Lesson Notes By Weeks and Term v3 - Primary 4
Length
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Length
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: General Mathematics
Class: Primary 4
Term: 3rd Term
Week: 5
Theme: Mensuration And Geometry
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Watch on YouTubeSee Facebook postestimate distances in kilometers and length in meters or centimeters and compare with measurements add and subtract length
This section provides the foundational knowledge and step-by-step reasoning required for teaching the topic of Length. 2.1 Definition of Length Length is the measurement or extent of something from end to end. It describes how long an object is or the distance between two points. 2.2 Standard Units of Length The international system of units (SI) provides standard units for measuring length, making it possible for people worldwide to understand and compare measurements consistently.
Millimetre (mm): Used for very small lengths, such as the thickness of a fingernail or the width of a small grain.
Centimetre (cm): Used for small lengths, such as the length of a pencil, an eraser, or the height of a book. (1 cm = 10 mm)
Metre (m): The base unit of length. Used for larger lengths like the height of a door, the length of a room, or the amount of fabric needed for a dress. (1 m = 100 cm)
Kilometre (km): Used for very long distances, such as the distance between cities or towns, or the length of a major road. (1 km = 1000 m) 2.3 Relationship Between Units (Conversion) To perform calculations involving different units of length, it is often necessary to convert them to a common unit.
Kilometers to Meters: To convert kilometres to metres, multiply by
1
0
0
0. Example: 2 km = 2 x 1000 m = 2000 m Meters to Kilometers: To convert metres to kilometres, divide by
1
0
0
0. Example: 3500 m = 3500 ÷ 1000 km = 3.5 km Meters to Centimeters: To convert metres to centimetres, multiply by
1
0
0. Example: 5 m = 5 x 100 cm = 500 cm Centimeters to Meters: To convert centimetres to metres, divide by
1
0
0. Example: 450 cm = 450 ÷ 100 m = 4.5 m Centimeters to Millimeters: To convert centimetres to millimetres, multiply by
1
0. Example: 8 cm = 8 x 10 mm = 80 mm Millimeters to Centimeters: To convert millimetres to centimetres, divide by
1
0. Example: 120 mm = 120 ÷ 10 cm = 12 cm 2.4 Estimation of Length and Distances Estimation involves making an educated guess about a measurement without using a measuring tool. It develops a sense of size and distance. Why Estimate? It helps in quickly determining approximate values, useful when exact measurements are not crucial or tools are unavailable.
Strategies for Estimation:
1. Benchmarking: Using a known length as a reference. For example, knowing one's own height (e.g., about 1.2 m) to estimate the height of a doorway (e.g., about twice my height = 2.4 m).
2. Visual Comparison: Directly comparing the object to be estimated with another object whose length is known or easily determined. Practical Examples for Estimation in Nigerian Contexts: Kilometres: "How far is the market from the school? (Is it about 1 km, 5 km, or 10 km?)" Meters: "How long is the classroom? (Is it about 5 m, 10 m?)" "How much fabric do you think Mama Chidi needs for a wrapper? (About 2m, 3m?)" Centimetres: "How long is your exercise book? (About 20 cm, 30 cm?)" "What is the length of your finger? (About 5 cm, 7 cm?)" 2.5 Measurement and Comparison After estimation, actual measurement is performed using appropriate tools (ruler, tape measure). The estimated value is then compared to the actual measured value to assess the accuracy of the estimation. 2.6 Addition and Subtraction of Lengths Lengths can be added or subtracted, but it is crucial that they are expressed in the same units before performing the operation. If units are different, conversion must occur first.
Worked Examples for Addition: Example 1 (Same Units): A tailor bought 5 m 60 cm of blue fabric and 3 m 25 cm of red fabric. What is the total length of fabric she bought?
Step 1: Arrange in columns. ``` m cm 5 60 + 3 25 ------- ``` Step 2: Add the centimetres. 60 cm + 25 cm = 85 cm * Step 3: Add the metres. 5 m + 3 m = the same units before performing the operation. If units are different, conversion must occur first.
Worked Examples for Addition: Example 1 (Same Units): A tailor bought 5 m 60 cm of blue fabric and 3 m 25 cm of red fabric. What is the total length of fabric she bought?
Step 1: Arrange in columns. ``` m cm 5 60 + 3 25 ------- ``` Step 2: Add the centimetres. 60 cm + 25 cm = 85 cm Step 3: Add the metres. 5 m + 3 m = 8 m Step 4: Combine the results. Total length = 8 m 85 cm Example 2 (With carrying over, Same Units): A builder used 7 m 85 cm of rope for one task and 4 m 30 cm for another. What is the total length of rope used?
