Question 1
What is meant by measurement?
Answer
Measurement is the process of comparison of the given quantity with the known standard quantity of the same nature.
Question 2
What do you understand by the term unit?
Answer
Unit is the quantity of a constant magnitude which is used to measure the magnitudes of other quantities of the same nature.
Question 3
How is a physical quantity expressed?
Answer
A physical quantity is expressed as:
Physical quantity = (numerical value) x (unit
1. The unit in which the quantity is being measured, and
2. The numerical value which expresses, how many times the above selected unit is contained in the given quantity.
Question 4
Name the three fundamental quantities.
Answer
The three fundamental quantities are —
1. Mass
2. Length
3. Time
Question 5
What is the S.I. unit of Luminous intensity?
Answer
The S.I. unit of Luminous intensity is candela (cd).
Question 6
Define one parsec.
Answer
One parsec is the distance from where the semi major axis of orbit of earth (1 A.U.) subtends an angle of one second.
Question 7
Define a fundamental unit.
Answer
A fundamental unit is that which is independent of any other unit or which can neither be changed nor can be related to any other unit.
Example – Units of mass, length, time, temperature, current as kg, second and kelvin respectively.
Question 8
Define a derived unit.
Answer
Derived units are those which depend on the fundamental units or which can be expressed in terms of fundamental units.
Example: For the measurement of area, we need to measure length and breadth in the unit of length and then express area in a unit which is length x length and unit is m².
Question 9
Define standard metre.
Answer
The standard metre is defined in terms of speed of light, according to which, one metre is the distance travelled by light in 1/299,792,458 of a second in air (or vacuum).
Question 10
How is nanometer related to Angstrom ?
Answer
Relation between nanometer (nm) and Angstrom (Ã…) is expressed as:
1 nanometer = 10 Ã…
Question 11
Complete the following —
1. 1 light year = ________ m
2. 1 m = ________ Ã…
3. 1 m = ________ µ
4. 1 micron = ________ Ã…
5. 1 fermi = ________ m
Answer
1. 1 light year = 9.46 x 10¹⁵ m
2. 1 m = 10¹⁰ Ã…
3. 1 m = 10⁶ µ
4. 1 micron = 10⁴Ã…
5. 1 fermi = 10-¹⁵ m
Question 12
Complete the following —
1. 1g = ______kg
2. 1mg = ________kg
3. 1 quintal = ________kg
4. 1 a.m.u (or u) = ________kg
Answer
1. 1 g = 10-³ kg
2. 1 mg = 10-⁶ kg
3. 1 quintal = 100 kg
4. 1 a.m.u (or u) = 1.66 x 10-²⁷ kg
Question 13
What is a leap year?
Answer
A leap year is the year in which the month of February is of 29 days.
1 Leap year = 366 days
Every fourth year (i.e., the year divisible by 4) has one day extra in the month of february (i.e., February has 29 days) and so it is the leap year.
Question 14
'The year 2024 will have February of 29 days'. Is this statement true?
Answer
Yes the statement is true.
We know that, if any year is divisible by 4, then it is a leap year and in a leap year, February has 29 days. As, the year 2024 is divisible by 4, so it will have 29 days in February.
Question 15
What is a lunar month?
Answer
A lunar month is the time of one lunar cycle, i.e., it is the amount of time it takes for the Moon to complete one orbit around the Earth and it is nearly equal to 29.5 days.
Question 16
Complete the following —
1. 1 nano second = ________s
2. 1 µs = _______s
3. 1 mean solar day = ________s
4. 1 year = ________s
Answer
1. 1 nano second = 10-⁹ s
2. 1 µs = 10-⁶ s
3. 1 mean solar day = 86400 s
4. 1 year = 3.15 x 10⁷ s
Question 17
Name the physical quantities which are measured in the following units —
1. u
2. ly
3. ns
4. nm
Answer
Physical quantity related to the unit are as follows —
Unit Physical quantity
u Mass
ly Distance
ns Time
nm Length
Question 18
Write the derived units of the following —
1. Speed
2. Force
3. Work
4. Pressure
Answer
The derived units of the following quantities are as follows —
Quanity Derived unit
Speed ms-¹
Force kg m s-²
Work kg m²s-²
Pressure kg m-¹s-²
Question 19
How are the following derived units related to the fundamental units?
