9th Class Physics Ch1 Physical Quantities & Measurement Short Questions

Non-physical quantities, such as emotions, intelligence, or satisfaction, cannot be directly measured using traditional instruments. However, they can be assessed indirectly through metrics, scales, or standardized tests. For example:
• Intelligence is measured using IQ tests.
• Customer satisfaction can be quantified through surveys or rating scales.
These methods convert abstract concepts into numerical values that allow for comparison and analysis.

A measurement is a process of comparison of an unknown quantity with a widely accepted standard quantity. A measurement consists of two parts: a number and a unit. A measurement without a unit is meaningless.

A standard unit ensures consistent and accurate measurements. It allows everyone to understand and compare results, which is important for science, trade, and construction.

Base Quantities: Fundamental physical quantities from which other quantities are derived:
• Length: Measured in meters (m)
• Mass: Measured in kilograms (kg)
• Time: Measured in seconds (s)

Derived Quantities: Formed by combining base quantities:
• Speed: Defined as distance/time (m/s)
• Area: Defined as length × width (m²)
• Density: Defined as mass/volume (kg/m³)

The height of a desk is typically expressed in centimeters (cm) or meters (m) for accurate and consistent measurement.

There are seven SI base units:
• Length: Meter (m)
• Mass: Kilogram (kg)
• Time: Second (s)
• Electric current: Ampere (A)
• Temperature: Kelvin (K)
• Amount of substance: Mole (mol)
• Intensity of light: Candela (cd)

Prefixes simplify the expression of very large or very small numbers by scaling the base unit. They make communication more efficient and measurements easier to read.

Sub-multiples:
• Milli (m): 10⁻³ (e.g., 1mm = 10⁻³m)
• Micro (μ): 10⁻⁶ (e.g., 1μm = 10⁻⁶m)
• Nano (n): 10⁻⁹ (e.g., 1ns = 10⁻⁹s)

Multiples:
• Kilo (k): 10³ (e.g., 1km = 10³m)
• Mega (M): 10⁶ (e.g., 1MJ = 10⁶J)
• Giga (G): 10⁹ (e.g., 1GHz = 10⁹Hz)

• (a) 5pm = 5×10⁻¹²m
• (b) 15ns = 15×10⁻⁹s
• (c) 6 μm = 6×10⁻⁶m
• (d) 5fs = 5×10⁻¹⁵m

(a) A Vernier Caliper is an instrument used to measure small lengths with high precision, down to 1/10th of a millimeter. It is commonly used to measure the thickness, diameter, width, or depth of an object.

(b) The Vernier Caliper has two parts:
• Main Scale: This scale has markings of 1 mm each.
• Vernier (sliding) Scale: The Vernier scale is 9 mm long and divided into 10 equal parts.

(c) The least count of a Vernier Caliper is the difference between the value of one main scale division (M.S) and one Vernier scale division (V.S).
Least count = 1 M.S div – 1 V.S div = 1 mm – 0.9mm = 0.1mm
Alternatively: Least count = 1 mm/10 = 0.1 mm

(d) Zero error exists if zero of vernier scale is not coinciding with zero of the main scale. There are two types of errors:
• Positive zero error: Zero of vernier scale is on the right side of the zero of the main scale.
• Negative zero error: Zero of vernier scale is on the left side of the zero of the main scale.

Least Count:
The least count of a vernier caliper is the value of the smallest measurement that can be taken using the vernier calipers.
Formula: Least count = One small division on main scale / No. of divisions on vernier scale
Least count = 1mm/10 = 0.1mm or 0.01cm

Length:
• The main scale reading is 2.6 cm.
• The Vernier scale division is 5th division.
Therefore: Length = Main scale reading + Vernier scale reading × L.C
Length = 2.6cm + 5×0.01cm = 2.65cm
Thus, the length measured is 2.65 cm.

The correct reading is B because the figure shows that the eye is exactly above the reading point, avoiding parallax error.

No, a non-physical quantity does not have dimensions because it is not related to fundamental physical units and is not measurable in physical terms.

The unit of charge in terms of base units is ampere-second (A.s), derived from the relationship Q = I × t.

The unit of pressure, pascal (Pa), can be expressed as: 1 Pa = 1 N/m²

The kilogram is the only base unit that has a prefix.

