Selina Solutions for Class 9 Physics Exercise 1(A) – Measurements and Experimentation (ICSE)

Introduction

If you’re studying from the Selina ICSE Physics book, Chapter 1 Measurements and Experimentation is one of the most important chapters for building a strong foundation.
In this article, you’ll get complete Selina Solutions for Class 9 Physics Exercise 1(A) with clear explanations, exam-focused answers, and step-by-step guidance.

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Study materials related to chapter 1 Measurements and Experimentation

(A) Multiple Choice Type

(Choose the correct answer from the options given below).

Question 1
The condition(s) essential for a unit to be accepted internationally :
(a) The unit should be of convenient size and without any ambiguity.
(b) The unit should be reproducible.
(c) The value of the unit should not change with space and time.
(d) All of the above.
Answer: 
(d) All of the above
Explanation: 
A standard unit must be convenient, reproducible, and constant everywhere.

Question 2
In mechanics, the three fundamental quantities are:
(a) Length, mass, velocity
(b) Mass, time, density
(c) Mass, time, pressure
(d) Length, mass, time
Answer:
(d) Length, mass, time
Explanation: In mechanics, all quantities are derived from these three fundamental quantities.

Question 3
How many units are fundamental and supplementary?
(a) 2 fundamental, 7 supplementary
(b) 3 fundamental, 6 supplementary
(c) 7 fundamental, 2 supplementary
(d) 4 fundamental, 5 supplementary
Answer:
(c) 7 fundamental, 2 supplementary
Explanation:
SI system has 7 base (fundamental) units and 2 supplementary units (radian, steradian).

Question 4
The fundamental unit is :
(a) Newton                    (b) Pascal
(c) Hertz                        (d) Second
Answer:
(d) Second
Explanation:
Second is a fundamental SI unit of time. Newton, Pascal, Hertz are derived units.

Question 5
Which of the following unit is not a fundamental unit?
(a) Metre                       (b) Litre
(c) Second                    (d) Kilogram
Answer:
(b) Litre
Explanation: Litre is not a fundamental SI unit; it is derived from cubic metre.

Question 6
One astronomical unit is equal to:
(a) 1.496 x 10¹¹ metre       (b) 1.596 x 10¹² metre
(c) 1.498 x 10¹⁰ metre       (d) 1.398 x 10¹¹ metre
Answer:
(a) 1.496 × 10¹¹ metre
Explanation:
One astronomical unit (AU) = average Earth–Sun distance.

Question 7
The distance of stars from earth is generally measured in:
(a) Kilometre                  (b) Light year
(c) Angstrom                  (d) Fermi
Answer:
(b) Light year
Explanation:
Distances of stars are extremely large, so light year is used.

Question 8
The unit of time is:
(a) light year                   (b) parsec
(c) leap year                   (d) angstrom
Answer:
(c) leap year
Explanation:
Leap year measures time. Others (light year, parsec, angstrom) measure length.

Question 9
1 Å is equal to:
(a) 0.1 nm                      (b) 10^–10 nm
(c) 10^–8 m                       (d) 10⁴ µ
Answer:
(a) 0.1 nm
Explanation:
1 Å = 10^–10 m = 0.1 nm

Question 10
One metric tonne is equal to:
(a) 10 quintal                   (b) 100 quintal
(c) 1000 quintal               (d) 25 quintal
Answer:
(a) 10 quintal
Explanation:
1 metric tonne = 1000 kg
1 quintal = 100 kg
So, 1000/100 = 10 quintal

Question 11
One solar mass is equal to :
(a) 3 x 10²⁰ kg            (b) 2 x 10²⁷ kg
(c) 2 x 10³⁰ kg            (d) 3 x 10³⁰ kg
Answer:
(c) 2 × 10³⁰ kg
Explanation:
Solar mass (mass of Sun) ≈ 2 × 10³⁰ kg

Question 12
One lunar cycle is nearly equal to :
(a) 28.5 days              (b) 29.5 days
(c) 30 days                 (d) 30.5 days
Answer:
(b) 29.5 days
Explanation:
Lunar cycle (synodic month) ≈ 29.53 days.