Step 1: Arrange in columns. ``` m cm 7 85 + 4 30 ------- ``` Step 2: Add the centimetres. 85 cm + 30 cm = 115 cm Step 3: Convert centimetres if greater than or equal to
1
0
0. Since 115 cm is 1 m 15 cm (because 100 cm = 1 m), write down 15 cm and carry over 1 m to the metres column. ``` m cm 7 85 + 4 30 ------- 15 (1m carried over) ``` Step 4: Add the metres, including the carried-over metre. 7 m + 4 m + 1 m (carried over) = 12 m Step 5: Combine the results. Total length = 12 m 15 cm Worked Examples for Subtraction: Example 3 (Same Units): A carpenter has a plank of wood 10 m 75 cm long. He cuts off 4 m 30 cm. What length of wood is left?
Step 1: Arrange in columns. ``` m cm 10 75 - 4 30 ------- ``` Step 2: Subtract the centimetres. 75 cm - 30 cm = 45 cm Step 3: Subtract the metres. 10 m - 4 m = 6 m Step 4: Combine the results. Length left = 6 m 45 cm Example 4 (With borrowing, Same Units): A roll of wire is 15 m 20 cm long. A worker uses 7 m 60 cm. How much wire is remaining?
Step 1: Arrange in columns. ``` m cm 15 20 - 7 60 ------- ``` Step 2: Subtract the centimetres. Notice that 20 cm is less than 60 cm. We need to borrow from the metres column. Borrow 1 m (which is 100 cm) from 15 m. The metres column becomes 14 m. The centimetres column becomes 20 cm + 100 cm = 120 cm. ``` m cm 14 120 (after borrowing) - 7 60 ------- ``` Step 3: Now subtract the centimetres. 120 cm - 60 cm = 60 cm Step 4: Subtract the metres. 14 m - 7 m = 7 m Step 5: Combine the results. Remaining wire = 7 m 60 cm This section outlines the practical activities for the teacher and students to engage with the topic.
Materials: Rulers, tape measures, string, chart paper, markers, various classroom objects (pencils, books, desks, blackboard), examples of local landmarks/distances.
Activity 1: Introduction and Prior Knowledge Activation (5 minutes)
Teacher Activity: Begin by asking students what they understand by 'length' and 'distance'. Ask for examples of when they might need to measure something (e.g., buying clothes, building a house, travelling). Display pictures of different measuring tools (ruler, tape measure) and ask students to identify them.
Student Activity: Students share their understanding and examples. Identify and discuss the measuring tools.
Activity 2: Estimating and Measuring Lengths in Centimetres and Metres (20 minutes)
Teacher Activity: Introduce the concept of estimation: making a smart guess. Explain why it's useful. Display various classroom objects (e.g., pencil, exercise book, blackboard, desk, door, classroom wall). Guide students to estimate the length of these objects using appropriate units (cm for small items, m for larger ones). For example, "How long do you think this pencil is in centimetres?" Demonstrate how to correctly use a ruler or tape measure to find the actual length. Organise students into small groups (3-4 students per group) and distribute rulers and tape measures. Assign each group a set of objects to estimate and measure.
Student Activity: Individually, students estimate the length of assigned objects and record their estimates in a table (e.g., Object | Estimated Length | Actual Length). In groups, students take turns measuring the actual lengths of the objects using the provided tools. Students record actual measurements and compare them with their estimates, noting how close their estimates were.
Activity 3: Estimating Distances in Kilometres (10 minutes)
Teacher Activity: Discuss familiar long distances relevant to students' lives (e.g., distance from school to a nearby market, distance to a neighbouring town, distance to the state capital). Introduce the unit 'kilometre' for long distances. Provide approximate real-world benchmarks (e.g., "It takes about 10-15 minutes to walk 1 km").
Ask students to estimate distances: "How far do you think it is from our school to the community health centre? (Is it 1 km, 5 km, or 10 km?)" "How far is it from our town to the nearest big city (e.g., Ibadan if in Oyo state)?". Share known actual distances for comparison.
Student Activity: Students participate in discussions, making and sharing their estimations of various distances in kilometres. They compare their estimates with the actual distances provided by the teacher.
Activity 4: Addition and Subtraction of Lengths (20 minutes)
Teacher Activity: Review the conversion factors (1m = 100cm, 1km = 1000m). Present clear, step-by-step examples of adding and subtracting lengths with and without carrying/borrowing, using the examples from Section 2.
6. Emphasize aligning units and carrying/borrowing correctly. Use word problems relevant to Nigerian scenarios (e.g., "A tailor uses X m Y cm of fabric, then Z m W cm more," or "A rope is X m Y cm long, and Z m W cm is cut off."). Guide students through solving a few problems on the board, encouraging participation.
Student Activity: Students observe the teacher's examples and ask questions. Students solve practice problems provided by the teacher in their notebooks, collaborating with peers if allowed. Students present their solutions and explain their steps.
Activity 5: Consolidation and Wrap-up (5 minutes)
Teacher Activity: Summarise the key concepts: importance of standard units, estimation, comparison, and performing operations on lengths. Address any remaining questions. Assign independent practice.
Student Activity: Students reiterate key learnings and clarify any lingering doubts. These practice questions are designed to reinforce understanding, with step-by-step solutions provided for teacher reference.
