# Units and Measurement

## Class 11 NCERT Physics

### NCERT

1   Fill in the blanks$\\$ (a) The volume of a cube of side $1 cm$ is equal to.... $m ^3$$\\ (b) The surface area of a solid cylinder of radius 2.0 cm and height 10.0 cm is equal to ... (mm ^2)$$\\$ (c) A vehicle moving with a speed of $18 km \ h^{- 1}$ covers....m in $1 s$$\\ (d) The relative density of lead is 11.3. Its density is ....g cm ^ 3 or . ...kg \ m ^{- 3} . ##### Solution : 1 cm = \dfrac{1}{100} m$$\\$ Volume of the cube $= 1cm ^3$$\\ But, 1 cm^ 3 = 1 cm * 1 cm * 1 cm =\\ (\dfrac{1}{100})m*(\dfrac{1}{100})m*(\dfrac{1}{100})m$$\\$ $\therefore 1 cm^3=10^{-6} m^3$$\\ Hence, the volume of a cube of side 1 cm is equal to 10^{-6} m^ 3$$\\$ The total surface area of a cylinder of radius $r$ and height h is $S = 2 \pi r ( r + h )$$\\ Give that r =2 cm = 2 * 1 cm = 2 * 10 mm = 20 mm\\ h = 10 cm = 10 = 10 mm = 100 mm\\ \therefore S = 2 * 3.14 * 20 ( 20 + 100 )= 15072 =1.5 * 10^ 4 mm ^2$$\\$ Using the conversion,$\\$ $1 km / hr=\dfrac{5}{18}m/s\\ 18 km/hr=18*\dfrac{5}{18}=5 m/s$$\\ Therefore, distance can be obtained using the relation: Distance = Speed × Time = 5 * 1 = 5m$$\\$ Hence, the vehicle covers $5 m$ in $1 s.$$\\ Relative density of a substance is given by the relation,\\ Relative density =\dfrac{\text{Density of substance}}{\text{ Density of water}}$$\\$ Density of water = $kg / cm ^3$$\\ Density of lead = Relative density of lead * Density of water\\ =11.3*1=11.3g /cm^3$$\\$ Again,$1g=\dfrac{1}{1000}kg\\ 1 cm^3=10^{-6} m^3\\ 1g.cm^3=\dfrac{10^{-3}}{10^{-6}} lg/m^3\\ =10^3 kg/m^3\\ \therefore 11.3 g /cm^3\\ =11.3*10^3 kg/m^3$

2   Fill in the blanks by suitable conversion of units:$\\$ $1 k g/m ^2 s ^{-2}= .... g\ cm ^2 s ^{- 2}\\ 1 m = ....1 y\\ 3.0 ms^{-2}- .... km /h^{-2}\\ G = 6.67 * 10^{-11} Nm ^2 ( kg)^{-2}= .... ( cm )^3s^{-2 } g{- 1}$

##### Solution :

$1 kg = 10^3 g\\ 1 m^2=10^4 cm^2\\ 1 kg/m^2 s^{-2}=1 kg * 1 m^2*1s^{-2}\\ =10^3 g * 10^4 cm^2*1s^{-2}=10^7 g/cm^2 s^{-2}$Light year is the total distance travelled by light in one year.$1\text{ly= Speed of light * One year}\\ =(3*10^8m/s)*(365*24*60*60s)\\ =9.46*10^{15}m\\ \therefore 1m=\dfrac{1}{9.46*10^{15}}=1.057*10^{-16}\text{ly}\\ 1 m=10^{-3}km$ Again,$1s=\dfrac{1}{3600}h\\ 1s^{-1}=3600h^{-1}\\ 1s^{-1}=(3600)^2h^{-2}\\ \therefore 3ms^{-2}\\ =(3*10^{-3 }km)*((36000)^2h^{-2})\\ =3.88*10^{-4} km/h^{-2}\\ 1N=1 kg/ms^{-2}\\ 1 kg =10^{-3} g^{-1}\\ 1 m^3=10^6 cm^3\\ \therefore 6.67*10^{-11}Nm^2 kg^{-2}\\ =6.67*10^{-11}*(1 kg /ms^{-2})(1 m^2)(1 s^{-2})\\ \therefore 6.67*10^{-11}*(1 kg *1 m^3 * 1 s^{-2})\\ =6.67 * 10^{-11}*(10^{-3} g^{-1})*(10^6 cm^3)*(1 s^{-2})\\ =6.67 * 10^{-8} cm^3 s^{-2}g^{-1}$

