Kerala Syllabus Class 8 Basic Science: Chapter 1 Measurement and Units - Questions and Answers
Std 8: Physics: Chapter 1: Measurement and Units: Questions and Answers
♦ Observe the following situations in our life. Find the physical quantities in each of them. Record the quantities you identified in the table.
| Situation | Inference | |
|---|---|---|
| 1 | Measuring the depth of a pit | Length |
| 2 | Measuring the weight of vegetables | Mass |
| 3 | Taking measurements by a tailor | Length |
| 4 | Using a stopwatch in a race | Time |
| 5 | Measuring blood pressure | Pressure |
| 6 | Measuring body heat | Temperature |
♦ Find and write more physical quantities that you are familiar with.
• Speed
• work
• Power
• Force
• Volume
• Area
♦ What is a fundamental quantity?
Fundamental quantities are quantities that exist independently and cannot be expressed in terms of other quantities.
♦ Fundamental Quantities
| Quantities | Units |
|---|---|
| 𝑙 - Length m - Mass t - Time 𝐼 - Intensity of current T - Temperature 𝑰ᵥ - Intensity of light n - Amount of substance | Metre - m Kilogram - kg Second - s Ampere - A Kelvin - K Candela - cd mol |
♦ Find out the physical quantities mentioned in Table 1.1 and list them below.
• Length
• Mass
• Time
• Temperature
♦ Look at the pictures. What are the physical quantities in these situations?
• Fig. 1.9 - Volume
• Fig. 1.10 - Volume
♦ Record how each of them is found out and complete the table appropriately.
| Situation | Physical Quantity | Method of Finding |
|---|---|---|
| For painting the wall | Area | Area = length x Width |
| Measurement of medicine/liquid | Volume | Volume = Circumference of the measuring jar x height |
♦ Which are the quantities used here to find area and volume?
The fundamental unit used to find out area and volume is length.
♦ What are derived quantities?
Quantities that can be expressed in terms of fundamental quantities are derived quantities.
• Mass marked on the cylinder = 14.2 kilograms (Textbook Page: 10).
♦ Here, the physical quantity of mass is indicated using a numerical value i.e., 14.2 (magnitude) and a unit i.e., kilogram.
Similarly, complete the table with the physical quantities shown in the pictures below, along with their numerical values and units.
A physical quantity is expressed by a number indicating its value followed by its unit.
♦ Units of Physical Quantities
Mark the height of a child in your class on the wall using a pencil as shown in the figure. Each one in the class may measure the height using two sticks of different lengths. Record the measurements in your Science Diary. Fill a bucket with water. Measure the water in it with two glasses of different sizes (Figure 1.16). Write down the measurements in the Science Diary.
• In both cases, are the measurements obtained the same?
While using 2 different scales, we get different measurements
• Why are the measurements not equal?
Measurements are not equal because scales are different.
• When everyone uses the same reference object, isn’t measurement the same?
To get the same measurements, we should use the same scales.
♦ Unit
A unit is a standardized reference accepted universally to measure a physical quantity.
♦ In the past, different units were used for measurements.
eg: Foot, hand span, finger span, furlong, yard for length.
♦ Different units were also used in other countries. What would be the practical problems of using these units?
• Low accuracy
• Difficulty for the people in other regions to analyse measurements
• Lack of uniformity and international communication.
♦ ‘SI’ units
International System of Units (‘SI’ units) is the world's most widely used system of measurements.
| Different units are required for the same physical quantity in various contexts. Larger units are used for larger quantities and smaller units for smaller quantities. |
|---|
♦ Different Units of Length
• The SI unit of length is metre. Its symbol is 'm'
• But we use centimetres (cm) when we need small units. eg: to draw in a notebook
• Kilometre (km) is used to measure the length of a road.
• To measure very small lengths, we use millimetres (mm).
♦ Take a metre scale from the science lab and examine it. You can see small and large lines on the metre scale. The distance between two consecutive large lines is one centimetre, and the distance between small lines is one millimetre.