Question 13
One shake is equal to :
(a) 10^–9 s                 (b) 10^–8 s
(c) 10^–10 s                (d) 10⁸ s
Answer:
(b) 10^–8 s
Explanation: One shake = 10^–8 seconds.

Question 14
Which of the following unit is equivalent to watt?
(a) Volt x Ampere           (b) Volt/Ampere
(c) Ampere/Volt              (d) Ohm
Answer:
(a) Volt × Ampere
Explanation:
Power = Voltage × Current
Watt = Volt × Ampere

Question 15
Which one of the following is a derived unit?
(a) Metre                        (b) Kelvin
(c) Metre³                       (d) Mole
Answer:
(c) Metre³
Explanation:
Metre³ (volume) is derived from metre → derived unit.

Question 16
The S.I. unit of energy is:
(a) Joule                         (b) Kg m²
(c) Kg m²s^–3                    (d) Kg m²s^–1
Answer:
(a) Joule
Explanation:
SI unit of energy is Joule.

Question 17
The S.I. unit of pressure is:
(a) g m^–2 s^–2                    (b) Kg m^–1 s^–2
(c) Kg m s²                      (d) Kg m s^–2
Answer:
(b) Kg m^–1 s^–2
Explanation:
Pressure = Force/Area
Force = Kg ms^–2 So Pressure = Kg m^–1 s^–2

Question 18
Which of the following is the smallest unit?
(a) Millimetre                (b) Angstrom
(c) Fermi                       (d) Metre
Answer:
(c) Fermi
Explanation: 1 fermi = 10^–15 m (smallest among options)

(B) Very Short Questions

Question 1
What is meant by measurement?
Answer:
Measurement is the process of comparing an unknown physical quantity with a known standard quantity of the same kind.

Question 2
What do you understand by the term unit?
Answer:
A unit is a standard quantity used for measuring a physical quantity.

Question 3
How is a physical quantity expressed?
Answer:
A physical quantity is expressed as the product of a numerical value and a unit.
Example: 5 metre, 10 kg

Question 4
Name the three fundamental quantities.
Answer:
The three fundamental quantities are:
● Length
● Mass
● Time

Question 5
What is the S.I. unit of Luminous intensity?
Answer:
The SI 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 a unit that is independent of other units and cannot be derived from them.
Example: metre, kilogram, second.

Question 8
Define a derived unit.
Answer:
Derived units are those which depend on the fundamental units are called fundamental units. 
Example: velocity (m/s), force (newton).

Question 9
Define standard metre.
Answer:
One metre is defined as the length of the path travelled by light in vacuum in 1/299,792,458 second.

Question 10
How is nanometer related to Angstrom?
Answer:
1 nanometer (nm) = 10 Å
Or 1 Å = 0.1 nm

Question 11
Complete the following :
(a) 1 light year = …………… m
(b) 1 m = …………… Å (c) 1 m = …………… µ
(d) 1 micron = …………… Å
(e) 1 Fermi = …………… m

Answer:

(a) 1 light year = 9.46 x 10¹⁵ m
(b) 1 m = 10¹⁰ Å
(c) 1 m = 10⁶ µ
(d) 1 micron = 10⁴ Å
(e) 1 Fermi = 10^–15 m

Question 12
Complete the following :
(a) 1 g = …………… kg
(b) 1 mg = …………… kg
(c) 1 quintal = …………… kg
(d) 1 a.m.u (or u) = …………… kg

Answer:

(a) 1 g = 10^–3 kg
(b) 1 mg = 10^–6 kg
(c) 1 quintal = 100 kg
(d) 1 a.m.u (or u) = 1.66 x 10^–27 kg

Question 13
What is a leap year?
Answer:
A leap year is a year having 366 days in which February has 29 days.