3   A calorie is a unit of heat or energy and it equals about $4.2 J$ where $1J = 1 kg/ m$ Suppose we employ a system of units in which the unit of mass equals $\alpha kg,$ the unit of length equals $\beta m$, the unit of time is $\gamma s$. Show that a calorie has a magnitude $4.2 \alpha ^{-1} \beta ^{-2 } \gamma^ 2$ in terms of the new units.

##### Solution :

Given that,$\\$ $1 \text{calorie}=4.2(1 kg)(1 m^2)(1 s^{-2})$$\\ New unit of mass = \alpha kg$$\\$ Hence, in terms of the new unit,$1 kg=\dfrac{1}{\alpha }=\alpha^{-1}$$\\ In terms of the new unit of length,\\ 1 m \dfrac{1}{\beta}=\beta^{-1} \ or \ 1 m^2 =\beta ^{-2}$$\\$ And, in terms of the new unit of time,$\\$ $1s=\dfrac{1}{\gamma}=\gamma^{-1}\\ 1 s^2=\gamma^{-2}\\ 1 s^{-2}=\gamma^2\\ \therefore 1 \text{Calories}=4.2(1 \alpha^{-1})(1 \beta^{-2})(1 \gamma^2)\\ =4.2 \alpha^{-1}\beta^{-2}\gamma^2$

4   Explain this statement clearly:$\\$ “To call a dimensional quantity ‘large’ or ‘small’ is meaningless without specifying a standard for comparison”. $\\$In view of this, reframe the following statements wherever necessary:$\\$ $\bullet$ atoms are very small objects$\\$ $\bullet$ a jet plane moves with great speed$\\$ $\bullet$ the mass of Jupiter is very large$\\$ $\bullet$ the air inside this room contains a large number of molecules$\\$ $\bullet$ a proton is much more massive than an electron$\\$ $\bullet$ the speed of sound is much smaller than the speed of light.

##### Solution :

The given statement is true because a dimensionless quantity may be large or small in comparison to some standard reference. For example, the coefficient of friction is dimensionless. The coefficient of sliding friction is greater than the coefficient of rolling friction, but less than static friction.$\\$ $\bullet$An atom is a very small object in comparison to a soccer ball.$\\$ $\bullet$A jet plane moves with a speed greater than that of a bicycle.$\\$ $\bullet$Mass of Jupiter is very large as compared to the mass of a cricket ball.$\\$ $\bullet$The air inside this room contains a large number of molecules as compared to that present in a geometry box.$\\$ $\bullet$A proton is more massive than an electron.$\\$ $\bullet$Speed of sound is less than the speed of light.

5   A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in tem of the new unit if light takes $8$ min and $20$ s to cover this distance?

