, mass and energy are doubled.Which of the  following is /are correct?

  1. unit of time is doubled

  2. unit of momentum is doubled

  3. unit of power is doubled

  4. Both (1) & (2)


Correct Option: B

In the equation $\left( P+\dfrac { a }{ { V }^{ 2 } }  \right) \left( v-b \right) =RT,$ the SI unit of a is

  1. N ${ m }^{ 2 }$

  2. ${ m }^{ 4 }$

  3. N${ m }^{ -3 }$

  4. ${ m }^{ -2 }$


Correct Option: C

The melting point of pure ice is

  1. $ 23.5^o C $

  2. $ 100^o C $

  3. 273.15 K

  4. 100 K


Correct Option: C
Explanation:

The melting point of pure ice is $0^0C$. To convert temperature from degree Celsius to Kelvin, we add $273.15$.


So , $0^0C = (0+273)\,K = 273\,K$

Then, melting point of pure ice is $273.15\,K$.

If $10 ^ { 7 }$ erg is taken as unit of energy, $10 ^ { 5 }$ dyne as theunti of force, one second as unit of time, what is the unit of length? 

  1. 1$m$

  2. 10$\mathrm { m }$

  3. 100$m$

  4. none


Correct Option: B

On a hypothetical scale X, the ice point in ${ 40 }^{ \circ  }$ and then steam point is ${ 120 }^{ \circ  }$. For another scale Y the ice point and stem points are ${ -30 }^{ \circ  }$ and ${ 130 }^{ \circ  }$ respectively. If X-reads ${ 50 }^{ \circ  }$ The read of Y is 

  1. $-{ 5 }^{ \circ }$

  2. $-{ 8 }^{ \circ }$

  3. $-{ 10 }^{ \circ }$

  4. $-{ 12 }^{ \circ }$


Correct Option: C

Which of the following temperature scale doesn't have negative number.

  1. celsius

  2. kelvin

  3. fahrenheit

  4. A.O.T


Correct Option: B

1 dyne $=$ _____ $N$.

  1. $10^5$

  2. $10^{-5}$

  3. $10^6$

  4. $10^{-6}$


Correct Option: B
Explanation:

SI unit of Force is $Newton.$
CGS unit of force is $dyne.$
$1\ N = 1\ kg m/s^{2} = 1000 g \times  100 cm/s^{2} = 100000\ dyne$

$1$ Newton$=$ $K$ dyne, then what is the value of $K$?

  1. $1$

  2. $10^2$

  3. $10^5$

  4. $10$


Correct Option: C
Explanation:

Newton is in SI units that is Kg metre per second whereas Dyne is in Gram centimetre per second.

We know that $1 Kg = 1000 gm$ where as $1 metre = 100 cm$.
Hence to convert from SI units to CGS unit, we will have to multiply by a factor that is $1000\times100$.
$K=10^{5}$

Express the speed $36 km h^{-1}$ in $m s^{-1}$.

  1. $3600 m s^{-1}$

  2. $10 m s^{-1}$

  3. $0.6 m s^{-1}$

  4. $36 m s^{-1}$


Correct Option: B
Explanation:

$36\ km h^{-1}=\dfrac{36 km}{1h}=\dfrac{36 \times 1000 m}{60 \times 60 s}$$=10 m s^{-1}$

$\displaystyle kg{ ms }^{ -1 }$ is the SI unit of

  1. Impulse

  2. Force

  3. Angular velocity

  4. None of these


Correct Option: A
Explanation:

Impulse $(I)$ imparted to the body is equal to the change in linear momentum $(\Delta p)$ of the body.


Impulse, $I=\Delta p$

Thus momentum has the same units as that of impulse. Since momentum has S.I. unit as $kgms^{-1}$, impulse also has S.I. unit as $kgms^{-1}$.

Density of mercury in MKS is 13600 kg m$^{-3}$ then in CGS is:

  1. 1.36 g m$^{-3}$

  2. 13.6 kg cm$^{-3}$

  3. 13.6 g cm$^{-3}$

  4. 1.36 kg m$^{-3}$


Correct Option: C
Explanation:

CGS unit of density is $g cm^{-3}$

Thus, $13600 kg /m^{3}=13600\times \dfrac{10^{-3}g}{10^6 cm^3}=13.6 g$ $cm^{-3}$

If 1 g cm$^{-3} = 10^3\,kg\, m^{-3}$ then $10^6kgm^{-3}=$

  1. $10^{-3}\,g\,cm^{-3}$

  2. $10^{6}\,g\,cm^{-3}$

  3. $10^{3}\,g\,cm^{-3}$

  4. $10^{-6}\,g\,cm^{-3}$


Correct Option: C
Explanation:

Given, $1 g cm^{-3}=10^3 kgm^{-3}$

If we multiply $10^3$ on both sides, $10^3 g cm^{-3}=10^3\times 10^3 kgm^{-3}$
or $10^3 g cm^{-3}=10^6 kgm^{-3}$
Thus, option C will be the right option. 

$1^o A$ is ____

  1. $10 nm$

  2. $0.1 nm$

  3. $100 nm$

  4. $1 nm$


Correct Option: B
Explanation:

$1 ^oA=10^{-10} m=10^{-1}\times 10^{-9} m=0.1 nm$   (as $1 nm=10^{-9}m$)

Which among the following is the smallest prefix?

  1. milli

  2. nano

  3. fermi

  4. deci


Correct Option: C
Explanation:

Here, $1$ milimeter $=10^{-3}$ meter ;

$1$ angstrom $=10^{-10}$ meter ;
$1$ fermi $=10^{-15}$ meter, 

Therefore, the smallest prefix is for fermi.

Temperature can be expressed as a derived quantity in terms of :

  1. length and mass

  2. mass and time

  3. length, mass and time

  4. none of these


Correct Option: D
Explanation:

Temperature cannot be expressed in the terms of mass, length and time. Temperature is expressed in kelvin.

The physical quantity having the same unit in all the systems of unit is :

  1. length

  2. mass

  3. time

  4. foot


Correct Option: C
Explanation:

Time has same unit, second, in the SI, FPS, CGS and  MKS system. Rest of the quantities have different units in  SI, FPS, CGS and MKS system.

A ______ is the interval from one noon to the next noon.

  1. mean solar day

  2. solar day

  3. lunar day

  4. day


Correct Option: B
Explanation:

A solar day is the interval from one noon to the next noon.

SI unit of Magnetic flux is weber and its CGS unit is maxwell. Then 1 Wb = :

  1. $10^6$ maxwell

  2. $10^7$ maxwell

  3. $10^8$ maxwell

  4. $10^9$ maxwell


Correct Option: C
Explanation:

Magnetic flux is the product of magnetic field and area.

As SI unit of magnetic flux is weber, so $1 \, Wb=Tesla (T)\times m^2$
where $1 \, T=10^{4} \, gauss$ and $1 m^2=10^4 cm^2$
(T and gauss is the SI and CGS unit of magnetic field )
So, $1 \, Wb=10^4 \, gauss \times 10^4cm^2=10^8 $ maxwell

$S = A(1 - e^{-Bxt})$, where S is speed and x is displacement. The unit of B is :

  1. $m^{-1}s^{-1}$

  2. $m^{-2}s$

  3. $s^{-2}$

  4. $s^{-1}$


Correct Option: A
Explanation:

It is given that $x$ is displacement and $t$ is time.
Unit of $x$ is $m$ and that of time t is $s$.
Since, the quantity $Bxt$ must be dimensionless.
$\implies$ Unit of B must be $m^{-1}s^{-1}$.

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