NCERT Solutions for Class 11 Physics Chapter 1 Units and Measurements

NCERT Solutions for Class 11 Physics Chapter 1 Units and Measurements

Multiple Choice Questions (MCQs)

Q.1.The number of significant figures in 0.06900 is
(a) 5              (b) 4                  (c) 2                    (d) 3
Thinking Process
If the number is less than 1, the zero(s) on the right of decimal point and before the first non-zero digit are not significant.
Ans. (b) In 0.06900, the underlined zeroes are not significant. Hence, number of significant figures are four (6900).

Q. 2 The sum of the numbers 436.32, 227.2 and 0.301 in appropriate significant figures is
(a) 663.821         (b) 664        (c) 663.8            (d) 663.82
Ans. (b) The sum of the numbers can be calculated as 663.821 arithmetically. The number with least decimal places is 227.2 is correct to only one decimal place.
The final result should, therefore be rounded off to one decimal place i.e., 664.

Note In calculating the sum, we should not confuse with the number of decimal places
and significant figures. The result should have least number of decimal places.

Q, 3 The mass and volume of a body are 4.237 g and 2.5 cm3, respectively. The density of the material of the body in correct significant figures is
(a) 1.6048 g cm’3 (b) 1.69 g cm’3 (c) 1.7 g cm-3 (d) 1.695

Thinking Process
In multiplication or division, the final result should retain as many significant figures as are there in the original number with the least significant figures.

Ans. (c) In this question, density should be reported to two significant figures.
Density = 4.237g/2.5 cm3 = 1.6948

As rounding off the number, we get density = 1.7

Q. 4 The numbers 2.745 and 2.735 on rounding off to 3 significant figures will give
(a) 2.75 and 2.74 (b) 2.74 and 2.73 (c) 2.75 and 2.73 (d) 2.74 and 2.74
Ans. (d) Rounding off 2.745 to 3 significant figures it would be 2.74. Rounding off 2.735 to 3 significant figures it would be 2.74.

Q. 5 The length and breadth of a rectangular sheet are 16.2 cm and 10.1 cm, respectively. The area of the sheet in appropriate significant figures and error is
(a) 164± 3 cm2 (b) 163.621 2.6 cm2 (c) 163.61 2.6 cm2 (d) 163.621 3 cm2








Q. 9 Which of the following measurement is most precise?
(a) 5.00 mm (b) 5.00 cm (c) 5.00 m (d) 5.00 km
Ans. (a) All given measurements are correct upto two decimal places. As here 5.00 mm has the <• smallest unit and the error in 5.00 mm is least (commonly taken as 0.01 mm if not specified), hence, 5.00 mm is most precise.
Note In solving these type of questions, we should be careful about units although their magnitude is same.

Multiple Choice Questions (More than One Option)

Very Short Answer Type Questions (VSAQ)

Q. 19 Why do we have different units for the same physical quantity?
Ans. The value of any given physical quantity may vary over a wide range, therefore, different units of same physical quantity are required.
e.g., The length of a pen can be easily measured in cm, the height of a tree can be measured in metres, the distance between two cities can be measured in kilometres and distance between two heavenly bodies can be measured in light year.

Q. 20 The radius of atom is of the order of 1 A and radius of nucleus is of the order of fermi. How many magnitudes higher is the volume of atom as compared to the volume of nucleus?

Q. 21 Name the device used for measuring the mass of atoms and molecules.
Ans. A mass spectrograph is used for measuring the mass of atoms and molecules.

Q. 22 Express unified atomic mass unit in kg.




Ans.Since, f(o) is a sum of different powers of 0 and it is a dimensionless quantity.
By principle of homogeneity as RHS is dimensionless, hence LHS should also be dimensionless.

Q. 24 Why length, mass and time are chosen as base quantities in mechanics?
Ans. Length, mass and time are chosen as base quantities in mechanics because
(i) Length, mass and time cannot be derived from one another, that is these quantities are independent.
(ii) All other quantities in mechanics can be expressed in terms of length, mass and time.

Q. 25 (a) The earth-moon distance is about 60 earth radius. What will be the diameter of the earth (approximately in degrees) as seen from the moon?
(b) Moon is seen to be of (1/2)° diameter from the earth. What must be the relative size compared to the earth?
(c) Fronj parallax measurement, the sun is found to be at a distance of about 400 times the earth-moon distance. Estimate the ratio of sun-earth diameters.

Q.26 Which of the following time measuring devices is more precise?
(a) A wall clock (b) A stop Watch (c) A digital watch (d) An atomic clock
Ans. A wall clock can measure time correctly upto one second. A stop watch can measure time correctly upto a fraction of a second. A digital watch can measure time up to a fraction of second. An atomic clock can measure time most precisely as its precision is in 10^-13 s.

Q. 28 The vernier scale of a travelling microscope has 50 divisions which coincide with 49 main scale divisions. If each main scale division is 0.5 mm, calculate the minimum inaccuracy in the measurement of distance.
Thinking Process
Inaccuracy will be measured by difference of 1 MSD and 1VSD, where MSD = mam scale division and VSD = verneir scale division.

Long Answer Type Questions

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