Distance between the Sun and the Earth:$\\$ = Speed of light × Time taken by light to cover the distance$\\$ Given that in the new unit, speed of light = 414 unit$\\$ Time taken, $t = 8$ min $20 s = 500 s$$\\ \therefore Distance between the Sun and the Earth = 1 × 500 = 500 units 6 Which of the following is the most precise device for measuring length:\\ a vernier callipers with 20 divisions on the sliding scale\\ a screw gauge of pitch 1 mm and 100 divisions on the circular scale\\ an optical instrument that can measure length to within a wavelength of light ? ##### Solution : A device with minimum count is the most suitable to measure length. Least count of vernier callipers\\ = 1 standard division (SD) - 1 vernier division (VD)\\ =1-\dfrac{9}{10}=\dfrac{1}{10}=0.01cm$$\\$ Least count of screw gauge =$\dfrac{\text{pitch}}{\text{Number of divisions}}$$\\ =\dfrac{1}{1000}=0.001 cm$$\\$ Least count of an optical device = Wavelength of light$\\$ $= 0.00001 cm$$\\ Hence, it can be inferred that an optical instrument is the most suitable device to measure length. 7 A student measures the thickness of a human hair by looking at it through a microscope of magnification 100. He makes 20 observations and finds that the average width of the hair in the field of view of the microscope is 3.5 mm. What is the estimate on the thickness of hair? ##### Solution : Magnification of the microscope = 100$$\\$ Average width of the hair in the field of view of the microscope = $3.5 mm$$\\ \therefore Actual thickness of the hair is \dfrac{3.4}{100}=0.035 mm 8 Answer the following:\\ You are given a thread and a metre scale How will you estimate the diameter of the thread?\\ A screw gauge has a pitch of 1.0 mm and 200 divisions on the circular scale. Do you think it is possible to increase the accuracy of the screw gauge arbitrarily by increasing the number of divisions on the circular scale?\\ The mean diameter of a thin brass rod is to be measured by vernier callipers. Why is a set of 100 measurements of the diameter expected to yield a more reliable estimate than a set of 5 measurements only? ##### Solution : Wrap the thread on a uniform smooth rod in such a way that the coils thus formed are very close to each other. Measure the length of the thread using a metre scale. The diameter of the thread is given by the relation.\\ Diameter =\dfrac{\text{Lenght of thread}}{\text{ Number of turns }}$$\\$ It is not possible to increase the accuracy of a screw gauge by increasing the number of divisions of the circular scale. Increasing the number divisions of the circular scale will increase its accuracy to a certain extent only.$\\$ A set of $100$ measurements is more reliable than a set of $5$ measurements because random errors involved in the former are very less as compared to the latter.

9   The photograph of a house occupies an area of $1.75cm ^2$ on a $35mm$ slide. The slide is projected on to a screen, and the area of the house on the screen $1.55m ^2$ is the linear magnification of the projector-screen arrangement?

##### Solution :

Area of the house on the slide $= 1.75cm ^2$$\\ Area of the image of the house formed on the screen =1.55m ^2 = 1.55 * 10^ 4 cm^ 2$$\\$ Arial magnification $m _a=\dfrac{\text{Area of image}}{\text{Area of object}}=\dfrac{1.55}{1.75}*10^4$$\\ \therefore Linear magnifications, m_1=\sqrt{m_a}$$\\$ $=\sqrt{\dfrac{1.55}{1.75}*10^4}=94.11$

10   State the number of significant figures in the following:$\\$ $0.007 m ^2\\ 2.64 *10 ^{24} kg\\ 0.2370 g cm^{-3}\\ 6.320 J\\ 6.032 Nm^{- 2}\\ 0.0006032 m ^2$

##### Solution :

The given quantity is $0.007m ^2$$\\ If the number is less than one, then all zeros on the right of the decimal point (but left to the first non-zero) are insignificant. This means that here, two zeros after the decimal are not significant. Hence, only 7 is a significant figure in this quantity.\\ The given quantity is 2.64 * 10^{ 24} kg .$$\\$ Here, the power of $10$ is irrelevant for the determination of significant figures. Hence, all digits i.e., $2, 6$ and $4$are significant figure$\\$ The given quantity is $0.2370 g cm ^{- 3}$$\\ For a number with decimals, the trailing zeroes are significant. Hence, besides digits 2, 3 and 7, 0 that appears after the decimal point is also a significant figure. The given quantity is 6.320J$$\\$ For a number with decimals, the trailing zeroes are significant. Hence, all four digits appearing in the given quantity are significant figures.$\\$ The given quantity is $6.032 Nm^{-2}$$\\ All zeroes between two non-zero digits are always significant\\ The given quantity is 0.0006032m ^2$$\\$ If the number is less than one, then the zeroes on the right of the decimal point (but left to the first non-zero) are insignificant. Hence, all three zeroes appearing before $6$ are not significant figures. All zeros between two non-zero digits are always significant. Hence the remaining four digits are significant figures.

11   .The length, breadth and thickness of a rectangular sheet of metal are 4.234 m, 1.005 m, and 2.01 cm respectively. Give the area and volume of the sheet to correct significant figures.