1. newton
2. watt
3. joule
4. pascal
Answer
Derived unit Fundamental unit
newton kg m s-²
watt kg m²s-³
joule kg m²s-²
pascal kg m-¹s-²
Question 20
Name the physical quantities related to the following units —
1. km²
2. newton
3. joule
4. pascal
5. watt
Answer
The physical quantities related to the following units are —
Unit Physical quantity
km² area
newton force
joule energy
pascal pressure
watt power
Exercise 1(A) — Short Answer Type
Question 1
What are the three requirements for selecting a unit of a physical quantity?
Answer
The three requirements for selecting a unit of a physical quantity are —
1. The unit should be of convenient size.
2. It should be possible to define the unit without any ambiguity.
3. The unit should be reproducible.
4. The value of unit should not change with space and time. (i.e. it must always remain same everywhere).
Question 2
What are the fundamental units in S.I. system? Name them along with their symbols.
Answer
The fundamental units in S.I. system along with their symbols are as follows —
Quantity Unit Symbol
Length. metre m
Mass kilogram kg
Time second s
Temperature kelvin K
Luminous intensity candela cd
Electric current ampere A
Amount of substance mole mol
Angle radian rd
Solid angle steradian st-rd
Question 3
Explain the meaning of derived unit with the help of one example.
Answer
The units of quantities other than those measured in fundamental units, can be expressed in terms of the fundamental units and they are called derived units.
Thus, derived units are those which depend on the fundamental units or which can be expressed in terms of the fundamental units.
Example – For the measurement of area, we need to measure length and breadth in the unit of length and then express area in a unit which is:
length x length and unit is m².
Question 4
Name two units of length which are bigger than a metre. How are they related to the metre?
Answer
The units of length which are bigger than a metre are —
1. Astronomical Unit (A.U.) — One astronomical unit is equal to the mean distance between the earth and the sun. Relation between metre and astronomical unit is expressed as:
A.U. = 1.496 x 10¹¹ m
2. Light year (ly) — A light year is the distance travelled by light in vacuum, in one year. Relation between metre and light year is expressed as:
1 light year = 9.46 x 10¹⁵m
Question 5
Name the three convenient units used to measure length ranging from very short to very long value. How are they related to the S.I. unit ?
Answer
The 3 convenient units used to measure length ranging from very short to very long value are —
1. centimeter (cm)
2. metre (m)
3. kilometer (km)
S.I. unit of length is meter (m). Relation between meter (m) and centimeter is —
1 m = 100 cm
Relation between meter (m) and kilometer is —
1 km = 1000 m
Question 6
Name the S.I. unit of mass and define it.
Answer
The S.I. unit of mass is kilogram (kg).
One kilogram is defined as the mass of a cylindrical piece of platinum-iridium alloy kept at International Bureau of Weights and Measures at Serves near Paris.
Question 7
State two units of mass smaller than a kilogram. How are they related to kilogram ?
Answer
The two units of mass smaller than a kilogram (kg) are:
1. gram (g) — Relation between gram and kilogram is:
1 g = 10-³kg
2. milligram (mg) — Relation between miligram and kilogram is:
1 mg = 10-⁶ kg
Question 8
State two units of mass bigger than a kilogram. Give their relationship with the kilogram.
Answer
The two units of mass bigger than a kilogram (kg) are:
1. Quintal — It is one hundred times a kilogram. Relation between quintal and kilogram is:
1 quintal = 100 kg
2. Metric tonne — It is one thousand times a kilogram. Relation between metric tonne and kilogram is:
1 metric tonne = 1000 kg
Question 9
Name the S.I. unit of time and define it.
Answer
The S.I. unit of time is second (s).
A second can be defined as 1/86400th part of a mean solar day,
i.e.,
1s=186400×one mean solar day 1s=864001×one mean solar day
Question 10
Name two units of time bigger than a second. How are they related to second?
Answer
The two units of time bigger than a second (s) are:
1. minute (min) — One minute is the duration of 60 seconds. Relation between minute and second is:
1 min = 60 s
2. Hour (h) — One hour is the duration of 60 minutes. Relation between hour and second is:
1 h = 3600 s
