# Assertion and Reason Questions for Class 11 Physics Chapter 2 Units and Measurements

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Directions: Each of these questions contain two statements, Assertion and Reason. Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Q.1. Assertion : Dimensional constants are the quantities whose values are constant.
Reason : Dimensional constants are dimensionless.

Q.2. Assertion : When we change the unit of measurement of a quantity, its numerical value changes.
Reason : Smaller the unit of measurement smaller is its numerical value.

Q.3. Assertion : Parallax method cannot be used for measuring distances of stars more than 100 light years away.
Reason : Because parallax angle reduces so much that it cannot be measured accurately.

Answer Answer: (a) As the distance of star increases, the parallax angle decreases, and great degree of accuracy is required for its measurement. Keeping in view the practical limitation in measuring the parallax angle, the maximum distance of a star we can measure is limited to 100 light year.

Q.4. Assertion : A.U. is much bigger than Å.
Reason : A.U. stands for astronomical unit and A stands for Angstrom.

Q.5. Assertion : The cesium atomic clocks are very accurate
Reason : The vibration of cesium atom regulate the rate of cesium atomic clock.

Q.6. Assertion: In the measurement of physical quantities direct and indirect methods are used.
Reason : The accuracy and precision of measuring instruments along with errors in measurements should be taken into account, while expressing the result.

Q.7. Assertion : Now a days a standard metre is defined in terms of the wavelength of light.
Reason : Light has no relation with length.

Q.8. Assertion : The error in the measurement of radius of the sphere is 0.3%.The permissible error in its surface area is 0.6%.
Reason : The permissible error is calculated by the formula ΔA/A=4Δr/r

Q.9. Assertion : The number of significant figures depends on the least count of measuring instrument.
Reason : Significant figures define the accuracy of measuring instrument.

Q.10. Assertion : Out of three measurements I = 0.7 m; I = 0.70 m and I = 0.700 m, the last one is most accurate.
Reason : In every measurement, only the last significant digit is not accurately known.

Answer Answer: (b) The last number is most accurate because it has greatest significant figure (3).

Q.11. Assertion : Number of significant figures in 0.005 is one and that in 0.500 is three
Reason : This is because zeros are not significant.

Answer Answer: (c) Since zeros placed to the left of the number are never significant, but zeros placed to right of the number are significant

Q.12. Assertion : Absolute error may be negative or positive.
Reason : Absolute error is the difference between the real value and the measured value of a physical quantity.

Q.13. Assertion : Energy cannot be divided by volume.
Reason : Dimensions for energy and volume are different.

Q.14. Assertion: ‘Light year’ and ‘Wavelength’ both measure distance.
Reason : Both have dimension of time.

Answer Answer: (c) Light year and wavelength both represent the distance, so both have dimension of length not of time.

Q.15. Assertion : Avogadro’s number is the number of atoms in one gram mole.
Reason : Avogadro’s number is a dimensionless constant.

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