• (a) 0.00534m = 5.34×10⁻³m
• (b) 2574.32kg = 2.57432×10³kg
• (c) 0.45m = 4.5×10⁻¹m
• (d) 0.004kg = 4.0×10⁻³kg
• (e) 186000s = 1.86×10⁵s

(i) Symbols, Not Abbreviations: Each unit is represented by a symbol, not an abbreviation (e.g., s, not sec).
(ii) No Plural Form: Symbols do not take a plural form (e.g., 10 mN, not 10 mNs).
(iii) Capitalization of Unit Names: Full unit names are written in lowercase, except Celsius (e.g., metre, second, newton).
(iv) Uppercase for Certain Symbols: Symbols appear in lowercase, except L for liter and symbols named after scientists (e.g., N for newton).
(v) Prefix Placement: Prefixes are written directly before the unit (e.g., ms, not m s).
(vi) Spacing Between Units: Units are written with one space apart (e.g., N m, not Nm).
(vii) No Compound Prefixes: Compound prefixes are not allowed (e.g., 7 ps, not 7 μμs).

Numbers must have the same exponent for addition or subtraction because the operation can only be performed on like terms. If the exponents are different, adjust the decimal point to make the exponents equal before performing the operation.

(i) Handle all apparatus and chemicals carefully and correctly.
(ii) Always check the label on the container before using the substance it contains.
(iii) Do not taste any chemical unless otherwise instructed by the teacher.
(iv) Do not eat, drink, or play in the laboratory.
(v) Do not tamper with the electrical mains and other fittings in the laboratory.
(vi) Never work with electricity near water.
(vii) Don’t place flammable substances near naked flames.
(viii) Wash your hands after all laboratory work.

The standard measurement of any quantity is called its unit.
There are two types of units:
(i) Base units
(ii) Derived units

Least count is the smallest measurement that can be taken accurately with an instrument.

Instrument	Range	Least Count
Measuring tape	1 cm to several metres	1 mm
Metre rule	1 mm to 1 m	1 mm
Vernier Callipers	0.1 mm to 15 cm	0.1 mm
Micrometer Screw Gauge	0.01 mm to 2.5 cm	0.01 mm

Zero error of an instrument: A systematic error that occurs when the instrument reads a value other than zero when the true value is zero.

(a) Least count of screw gauge:
Least count = Pitch of screw gauge / No. of divisions on circular scale

(b) Least count of Vernier calipers:
Usually, the least count is found by dividing the length of one small division of main scale by the total number of divisions on the Vernier scale:
Least count = 1 mm/10 = 0.1 mm

When the thimble makes one complete turn, the spindle moves 0.5 mm (1 scale division) on the main scale. This movement is called the pitch of the screw gauge.

Area is a derived quantity because it is calculated by multiplying two base quantities, length and width. It is expressed in terms of square units, such as m².

(i) Base Units:
• Definition: Fundamental units that cannot be expressed in terms of other units.
• Examples: Meter (m), kilogram (kg), second (s), Kelvin (K), etc.

(ii) Derived Units:
• Definition: Units that can be expressed in terms of base units.
• Examples: Speed (m/s), force (N = kg m/s²), energy (J = kg m²/s²), etc.

(i) The rotation of the Earth on its axis.
(ii) The revolution of the Earth around the Sun.
(iii) The vibration of a cesium-133 atom.
(iv) The oscillation of a pendulum.

(i) Your eye level may move a bit while reading the meniscus. → Personal Error
(ii) Air current may cause the balance to fluctuate. → Random Error
(iii) The balance may not be properly calibrated. → Systematic Error
(iv) Some of the liquid may have evaporated while it is being measured. → Random Error

(i) 1.25×10³ → 3 significant figures
(ii) 12.5 cm → 3 significant figures
(iii) 0.125 m → 3 significant figures
(iv) 0.000125 km → 3 significant figures

Systematic Errors	Random Errors
Influence all measurements of a particular type in the same way, resulting in consistent differences in readings.	Occur when repeated measurements of the same quantity produce different results under the same conditions.
Arise from factors such as zero error, poor calibration, or incorrect markings on the scale.	Caused by unpredictable factors; experimenter has little or no control. Arise due to fluctuations in temperature, pressure, humidity, or voltage.
Minimizing: Compare instrument with a more accurate one or apply a correction factor.	Minimizing: Take multiple readings and calculate the average or mean value.