Now, complete the following relationship (Textbook Page: 13) given below.
• 1 metre = 100 centimetre
• 1 centimetre = 10 millimetre
• 1 metre = 1000 millimetre
♦ How many micrometres would make one metre?
1 metre = 1000000 micrometre
or
1 micrometre = 1/1000000 m = 10⁻⁶ m
Micrometre is known as a micron.
Micron is used to measure the thickness of plastic carry bags.
♦ Bigger units
Kilometre (km)
1 kilometre = 1000 metre = 10³ m
♦ Astronomical Unit (AU)
Astronomical Unit (AU) is the average distance from the Earth to the Sun. It is approximately 150 million kilometre.
♦ Light year
A light year is the distance light travels in a year in a vacuum. Light travels at a speed of approximately 300,000 km/s.
Distance travelled by light in 1s = 3 lakh km
Distance travelled by light in one day = 3 lakh X 60 X 60 24 X 365 km
1 Light year (ly) = 9.46 X 10¹² km.
♦ Mass
The amount of matter contained in a substance is its mass. The unit of mass is the kilogram. Its symbol is ‘kg’.
♦ SI unit of mass is the kilogram (kg).
Common balances compare the unknown mass of the object with known standard masses.
Milligram and gram are the smaller units of mass.
Gram, milligram → to measure small quantities.
♦ Gram
1 gram = 1000 milligrams
1000 g = 1 kg
1 g = 1/1000 kg or 10⁻³ kg
♦ Milligram
1000 milligram = 1 g
1000000 milligram = 1 kg
Quintal, Tonne are the larger unit of mass.
♦ Identify the relationship between the units of mass and kilogram from the table given below.
| Unit | Relation to kilograms |
|---|---|
| Milligram | 1 kilogram = 1000000 milligram |
| Gram | 1 kilogram = 1000 gram |
| Quintal | 1 quintal = 100 kilogram |
| Tonne | 1 tonne = 1000 kilogram |
♦ Different Units of Time
The SI unit of time is the second. Its symbol is ‘s’.
Minute → 1 minute = 60s
Hour → 1 Hour = 60 minuts
1 hr = 60 X 60 seconds = 3600 s
♦ Minute and hour are the other units used to denote time. Identify the relationship between these units and ‘second’.
| Unit | Relationship with second |
|---|---|
| Minute | 1 minute = 60 seconds |
| Hour | 1 hour = 3600 seconds |
♦ Volume
The volume of an object is the amount of space it occupies.
♦ What is the unit of volume?
The SI unit of volume is cubic metre. Its symbol is m³.
1 litre = 1000 cm³
1 litre = 1000 millilitre (1000 mL)
1 cm³ = 1 millilitre
1 m³ = 1000000 cm³ or 1m³ = 1000 litre
♦ Density
The mass of a substance per unit volume is called its density.
Density = Mass / Volume.
Unit of density = g/cm³ or kg/m³ (SI unit)
♦ Take a cardboard box and calculate its volume. Fill it with sawdust and measure its mass. Then replace it with sand and find its mass. Tabulate the findings.
Length of the box = 25 cm, Breadth = 20 cm, Height = 10 cm
Volume = 25 X 20 X 10 = 5000 cm³
Mass of sawdust = 2 kg = 2000g
Mass / Volume = 2000 g / 5000 cm³ = 0.4 g/cm³
Mass of sand = 7.5 kg = 7500 g
Mass / Volume = 7500 g / 5000 cm³ = 1.5 g/cm³
Mass / Volume represents the mass of a substance per unit volume. The volume of sawdust and sand is the same, but the mass per unit volume is different.
♦ Fundamental Units
Fundamental units are the units of fundamental quantities.
♦ Note the fundamental units and their symbols given below.
• They are standardised units.
• They are internationally accepted.
• Units of all other quantities can be expressed in terms of these units.