##### Solution :

Length of sheet, l= 4.234 m$\\$ Breadth of sheet b = 1.005 m$\\$ Thickness of sheet h = 2.01 cm = 0.0201 m$\\$ The given table lists the respective significant figures:

Hence, area and volume both must have least significant figures i.e., 3.$\\$ Surface area of the sheet = $2 ( l * b + b * h +h * l )\\ = 2 ( 4.234 * 1.005 + 1.005 *0.0201 + 0.0201 *4.234 )\\ = 2 ( 4.25517 +0.02620 + 0.08510 )\\ = 2 * 4.360\\ = 8.72m^ 2\\$$\\ Volume of the sheet = l * b * h\\ = 4.234 * 1.005 * 0.0201\\ = 0.0855m ^3$$\\$ This number has only 3 significant figures i.e., 8, 5, and 5.

12   The mass of a box measured by a grocer’s balance is 2.300 kg. Two gold pieces masses 20.15 g and 20.17 g are added to the box. What is (a) the total mass of the box, (b) the difference in the masses of the pieces to correct significant figures?

##### Solution :

Mass of grocer’s box = 2.300 kg$\\$ Mass of gold piece I = 20.15g = 0.02015 kg$\\$ Mass of gold piece II = 20.17 g = 0.02017 kg$\\$ Total mass of the box = 2.3 + 0.02015 + 0.02017 = 2.34032 kg$\\$ In addition, the final result should retain as many decimal places as there are in the$\\$ number with the least decimal places. Hence, the total mass of the box is 2.3 kg.$\\$ Difference in masses = 20.17 - 20.15 = 0.02g$\\$ In subtraction, the final result should retain as many decimal places as there are in the number with the least decimal places.$\\$

13   A physical quantity P is related to four observables a . b . c and d as follows:$\\$ $P=\dfrac{a^3b^2}{\sqrt{cd}}$$\\The percentage errors of measurement in a , b , c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result? ##### Solution : P=\dfrac{a^3b^2}{\sqrt{cd}}\\ \dfrac{\Delta P}{P}=\dfrac{3\Delta a}{a}+\dfrac{2\Delta b}{b}+\dfrac{1\Delta c}{2c}+\dfrac{\Delta d}{d}\\ (\dfrac{\Delta P}{P}*100)%=(3*\dfrac{3\Delta a}{a}*100+2*\dfrac{2\Delta b}{b}*100+\dfrac{1\Delta c}{2c}*100+\dfrac{\Delta d}{d}*100\\)%\\ =3*1+2*3+\dfrac{1}{2}*4+2\\ =3+6+2+2=13%$$\\$ Percentage error in P = 13 %$\\$ Value of P is given as 3.763$\\$ By rounding off the given value to the first decimal place, we get P = 3.8.$\\$

14   A book with many printing errors contains four different formulas for the displacement y of a particle undergoing a certain periodic motion :$\\$ (a) y = a sin 2$\pi$ t/T$\\$ (b) y = a sin vt$\\$ (c) y = (a/T) sin t/a$\\$ (d) y = (a $\sqrt{2}$) (sin 2$\pi$t /T + cos 2$\pi$t /T)$\\$ (a = maximum displacement of the particle, v = speed of the particle. T = time-period of motion). Rule out the wrong formulas on dimensional grounds.

