9th Class Physics
Chapter-1: Physical Quantities & Measurements
A. Multiple Choice Questions
Tick (✓) the correct option
1.1 The instrument that is most suitable for measuring the thickness of a few sheets of cardboard is a:
Explanation: A micrometer screw gauge can measure very small thicknesses (up to 0.01 mm) accurately, making it suitable for measuring the thickness of a few sheets of cardboard.
1.2 One femtometre is equal to:
Explanation: Femto- is the SI prefix for 10⁻¹⁵, so one femtometre equals 10⁻¹⁵ meters.
1.3 A light-year is a unit of:
Explanation: A light-year is the distance that light travels in one year, approximately 9.46 trillion kilometers.
1.4 Which one is a non-physical quantity?
Explanation: Colour is a perceptual property and cannot be measured with standard units, making it a non-physical quantity.
1.5 When using a measuring cylinder, one precaution to take is to:
Explanation: To avoid parallax error, the eye should be positioned in line with the bottom of the meniscus when reading the volume.
1.6 Volume of water consumed by you per day is estimated in:
Explanation: Daily water consumption is typically measured in liters, as it's a practical unit for this purpose (1 liter = 1000 mL).
1.7 A displacement can is used to measure:
Explanation: A displacement can measures the volume of irregular solids by measuring the volume of liquid displaced when the solid is immersed.
1.8 Two rods with lengths 12.321 cm and 10.3 cm are placed side by side; the difference in their lengths is:
Explanation: 12.321 cm - 10.3 cm = 2.021 cm. When subtracting, the result should have the same number of decimal places as the measurement with the fewest decimal places (10.3 has one decimal place).
1.9 Four students measure the diameter of a cylinder with Vernier Callipers. Which of the following readings is correct?
Explanation: Vernier Callipers typically have a precision of 0.01 cm, so a reading of 3.47 cm is appropriate. 3.475 cm would require higher precision instruments.
1.10 Which of the following measures is likely to represent the thickness of a sheet of this book?
Explanation: A typical sheet of paper is about 0.1 mm thick, so 0.04 mm is a reasonable estimate for a thin sheet in a book.
1.11 In a Vernier Callipers ten smallest divisions of the Vernier scale are equal to nine smallest divisions of the main scale. If the smallest division of the main scale is half a millimetre, the Vernier constant is equal to:
Explanation: Vernier constant = (1 MSD - 1 VSD) = (0.5 mm - (9×0.5 mm)/10) = 0.5 mm - 0.45 mm = 0.05 mm.
Common measuring instruments used in physics
B. Short Answer Questions
1.1 Can a non-physical quantity be measured? If yes, then how?
Answer: No.
A non-physical quantity (like love, honesty, intelligence) cannot be measured because it has no physical existence or numerical value.
Only physical quantities—those that can be expressed in numbers and units—can be measured.
1.2 What is measurement? Name its two parts.
Answer: Measurement is the process of finding how many times a standard quantity is contained in the quantity being measured.
It has two parts:
- Numerical value (magnitude)
- Unit
Example: 5 m → 5 = number, m = unit.
1.3 Why do we need a standard unit for measurements?
Answer: We need standard units so that measurements are uniform, accurate, and comparable everywhere in the world.
Without standard units, results would vary from person to person or country to country.
1.4 Write the names of 3 base quantities and 3 derived quantities.
Answer:
Base Quantities:
- Length
- Mass
- Time
Derived Quantities:
- Speed (m/s)
- Force (N)
- Area (m²)
1.5 Which SI unit will you use to express the height of your desk?
Answer: Metre (m) — or centimetre (cm) if the desk is short.
Both are SI-based units of length.
1.6 Write the name and symbols of all SI base units.
| Base Quantity | SI Unit Name | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
1.7 Why is prefix used? Name three sub-multiple and three multiple prefixes with their symbols.
Answer: Prefixes are used to express very large or very small quantities easily by changing the size of the unit.
