1.0 Introduction to Physical Quantities and Measurement
Physics begins with observation, but observation alone is not enough. To study anything scientifically, we must measure it. For example, saying "this table is long" is not very useful in science. But saying "this table is 2 metres long" gives clear and exact information.
A quantity that can be measured is called a physical quantity. Length, mass, time, temperature, area and volume are examples of physical quantities. These quantities help us describe objects and events accurately.
Statement: A physical quantity is any quantity that can be measured and expressed using a number and a unit.
Measurement means comparing an unknown quantity with a known standard quantity. For example, when we measure the length of a pencil with a ruler, we compare the pencil with standard centimetre marks on the ruler.
Measurement = Numerical Value + Unit
- Numerical Value: The number obtained during measurement
- Unit: The standard used for comparison
- Example: 5 m means numerical value = 5 and unit = metre
Why is saying "the table is 2" incomplete? Because 2 can mean 2 metres, 2 centimetres, 2 feet or even 2 kilometres. A number without a unit does not give complete scientific information. That is why every measurement must include both a number and a unit.
Measurement is used everywhere. A tailor measures cloth before stitching clothes. A doctor measures body temperature. A shopkeeper measures mass while selling rice or fruits. A sports teacher measures time during a race. Accurate measurement makes daily work easier and fairer.
| Physical Quantity | What It Measures | Daily-Life Example |
|---|---|---|
| Length | How long an object is | Length of a pencil |
| Mass | Amount of matter | Mass of fruits |
| Time | Duration of an event | Time taken to run a race |
| Temperature | How hot or cold something is | Body temperature |
Never write a measurement without a unit. Writing "length = 10" is incomplete. Write "length = 10 cm" or "length = 10 m" depending on the correct unit.
NU = Complete Measurement
N = Number
U = Unit
Without a unit, the number is not scientifically complete.
- A physical quantity is something that can be measured.
- A complete measurement has a numerical value and a unit.
- Measurement makes Physics accurate and useful in daily life.
Why is saying "the table is 2" incomplete in Physics?
2.0 Standard Units and SI System
A unit is a fixed standard used for measurement. When we measure any physical quantity, we compare it with a standard unit. For example, when we measure length in metres, we compare the object with the standard unit called metre.
In earlier times, people used body parts like handspan, footstep and cubit to measure length. But these were not accurate because the size of body parts differs from person to person. A child’s handspan is smaller than an adult’s handspan. So, the same table may have different measurements if different people use their hands to measure it.
Statement: A standard unit is an accepted fixed quantity used by everyone to measure a physical quantity accurately.
Why do we need standard units? Suppose one student says the classroom is 20 footsteps long, and another student says it is 16 footsteps long. Both may be correct according to their own footstep size, but the measurement is not standard. To avoid confusion, scientists use standard units such as metre, kilogram and second.
The internationally accepted system of units is called the SI system. SI stands for International System of Units. It helps scientists, students, engineers, doctors and people around the world use the same units for measurement.
MLS helps you remember three important SI units:
M = metre for length
L = kilogram for mass, remember "load/mass"
S = second for time
| Physical Quantity | SI Unit | Symbol | Common Daily Unit |
|---|---|---|---|
| Length | metre | m | centimetre, kilometre |
| Mass | kilogram | kg | gram |
| Time | second | s | minute, hour |
| Temperature | kelvin | K | degree Celsius |
Standard units make trade, travel, medicine and construction accurate. A doctor gives medicine in millilitres or grams. A builder measures walls in metres. A shopkeeper sells rice in kilograms. A train schedule uses hours and minutes. Without standard units, daily life would become confusing.
Unit symbols must be written correctly. Write kg, not Kg or KG. Write m, not M. Write s, not sec in formal SI symbol form. Small letters and capital letters can change meaning in science.
| Non-Standard Unit | Problem | Better Standard Unit |
|---|---|---|
| Handspan | Different people have different hand sizes | centimetre or metre |
| Footstep | Steps are not equal for everyone | metre |
| Cubit | Arm length varies from person to person | metre |
SI units are used worldwide in science. This helps scientists from different countries understand each other’s measurements without confusion.