##### Solution :

(a = ma×imum displacement of the particle, v = speed of the particle. T= timeperiod of motion).$\\$ Rule out the wrong formulas on dimensional grounds.$\\$$\\ (a) Correct Y=a \sin^2\pi t/T$$\\$ Dimension of $y=M^ 1 L ^1 T ^0$$\\ Dimension of a= M ^1 L ^1 T ^0$$\\$ Dimension of $\sin^2\pi t/T = M ^1 L^ 0 T ^0$$\\ Dimension of L.H.S = Dimension of R.H.S\\ Hence, the given formula is dimensionally correct.\\ (b) Incorrect\\ Y = a sin vt\\ Dimension of y= M^ 0 L^ 1 T^ 0$$\\$ Dimension of a = $M^ 0 L^ 1 T^ 0$$\\ Dimension of vt = M^ 0 L^ 1 T ^{-1} × M ^0 L^ 1 T^ 1 = M^ 0 L ^1 T ^0$$\\$ But the argument of the trigonometric function must be dimensionless, which is not so in the$\\$ given case. Hence, the given formula is dimensionally incorrect.$\\$ (c) Incorrect$\\$ Y= (a/T) sin(t/a)$\\$ Dimension of y= $M ^0 L^ 1 T ^0$$\\ Dimension of a/T = M ^0 L^ 1 T ^{-1}$$\\$$\\ Dimension of =t/a = M ^0 L ^{-1} T ^1$$\\$ But the argument of the trigonometric function must be dimensionless, which is not so in the given case. Hence, the formula is dimensionally incorrect.$\\$ (d) Correct$\\$ Y=( a $\sqrt{2} )\sin^2\pi t/T+ cos^2\pi t/T$$\\ Dimension of y= M^ 0 L ^1 T ^0$$\\$ Dimension of a= $M ^0 L^ 1 T ^0$$\\ Dimension of t/T = M ^0 L ^0 T^ 0$$\\$ Since the argument of the trigonometric function must be dimensionless (which is true in the given case), the dimension of y and a are the same. Hence, the given formula is dimensionally correct

15   A famous relation in physics relates ‘moving mass’m to the ‘rest mass’ m$_ 0$ of a particle in terms of its speed v and the speed of light,c. (This relation first arose as a consequence of special relativity due to Albert Einstein). A boy recalls the relation almost correctly but forgets where to put the constant c. He writes:$\\$ $m=\dfrac{m_0}{(1-v^2)^{\frac{1}{2}}}$

Given the relation,$\\$ $m=\dfrac{m_0}{(1-v^2)^{\frac{1}{2}}}$$\\ Dimension of m = M^ 1 L ^0 T ^0$$\\$ Dimension of m 0 =$M^ 1 L ^0 T^0$$\\ Dimension of v = M ^0 L^{-1}T^{-1}$$\\$ Dimension of v 2 =$M ^0 L ^2 T ^{- 2}$$\\ Dimension of c = M ^0L^{-1}T^{-1}$$\\$ The given formula will be dimensionally correct only when the dimension of L.H.S is the same as that of R.H.S. This is only possible when the factor, ${(l-v^2)^{\frac{1}{2}}}$ is dimensionless i.e.,${(l-v^2)^{\frac{1}{2}}}$ is dimensionless. This is only possible if $\lambda ^2$ is divided by c$^ 2$ . Hence, the correct relation is$m=\dfrac{m_0}{(l-\dfrac{v^2}{c^2})^{\frac{1}{2}}}$$\\ 16 The unit of length convenient on the atomic scale is known as an angstrom and is denoted by A :1 A = 10^{- 10 } m The size of a hydrogen atom is about 0.5A what is the total atomic volume in m^3 of a mole of hydrogen atoms? ##### Solution : Radius of hydrogen atom, r = 0.5 A = 0.5 * 10 ^{- 10 } m\\ Volume of hydrogen atom =\dfrac{4}{3}\pi r^3\\ =\dfrac{4}{3}*\dfrac{22}{7}*(0.5*10 ^{- 10 })^3\\ = 0.524 * 10 ^-{ 30} m ^3$$\\$ 1 mole of hydrogen contains $6.023 * 10 ^-{23}$hydrogen atoms = Volume of 1 mole of hydrogen atoms $6.023 * 10 ^-{23} = 0.524 * 10 ^-{ 30} m ^3$

17   One mole of an ideal gas at standard temperature and pressure occupies 22.4 L (molar volume). What is the ratio of molar volume to the atomic volume of a mole of hydrogen? (Take the size of hydrogen molecule to be about 1A). Why is this ratio so large?

##### Solution :

18   Explain this common observation clearly : If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train’s motion, but the distant objects (hill tops, the Moon, the stars etc.) seem to be stationary. (In fact, since you are aware that you are moving, these distant objects seem to move with you).

##### Solution :

Line of sight is defined as an imaginary line joining an object and an observer’s eye. When we observe nearby stationary objects such as trees, houses, etc. while sitting in a moving train, they appear to move rapidly in the opposite direction because the line of sight changes very rapidly. On the other hand, distant objects such as trees, stars, etc. appear stationary because of the large distance. As a result, the line of sight does not change its direction rapidly.