# Class 11 Physics Chapter 2 Important Questions Units and Measurement

Q.1. What are fundamental units? Ans. Fundamental units are those units, which can neither be derived from one another nor can they be further resolve

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 2 Units and Measurement. In this lesson, students will learn about Units and Measurement. This will not only help the students to know the important questions but will also help them during revision.

Q.1. What are fundamental units?
Ans. Fundamental units are those units, which can neither be derived from one another nor can they be further resolved into any other units.

Q.2. What are derived units?
Ans. The units of all such physical quantities, which can be expressed in terms of the fundamental units of mass, length and time are called Derived units.

Q.3. What is meant by unit?
Ans. The unit of a physical quantity is a standard of the same kind choose and in order to measure that quantity.

Q.4. What should we know in order to measure a physical quantity?
Ans. It's your net and the number of times the unit is contained in the physical quantity.

Q.5. Is the measure of a physical quantity dependent upon the choice of unit?
Ans. If the size of the unit used to measure the physical quantity is bigger, the numerical value of the physical quantity is smaller and vice-versa. However, the measure of a physical quantity remains the same.

Q.6. How many nanometre are there in one metre?
Ans. Now, 1 nanometre = 10-9 m Therefore, 1m = 109 nm.

Q.7. How many fermi are there in one metre?
Ans. Now, 1 fermi = 10-15 m Therefore, 1 m = 1015 f.

Q.8. Is light year is a unit of time?
Ans. No, it is a unit of distance.

Q.9. Define light year.
Ans. One light year is defined as the distance travelled by light in one year.

1 light year = 9.46 × 1015 m

Q.10. Define astronomical unit.
Ans. It is the mean distance of the Sun from Earth.

1 AU = 1.496 × 1011 m

Q.11. Define a parsec.
Ans. It is the distance at which an arc of length one astronomical unit subtends an angle of one second of an arc.

1 parsec = 3.08 × 1016 m

Q.12. arrange the following units of length in descending order :Light year, Astronomical unit and Parsec.
Ans. Parsec > light year >astronomical unit.

Q.13. How many kilograms are there in 1 a.m.u. ?
Ans. 1 a.m.u. = 1.66 × 10-27 kg.

Q.14. Express 0.000003 m as a power of 10.
Ans. 0.000003 = 3 × 10-6

Q.15. What necessitated the selection of some fundamental units?
Ans. For measuring mass, length and time, three independent units i.e. kilogram, metre and second are used. For measuring other physical quantities, if a separate unit is defined for each of them, then it will become very difficult to remember all of them as they will be quite unrelated to each other.

Q.16. What are the characteristics of a physical standard?
Ans.

• (a) It should be well defined.
• (b) It should be of suitable size neither too large or too small in comparison to the quantity to be measured.
• (c) It should be easily reproducible at all places.
• (d) It should not change with time and from place to place.
• (e) It should not change with change in its physical conditions such as temperature pressure, etc.
• (f) It should be easily accessible.

Q.17. What is the advantage in choosing the wavelength of a light radiation as a standard of length?
Ans. The advantage in choosing the wavelength of a light radiation as a standard of length are as follows:

• (a) The wavelength of light is not affected by time and environment.
• (b) This standard of length does not undergo any change places.

Q.18. Both mks and SI systems have metre, kilogram and second as the basic units how does mks system differ from SI system?
Ans. SI system covers the units of physical quantities from all the branches of physics, whereas mks system is confined to mechanics only.

Q.19. Define One lunar month.
Ans. One lunar month is the time taken by the moon to complete one revolution around the earth in its orbit.

Q.20. What is meant by one solar day?
Ans. One solar day is the time taken by the earth to complete one rotation about its axis. 1 solar day = 86,400 s.

Q.21. Define second.
Ans. Second (s) is defined as to be equal to the duration of 9,192,631,770 vibrations corresponding to the transition between two hyperfine levels of caesium-133 atom in the ground state.

Q.22. Define ampere.
Ans. Ampere (A) is defined as the current which generates a force of 2×10-7 Nm-1 between two straight parallel conductors of infinite length and negligible circular cross-sections when placed 1 m apart in vacuum.

Q.23. Define mole.
Ans. Mole (mol) is the amount of a substance containing the same number of elementary units as there are atoms in 0.012 kg of carbon-12.

Q.24. What is the SI unit of temperature?
Ans. The SI unit of temperature is Kelvin (K).

Q.25. If an atom were enlarged to the size of the earth (~107 m), how large would its nucleus be?
Ans. We know,

size of an atom = 10-11 m ;

Size of the earth = 107 m

Thus, we are scaling up atom by a factor of 107 / 10-11 i.e. 1018

Actual size of the nucleus = 10-14 m

Scaling up by a factor 1018 , the apparant size of the nucleus = 10-14 × 1018 = 104 m

The apparent size of the nucleus (when scaled up) is that of the height of Mount Everest.

Q.26. What do you mean by order of magnitude of a quantity?
Ans. The order of magnitude of a quantity means its value in suitable power of 10 nearest to the actual value of quantity.

Q.27. How do you find the order of magnitude of a quantity?
Ans. If a number is less than 5, it is taken as 1 and if it lies between 5 and 10, it is taken as 10.

Q.28. What is the order of the size of a galaxy?
Ans. 1021 m approximately.

Q.30. Which type of waves are used in SONAR?
Ans. Ultrasonics.

Q.31. Which method is used to find the height of a tower or that of a mountain?
Ans. Triangulation method.

Q.32. what does LASER mean?
Ans. LASER is the abbreviation for Light Amplification Stimulated Emission Radiation.

Q.33. What is meant by angular diameter of moon?
Ans. The angle subtended by two diametrically opposite ends of the moon at a point on the earth is called angular diameter of the moon. Its value is about 0.5°.

Q.34. Are inertial and gravitational mass of an object differ from one another?
Ans. No, initial and gravitational mass of an object are the same.

Q.35. What is the most accurate clock?
Ans. A cesium clock is the most accurate one. Two cesium clocks may differ by only one second after running for 5,000 years.

Q.36. Suggest distance corresponding to each of the following orders of length : (i) 107 m (ii) 104 m (iii) 103 m (iv) 102 m (v) 10-3 m (vi) 10-6 m (vii) 10-14 m.
Ans.

• (i) 107 m = radius of the earth
• (ii) 104 m = height of Mount Everest
• (iii) 103 m = distance travelled by the sound in water in one second
• (iv) 102 m = length of a college playground
• (v) 10-3 m = thickness of a card-board
• (vi) 10-6 m = mean free path of an air molecule
• (vii) 10-14 m = size of the atomic nucleus.

Q.37. What do you mean by gravitational mass?
Ans. The mass of the body measured, when the body is under the effect of gravity but without motion is called its gravitational mass.

Q.38. What do we mean by inertial mass?
Ans. The mass of the body measured, when the body is in translational motion under the effect of an external force other than gravity, is called as inertial mass.

Ans. The actual volume occupied by the atoms in one gram of a substance is two thirds of the volume occupied by one gram of the substance.

Q.40. Define weight.
Ans. Weight of a body is defined as the gravitational pull on it due to the earth.

W = mg