Question 14
‘The year 2024 will have February of 29 days’. Is this statement true?
Answer:
Yes, the statement is true. The year 2024 is a leap year, and in a leap year February has 29 days.

Question 15
What is a lunar month?
Answer:
A lunar month is the time taken by the Moon to complete one cycle of phases. It is approximately 29.5 days.

Question 16
Complete the following :
(a) 1 nano second = …………… s
(b) 1 µs = …………… s
(c) 1 mean solar day = …………… s
(d) 1 year = …………… s

Answer:

(a) 1 nanosecond = 10^–9 s
(b) 1 µs = 10^–6 s
(c) 1 mean solar day = 86400 s
(d) 1 year = 3.15 × 10⁷ s

Question 17
Name the physical quantities which are measured in the following units :
(a) u
(b) ly
(c) ns
(d) nm

Answer:

(a) u → atomic mass (mass of atoms)
(b) ly → distance
(c) ns → time
(d) nm → length

Question 18
Write the derived units of the following :
(a) speed
(b) force
(c) work
(d) pressure

Answer:

(a) Speed → m s^–1
(b) Force → newton (N)
(c) Work → joule (J)
(d) Pressure → pascal (Pa)

Question 19
How are the following derived units related to the fundamental units?
(a) Newton
(b) Watt
(c) Joule
(d) Pascal

Answer:

(a) Newton = kg m s^–2
(b) Watt = kg m² s^–2
(c) Joule = kg m² s^–2
(d) Pascal = kg m^–1 s^–2

Question 20
Name the physical quantities related to the following units :
(a) km²
(b) Newton
(c) Joule
(d) Pascal
(e) Watt

Answer:

(a) km² → area
(b) Newton → force
(c) Joule → work / energy
(d) Pascal → pressure (e) Watt → power

(C) Short Questions

Question 1
What are the three requirements for selecting a unit of a physical quantity?
Answer:
The three requirements are:
● It should be of convenient size.
● It should be well-defined and unambiguous.
● It should be constant and reproducible.

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:

Question 3
Explain the meaning of derived unit with the help of one example.
Answer:
A derived unit is a unit obtained by combining fundamental units.
Example: \(Speed=\frac{distance}{time}\), so unit of speed = m/s, which is a derived unit.

Question 4
Name two units of length which are bigger than a metre. How are they related to the metre?
Answer:
Two units bigger than metre are kilometre and hectometre.
● 1 km = 1000 m
● 1 hm = 100 m

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 three convenient units are Angstrom, nanometre and kilometre.
Their relation with SI unit metre is:
● 1 Å = 10^–10 m
● 1 nm = 10^–9 m
● 1 km = 10³ 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 the standard prototype of platinum-iridium kept at the International Bureau of Weights and Measures, France.

Question 7
State two units of mass smaller than a kilogram. How are they related to kilogram ?
Answer:
Two units smaller than kilogram are gram and milligram. 1 g = 10^–3 kg and 1 mg = 10^–6 kg.

Question 8
State two units of mass bigger than a kilogram. Give their relationship with the kilogram.
Answer:
Two units bigger than kilogram are quintal and metric tonne.
1 quintal = 100 kg and 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.,
1 s =  \(\frac{1}{86400}\)× one mean solar day

Question 10
Name two units of time bigger than a second. How are they related to second?
Answer:
Two units bigger than second are minute and hour.
1 minute = 60 s and 1 hour = 3600 s.