♦ Derived Units
Derived units are units that can be stated using fundamental units or that depend on fundamental units.
♦ See how derived units are formulated in the table given.
| Derived Quantities | Equation | Unit |
|---|---|---|
| Area | Area = length x breadth | m x m = m² |
| Volume | Volume = length x breadth x height | m x m x m = m³ |
| Density | Density = Mass / Volume | kg/m³ |
♦ Rules for writing the units
| Unit written incorrectly | General rules | Correct method |
|---|---|---|
| 1000 KG/M³ 1.5 KG | Use lower case of the English alphabet | 1000 kg/m³ 1.5 kg |
| 1000 kgs/m³ 1.5 kgs | kgs (wrong) - don't use plurals | 1000 kg/m³ 1.5 kg |
| 1000 kg/m³ 1.5 kg | Leave a space between the number and the symbol | 1000 kg/m³ |
| 1000 kg/m/m/m | Combine the same units | 1000 kg/m³ |
| 1000 kg/cubic metre 1000 kilograms per m³ | Do not mix a symbol of a unit with the name of the unit. | 1000 kg/m³ |
| 1 kg 500 g | Don't use different units of the same measurement | 1.5 kg |
| 273 Kelvin 250 Metre | Use only lowercase letters when writing the name of a unit instead of its symbol | 273 kelvin 250 metre |
♦ Measuring Instruments
What are the instruments used to measure length?
To measure length, we commonly use a scale, tape, etc.
♦ Measure the length of a pen using a scale and determine your height using a measuring tape.
Example:
• Length of the pen = 14.2 cm
• Your height = 153 cm
• What is the smallest measurement possible using the scale / measuring tape?
The least count that can be measured using a tape or scale is a millimetre.
♦ Least count
The smallest value that can be measured using an instrument is called its least count.
The least count of a commonly used scale is 0.1 cm.
♦ What are the instruments with a least count below 0.1 cm?
• Vernier calliper
• Screw gauge
♦ Measuring the thickness of paper using a scale
Measure the thickness of a stack of paper and count the number of papers in it. Then calculate the thickness.
Number of papers in the paper stack = 500
Thickness of the paper stalk = 55 mm
Thickness of one paper = Thickness of the paper stalk / Number of papers = 55 mm / 500 = 0.11 mm
♦ Measuring the volume using a measuring jar
Take 250 mL of water in a measuring jar. Tie the stone with a thread and dip it into the water. Observe the rise in the water level. From this, we can calculate the volume of the stone which is equal to the volume of water displaced.
• What unit is used in the measuring jar?
Millilitre
• What is the least count of the measuring jar?
10 mL
• Initial water level before dipping the stone = 250 mL
• Water level after dipping the stone = 375 mL
• Volume of the stone = 375 - 250 = 125 mL or 125 cm³
♦ Let’s assess
1. Identify the odd one out in each group and explain common features of the others.
I. a) Kilogram b) Kilometre c) Second d) Mole
II. a) Time b) Area c) Mass d) Electric current
III a) Metre b) Kilogram c) Second d) Degree Celsius
Answer:
I. b) Kilometre. Others are SI units. km is the bigger unit of length
II. b) Area. It is a derived unit. Others are basic units
III. d) Degree Celsius. Others are SI units. SI unit of temperature is kelvin.
2. Different units of length are given below. Fill in the table below.
| Unit | Relationship with metre |
|---|---|
| Kilometre | 1 km = 1000 metre |
| Millimetre | 1 m = 1000 millimetre |
| Centimetre | 100 cm = 1 metre |
3. Convert the following measurements to SI units without changing their values.
a) 2000 g
b) 1 h
c) 1.5 km
d) 200 cm
Answer:
a) 2 kg
b) 3600 s
c) 1500 m
d) 2 m
4. Different units of mass are given below. Arrange them in the ascending order of their values.
a) Kilogram
b) Milligram
c) Quintal
d) Gram
Answer: Milligram → Gram → Kilogram → Quintal
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