Question 1: Estimation and Measurement Estimate the length of the teacher's table in metres. Then, measure it using a tape measure and compare your estimate to the actual measurement.
Estimated Length: (Students' individual estimations will vary, e.g., 2 metres)
Actual Length: (Teacher measures the table with students watching, e.g., 2 metres 15 centimetres)
Comparison: My estimate of 2 metres was close to the actual length of 2 metres 15 centimetres. (
Commentary: Encourage students to reflect on how close their estimate was and why it might have been different).
Question 2: Addition of Lengths A carpenter needs two pieces of wood. One piece is 3 m 45 cm long, and the other is 2 m 80 cm long. What is the total length of wood the carpenter needs?
Solution: Step 1: Write the lengths in columns, aligning metres and centimetres. ``` m cm 3 45 + 2 80 ``` Step 2: Add the centimetres column. 45 cm + 80 cm = 125 cm Step 3: Convert 125 cm to metres and centimetres. 125 cm = 1 m 25 cm (since 100 cm = 1 m) Write down 25 cm and carry over 1 m to the metres column. ``` m cm 3 45 + 2 80 25 (carry 1m) ``` Step 4: Add the metres column, including the carried-over metre. 3 m + 2 m + 1 m (carried) = 6 m Step 5: Combine the results. Total length = 6 m 25 cm
Commentary: This problem demonstrates addition with carrying over from centimetres to metres, a common challenge for students. Emphasize the conversion of 100 cm to 1 m.
Question 3: Subtraction of Lengths A roll of fishing net is 12 m 10 cm long. A fisherman uses 5 m 30 cm of it. How much net is left?
Solution: Step 1: Write the lengths in columns, aligning metres and centimetres. ``` m cm 12 10 5 30 ``` Step 2: Subtract the centimetres column. Since 10 cm is less than 30 cm, we need to borrow 1 m (100 cm) from the metres column. The 12 m becomes 11 m. The 10 cm becomes 10 cm + 100 cm = 110 cm. ``` m cm 11 110 (after borrowing) 5 30 ``` Step 3: Subtract the new centimetres column. 110 cm - 30 cm = 80 cm Step 4: Subtract the metres column. 11 m - 5 m = 6 m Step 5: Combine the results. Remaining net = 6 m 80 cm
Commentary: This problem involves borrowing from the metres column to enable subtraction in the centimetres column. Highlight the conversion of 1 m to 100 cm when borrowing.
Question 4: Combining Units and Operations A delivery van travels 4 km 500 m to deliver goods in the morning and then 2 km 800 m in the afternoon. What is the total distance covered by the van?
Solution: Step 1: Write the distances in columns, aligning kilometres and metres. ``` km m 4 500 + 2 800 ``` Step 2: Add the metres column. 500 m + 800 m = 1300 m Step 3: Convert 1300 m to kilometres and metres. 1300 m = 1 km 300 m (since 1000 m = 1 km) Write down 300 m and carry over 1 km to the kilometres column. ``` km m 4 500 + 2 800 300 (carry 1km) ``` Step 4: Add the kilometres column, including the carried-over kilometre. 4 km + 2 km + 1 km (carried) = 7 km Step 5: Combine the results. Total distance = 7 km 300 m
Commentary: This example applies the addition concept to kilometres and metres, reinforcing the carrying-over process and the conversion factor for kilometres.
Understanding length has profound relevance and numerous practical applications in daily Nigerian life. Teachers should explicitly link the topic to these contexts. Tailoring and Fashion Design (Cultural and Economic): Nigerian fashion is vibrant and requires precise measurements. Tailors (like Mama Nike or Baba Tunde) constantly measure clients for clothes (wrapper, Agbada, blouse, trousers) and then measure fabric (e.g., Ankara, Adire, Lace) in metres and centimetres. Students can relate to this when their parents buy fabrics or get clothes sewn. The ability to estimate fabric needed prevents wastage and ensures a good fit.
Example:* Estimating how many metres of Ankara fabric are needed for a child's dress versus an adult's Agbada. Construction and Building (Economic and Community): From building simple mud houses in rural areas to modern high-rises in cities like Lagos or Abuja, measurement of length is fundamental. Carpenters measure wood, masons measure blocks and wall dimensions, and engineers measure land plots and building heights. Knowing how to measure and calculate lengths is essential for accurate blueprints, material procurement, and ensuring structural integrity.
Example:* A builder needing to calculate the total length of fencing wire required to secure a compound that is 20m long and 15m wide. Transportation and Travel Planning (Community and Economic): Whether travelling by car, bus, or even walking, understanding distances in kilometres is vital. People estimate journey times based on distances, plan fuel consumption, and determine fares. This is relevant for interstate travel (e.g., how many kilometres from Kano to Kaduna?) or local commutes (e.g., distance to the nearest market).
Example:* Estimating the distance from a rural village to the nearest town where the weekly market is held, to decide if walking or taking a motorcyle (okada) is more feasible.