(D) Numericals

Question 1
The wavelength of light of a particular colour is 5800 Å.
Express it in :
(a) nanometer and
(b) metre.
Solution:
Given,
wavelength = 5800 Å
We know: 1 Å = 0.1 nm
1 Å = 10^–10 m
(a) In nanometer,
5800 Å = 5800 × 0.1 nm = 580 nm
(b) In metre,
5800 Å = 5800 × 10^–10 m = 5.8 × 10^–7 m

Question 2
The size of a bacteria is 1 µ. Find the number of bacteria in 1 m length.
Solution:
Given,
Size of a bacteria = 1 µ
Total length = 1m
We know:
1 µm = 10⁻⁶ m
∴ Number of bacteria in 1m length  \(=\ \frac{1\operatorname{m}}{1\ \mu} =\ \frac{1\operatorname{m}}{{10}^{-6}\ m}={10}^6\)
Hence, number of bacteria in 1 m length is 10⁶ bacteria.

Question 3
The distance of a galaxy from the earth is 5.6 x 10²⁵ m. Assuming the speed of light to be 3 x 10⁸ m s^–1 find the time taken by light to travel this distance.
\(\left[Hint:\ Time\ taken=\frac{Distance\ travelled\ }{speed}\ \right]\)
Solution:
Given,
Distance = 5.6 x 10²⁵ m
Speed = 3 x 10⁸ m s^–1
Time taken \(=\frac{Distance\ travelled\ }{speed}=\frac{5.6\ \ times\ \ {10}^{25}}{3\ \times\ {10}^8}\)
⇒ Time taken = 1.87 × 10¹⁷  s

Question 4
The wavelength of light is 589 nm. What is its wavelength in Å?
Solution:
Wavelength of light = 589 nm
We know:
1 nm = 10 Å
So,
589 nm = 589 × 10 Å = 5890 Å
∴ The wavelength = 5890 Å

Question 5
The distance of the nearest star, Proxima Centauri, from the Earth is 4.0 x 10¹³ km. Express it in light year.
Solution:
Distance of Proxima Centauri from Earth = 4.0 x 10¹³ km
We know:
1 light year = 9.46 x 10¹² km
9.46 x 10¹² km = 1 Light year
∴ 4.0 x 10¹³ km \(=\frac{1}{9.46\ \times\ {10}^{12}}\times4\times{10}^{13}\)
                         = 4.2 light years.
Hence, distance of the nearest star from earth is 4.2 light years.

Question 6
It takes time 8 min for light to reach from the sun to the earth surface. If speed of light is taken to be 3 x 10⁸ m s^–1, find the distance from the sun to the earth in km.
Solution:
Given, 
Time taken by light = 8 min = 8 × 60 s = 480 s
Speed of light = 3 × 10⁸ m s^–1
Distance = speed × time Distance = 3 × 10⁸ × 480
Distance = 1.44 × 10¹¹ m
Convert metre to kilometre
1 km = 10³ m
Distance
Distance = 1.44 × 10⁸ km
∴ Distance from Sun to Earth = 1.44 × 10⁸ km

Question 7
‘The distance of a star from the earth is 8.33 light minutes’. What do you mean by this statement? Express the distance in metre.
Solution:
“The distance of a star from the earth is 8.33 light minutes” means that light takes 8.33 minutes to travel from the star to the Earth.
Given,
Speed = 3 x 10⁸ m s^–1
Time = 8.33 min = 8.33 x 60 s = 499.8 s ≈ 500 s
Distance = speed of light × time
Distance = 3 x 10⁸ x 500
= 1500 x 10⁸
= 1.5 x 10¹¹ m
∴ The distance from the star to the earth is 1.5 x 10¹¹ m.

Download PDF of Selina Solutions for Class 9 Physics Exercise 1(A) – Measurements and Experimentation

📝 Exam Tips (Very Important 🔥)

✔ Always write units with answers
✔ Learn SI units properly
✔ Practice significant figures questions
✔ Avoid parallax error in practical exams
✔ Revise definitions regularly

You can also visit :

ICSE Class 9 Physics Notes

📥 Conclusion

The chapter Measurements and Experimentation is extremely important for ICSE Class 9 Physics. It forms the base for all numerical problems and practical applications.
Make sure to understand concepts like least count, errors, and significant figures clearly for better performance in exams.

Official Website:

Students can visit the official CISCE website for more details and updates.

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