Sub-multiples (smaller than 1):
- milli (m) = 10⁻³
- micro (µ) = 10⁻⁶
- nano (n) = 10⁻⁹
Multiples (greater than 1):
- kilo (k) = 10³
- mega (M) = 10⁶
- giga (G) = 10⁹
1.8 What is meant by:
Answer:
- (a) 5 pm → 5 picometres = 5 × 10⁻¹² m
- (b) 15 ns → 15 nanoseconds = 15 × 10⁻⁹ s
- (c) 6 µm → 6 micrometres = 6 × 10⁻⁶ m
- (d) 5 fs → 5 femtoseconds = 5 × 10⁻¹⁵ s
1.9 (a) For what purpose is a Vernier Callipers used?
Answer: It is used to measure the external & internal diameters and depth of small objects to an accuracy of 0.1 mm (0.01 cm).
1.9 (b) Name its two main parts.
Answer:
- Main scale (fixed jaw & scale)
- Vernier scale (sliding jaw & scale)
1.9 (c) How is least count found?
Answer: Least count = 1 M.S.D. – 1 V.S.D. = (value of 1 main-scale division) ÷ (total Vernier divisions).
1.9 (d) What is meant by zero error?
Answer: The non-zero reading shown when the jaws are fully closed; it must be subtracted/added from the final reading.
1.10 State least count and Vernier scale reading as shown in the figure and hence find the length.
Answer:
Least count = 0.1 mm = 0.01 cm
Vernier coincidence = 5th division
Vernier scale reading = 5 × 0.01 cm = 0.05 cm
Main-scale reading = 5.9 cm
Total length = 5.9 cm + 0.05 cm = 5.95 cm
1.11 Which reading out of A, B and C shows the correct length and why?
Answer: Reading B is correct because the eye is placed exactly perpendicular to the scale, eliminating parallax error.
Correct eye position when reading a meniscus in a measuring cylinder
C. Constructed Response Questions
1.1 Write the most appropriate unit for measuring:
- (a) Thickness of a five-rupee coin: millimetre (mm)
- (b) Length of a book: centimetre (cm)
- (c) Length of football field: metre (m)
- (d) The distance between two cities: kilometre (km)
- (e) Mass of five-rupee coin: gram (g)
- (f) Mass of your school bag: kilogram (kg)
- (g) Duration of your class period: minute (min)
- (h) Volume of petrol filled in the tank of a car: litre (L)
- (i) Time to boil one litre milk: minute (min)
1.2 Why might a standard system of measurement be helpful to a tailor?
- Guarantees consistent body & cloth dimensions.
- Avoids fitting errors and material waste.
- Enables global trade without re-measuring.
1.3 Least count & thickness of the steel rod
Pitch = 1 mm, Circular divisions = 100
Least count = 1 mm ÷ 100 = 0.01 mm
Reading shown: 1 mm + 47 × 0.01 mm = 1.47 mm
1.4 Measuring pencil diameter with only a metre scale
Tightly bundle N identical pencils with tape.
Measure total width (W) of the bundle across the hexagon flats.
Diameter = W / N (precision ↑ as N ↑).
1.5 Worn-out end of metre scale – where to place the pencil?
Place the pencil's one end at the 1 cm mark and subtract 1 cm from the final reading.
1.6 Why place the object close to the metre scale?
Minimises parallax error; line of sight stays perpendicular, giving true length.
1.7 Why is a standard unit needed to measure correctly?
Ensures universally reproducible, comparable & error-free results across people, places & time.
1.8 Natural phenomena usable as accurate time standards
- Pendulum swing (fixed length)
- Earth's rotation (solar day)
- Heart-beat (short intervals)
- Vibrations of a quartz crystal
1.9 Why is the meniscus hard to locate in a wider vessel?
Curvature is flatter; small tilt causes large horizontal shift, so the exact bottom of the meniscus is unclear.
1.10 Which instrument can be used to measure:
(i) Internal diameter of a test tube?
Answer: Vernier Callipers (use the inside/upper jaws).
(ii) Depth of a beaker?
Answer: Vernier Callipers (use the depth rod at the tail end).
Benefits of standard measurement system for a tailor