- Standard units are fixed units accepted by everyone.
- SI units help make measurements uniform across the world.
- Non-standard units like handspan and footstep are not reliable for scientific measurement.
Why is a metre better than a handspan for measuring the length of a classroom?
3.0 Measurement of Length, Mass and Time
Length, mass and time are three very important physical quantities. We measure length to know how long, wide or tall something is. We measure mass to know how much matter an object contains. We measure time to know how long an event takes.
Statement: Length is the distance between two points. Mass is the amount of matter in an object. Time is the duration between two events.
Length is measured using instruments such as a ruler, metre scale or measuring tape. Small lengths are usually measured in centimetres or millimetres. Longer distances are measured in metres or kilometres.
Why must we keep the eye exactly above the mark while reading a ruler? If the eye is placed to the left or right side, the reading may look different from the actual value. This mistake is called parallax error. To avoid it, the eye should be vertically above the mark being read.
Mass is measured using a beam balance, physical balance or electronic balance. In daily life, shopkeepers use weighing machines to measure the mass of fruits, vegetables, rice and other items. The SI unit of mass is kilogram, but gram is also commonly used for smaller objects.
Time is measured using a clock, watch or stopwatch. A stopwatch is useful when we need to measure short time intervals, such as the time taken by a runner to complete a race or the time taken by a pendulum to complete a swing.
| Quantity | Common Instrument | Common Unit | Example |
|---|---|---|---|
| Length | Ruler, metre scale, measuring tape | mm, cm, m, km | Length of a pencil |
| Mass | Beam balance, electronic balance | g, kg | Mass of a school bag |
| Time | Clock, watch, stopwatch | s, min, h | Time taken for a race |
LMT helps you remember three basic measurements:
L = Length
M = Mass
T = Time
A carpenter measures length before cutting wood. A shopkeeper measures mass before selling rice. A coach measures time to check how fast an athlete runs. These examples show that measurement is not only for exams; it is used in real life every day.
The least count of an instrument is the smallest value it can measure. For a common ruler, the smallest division is usually 1 millimetre. So, the least count of the ruler is 1 mm. An instrument with a smaller least count can measure more precisely.
While measuring length, do not start from the broken edge of a ruler. If the zero mark is damaged, start from another clear mark such as 1 cm and subtract the starting reading from the final reading.
Q: A pencil starts at the 2 cm mark and ends at the 14 cm mark on a ruler. What is its length?
Solution: Length = final reading - initial reading
Length = 14 cm - 2 cm = 12 cm
Answer: The length of the pencil is 12 cm.
💡 Unit Tip: Always write the unit with the answer. Here the answer is 12 cm, not just 12.
- Length is measured using a ruler, metre scale or measuring tape.
- Mass is measured using a balance.
- Time is measured using a clock, watch or stopwatch.
Why should your eye be exactly above the mark while reading a ruler?
4.0 Measurement of Area, Volume and Temperature
In Physics, we do not measure only length, mass and time. We also measure area, volume and temperature. These quantities are useful in daily life, science experiments, construction, cooking, medicine and weather study.
Statement: Area is the amount of surface covered by an object. Volume is the amount of space occupied by an object. Temperature tells how hot or cold a body is.
Area is used when we measure surfaces such as the top of a table, a classroom floor, a notebook page or a wall. Volume is used when we measure space occupied by objects or liquids, such as water in a bottle or a box in a cupboard. Temperature is measured when we want to know how hot or cold something is.
Area = length x breadth
- Length: Longer side of the rectangle (cm or m)
- Breadth: Shorter side or width of the rectangle (cm or m)
- Area: Surface covered by the rectangle (cm² or m²)
Volume = length x breadth x height
- Length: Long side of the cuboid (cm or m)
- Breadth: Width of the cuboid (cm or m)
- Height: Vertical size of the cuboid (cm or m)
- Volume: Space occupied by the cuboid (cm³ or m³)
Q: A rectangular notebook has length 20 cm and breadth 15 cm. Find its area.
Solution: Area = length x breadth
Area = 20 x 15 = 300 cm²
Answer: The area of the notebook is 300 cm².
💡 Unit Tip: Area is written in square units such as cm² or m².
Q: A box has length 6 cm, breadth 4 cm and height 3 cm. Find its volume.
Solution: Volume = length x breadth x height
Volume = 6 x 4 x 3 = 72 cm³
Answer: The volume of the box is 72 cm³.
💡 Unit Tip: Volume is written in cubic units such as cm³ or m³.
A painter calculates the area of a wall before buying paint. A carpenter calculates the volume of a wooden box. A doctor measures body temperature using a thermometer. These measurements help people make correct decisions in daily life.
The volume of liquids is measured using a measuring cylinder or measuring jar. The common units used for liquid volume are millilitre (mL) and litre (L). While reading the level of liquid in a measuring cylinder, the eye should be at the same level as the liquid surface.
Temperature is measured using a thermometer. The common unit of temperature in daily life is degree Celsius (°C). Body temperature, room temperature and weather temperature are usually measured in degree Celsius.
| Quantity | Meaning | Common Unit | Instrument / Method |
|---|---|---|---|
| Area | Surface covered | cm², m² | Calculated using formula |
| Volume | Space occupied | cm³, m³, mL, L | Formula or measuring cylinder |
| Temperature | Hotness or coldness | °C, K | Thermometer |
Do not confuse area and volume units. Area uses square units like cm², while volume uses cubic units like cm³. Writing area in cm³ or volume in cm² is incorrect.
Area = Flat, Volume = Space
Area measures a flat surface. Volume measures the space inside or occupied by an object.
- Area measures surface covered and is written in square units.
- Volume measures space occupied and is written in cubic units or litres.
- Temperature tells how hot or cold a body is and is measured using a thermometer.
Why is the unit of area cm², but the unit of volume cm³?
5.0 Accuracy, Applications and Exam Readiness
Measurement becomes useful only when it is done carefully. In Physics, accurate measurement means using the correct instrument, reading it properly and writing the answer with the correct unit. A small mistake in measurement can lead to a wrong result.
Statement: Accurate measurement means measuring a physical quantity correctly using a suitable instrument and proper unit.
Why should measurement be accurate? Imagine a tailor measuring cloth wrongly. The dress may not fit. If a doctor measures medicine wrongly, it can be unsafe. If a builder measures a wall wrongly, the construction may become defective. Accurate measurement helps us avoid mistakes.
Measurement is used in tailoring, cooking, sports, medicine, travel and construction. A cook measures ingredients, an athlete measures race time, a doctor measures temperature, a driver checks distance and speed, and an engineer measures length and area before building structures.
Always write both number and unit. Writing "area = 40" is incomplete. Correct answer: "area = 40 cm²" or "area = 40 m²" depending on the given measurements.
| Topic | Quick Revision Point | Example / Unit |
|---|---|---|
| Physical Quantity | A quantity that can be measured | Length, mass, time |
| Measurement | Numerical value plus unit | 5 m, 2 kg, 10 s |
| SI Units | Internationally accepted units | m, kg, s, K |
| Length | Distance between two points | metre (m) |
| Mass | Amount of matter in an object | kilogram (kg) |
| Time | Duration of an event | second (s) |
| Area | Surface covered by an object | cm², m² |
| Volume | Space occupied by an object | cm³, m³, mL, L |
Measurement = Numerical Value + Unit
Area of rectangle = length x breadth
Volume of cuboid = length x breadth x height
- Area unit: square units such as cm² or m²
- Volume unit: cubic units such as cm³ or m³
- Measurement rule: Always write both number and unit
CUC Rule for Measurement
C = Choose correct instrument
U = Use correct unit
C = Check reading carefully
- Define physical quantity.
- Why do we need standard units?
- Name the SI unit of length, mass and time.
- What is parallax error?
- Find the area of a rectangle of length 12 cm and breadth 5 cm.
- Find the volume of a cuboid of length 4 cm, breadth 3 cm and height 2 cm.
- Every measurement must have a numerical value and a unit.
- Standard units make measurements accurate and uniform.
- Correct instrument, correct eye level and correct unit are important for accurate measurement.
Why should every measurement have a unit?