📌 Chapter Overview
This chapter explains measurement of length, standard units, how to correctly measure lengths, the concept of reference points, what motion is, and the different types of motion.
🔑 Key Points
- Measurement using body parts (handspan, foot) is non-standard — results differ person to person
- The world uses SI Units (International System of Units) as standard
- SI unit of length = Metre (m)
- Reference point is needed to describe position and motion
- An object is in motion if its position changes with respect to a reference point
- Types of motion: Linear, Circular, Oscillatory
📐 5.1 How Do We Measure?
- In the past, people used body parts like handspan (balisht), arm length, foot, strides to measure
- Problem: Different people have different-sized body parts → different results for same length!
- A measurement consists of two parts: a number + a unit
- Example: "13 handspans" → 13 is the number, handspan is the unit
🏛️ India's Ancient Measurement System: Angula (finger width), dhanusa and yojana were used in ancient India. The angula is still used by traditional carpenters and tailors. Scales have been found at Harappan Civilisation excavation sites!
📏 5.2 Standard Units
- Different countries had different measurement systems → created confusion
- Countries came together to adopt SI Units (International System of Units)
- SI Unit of Length = Metre (m)
Unit Conversions:
1 km = 1000 m
1 m = 100 cm
1 cm = 10 mm| Unit | Full Name | Use |
|---|---|---|
| km | Kilometre | Long distances (city to city) |
| m | Metre | Medium lengths (room height) |
| cm | Centimetre | Short lengths (pencil, book) |
| mm | Millimetre | Very small lengths (coin thickness) |
💡 1 inch = 2.54 cm (still used by some people)
✅ 5.3 Correct Way of Measuring Length
Choosing the Right Tool:
- 15-cm scale → small objects (pencil, eraser)
- Metre scale or tape → height of a room, furniture
- Flexible tape → curved surfaces (chest size, tree girth)
Rules for Correct Measurement:
Rule 1 — Placing the scale:
- Place the scale in contact with the object along its length (not tilted)
Rule 2 — Position of eye:
- Eye should be directly above the point being measured (position B)
- Reading from the side (positions A or C) gives wrong results due to parallax error
Rule 3 — Broken scale:
- If zero marking is unclear, start from another mark (e.g., 1.0 cm)
- Length = Larger reading − Smaller reading
- Example: 10.4 cm − 1.0 cm = 9.4 cm
♿ Visually challenged students use scales with raised markings that can be felt by touch.
〰️ 5.4 Measuring the Length of a Curved Line
- A straight scale cannot directly measure a curved line
- Methods:
- Flexible measuring tape — directly place along the curve
- Thread method — place thread along the curve → straighten it → measure with scale
📍 5.5 Describing Position
- The position of an object must be described with respect to a Reference Point
- Reference Point = A fixed object or point with respect to which distance is measured
- Example: "Delhi 70 km" on a milestone means Delhi is the reference point
- Different people can have different reference points → same object appears at different distances
- Using the same reference point gives the same answer for everyone
🚗 5.6 Moving Things
Motion and Rest:
- Motion = An object whose position changes with respect to reference point with time
- Rest = An object whose position does NOT change with respect to reference point
💡 Same object can be in motion AND at rest depending on the reference point! Example: A passenger on a moving bus — at rest w.r.t. the bus, but in motion w.r.t. a building outside.
🔄 5.7 Types of Motion
1. ➡️ Linear Motion
- Object moves along a straight line
- Examples: A falling eraser, Republic Day march-past, a box being pushed, ball rolling on a straight road
2. 🔵 Circular Motion
- Object moves along a circular path
- Examples: Merry-go-round, blade of a fan, an eraser tied to a thread and whirled
3. 🔁 Oscillatory Motion
- Object moves to and fro about a fixed position
- Examples: Swing, pendulum, metal strip vibrating, sewing machine needle
💡 Periodic Motion: If an object repeats its path after a fixed time interval. Both Circular and Oscillatory motions are periodic in nature!
Types of Motion in a Children's Park:
| Object | Type of Motion |
|---|---|
| Swing | Oscillatory |
| Merry-go-round | Circular |
| Slide (sliding down) | Linear |
| See-saw | Oscillatory |
| Skipping rope (rope) | Circular |
📖 Important Definitions
| Term | Definition |
|---|---|
| Measurement | Comparing an unknown quantity with a known standard |
| Unit | A standard quantity used for measurement |
| SI Units | International System of Units — used worldwide |
| Metre | SI unit of length (symbol: m) |
| Reference Point | A fixed point with respect to which position is described |
| Motion | Change in position of an object with respect to reference point with time |
| Rest | No change in position with respect to reference point with time |
| Linear Motion | Motion along a straight line |
| Circular Motion | Motion along a circular path |
| Oscillatory Motion | To and fro motion about a fixed position |
| Periodic Motion | Motion that repeats after a fixed interval of time |
❓ NCERT Exercise Questions & Answers
Q1. Match the lengths with units:
| Length | Unit |
|---|---|
| Distance between Delhi and Lucknow | Kilometre |
| Thickness of a coin | Millimetre |
| Length of an eraser | Centimetre |
| Length of school ground | Metre |
Q2. True (T) or False (F): (i) Motion of a car on straight road = linear motion → T (ii) Object changing position w.r.t. reference point = in motion → T (iii) 1 km = 100 cm → F (1 km = 1,00,000 cm)
Q3. Which is NOT a standard unit? (i) millimetre (ii) centimetre (iii) kilometre (iv) handspan ✅ Answer: handspan
Q5. Express 1.5 km in metres: Answer: 1.5 km = 1.5 × 1000 = 1500 m
Q9. Two examples each of linear, circular, oscillatory motion:
Linear: Falling stone, Car moving on straight road Circular: Merry-go-round, Fan blade Oscillatory: Swing, Pendulum
Q12. Materials Tasneem should NOT use for metre scale:
- Paper — tears easily
- Cloth — stretches and is flexible
- Stretchable rubber — it stretches, giving wrong measurements
A scale must be made of a rigid, non-stretchy material like plywood or steel.
✅ MCQ Questions
1. SI unit of length is: a) Centimetre b) Inch c) Metre d) Kilometre ✅ Answer: c) Metre
2. Motion of a swing is: a) Linear b) Circular c) Oscillatory d) Random ✅ Answer: c) Oscillatory
3. Which is NOT used to measure a curved line? a) Thread b) Flexible tape c) Metre scale (straight) d) Thread + scale ✅ Answer: c) Straight metre scale
4. 1 m = _____ cm: a) 10 b) 100 c) 1000 d) 1 ✅ Answer: b) 100
5. A merry-go-round shows which type of motion? a) Linear b) Oscillatory c) Circular d) Periodic ✅ Answer: c) Circular
6. An object is at rest when: a) It is sleeping b) Position does not change w.r.t. reference point c) It is very slow d) It is underground ✅ Answer: b)
🧠 Quick Revision
- 📏 SI Unit of Length = Metre (m)
- 🔢 Conversions: 1 km=1000m | 1m=100cm | 1cm=10mm
- 👁️ Eye position = directly above the point being read
- 📍 Reference Point = fixed point to measure distances from
- 🚶 Motion = position changes w.r.t. reference point
- ➡️ Linear = straight line (falling stone, march-past)
- 🔵 Circular = circular path (fan, merry-go-round)
- 🔁 Oscillatory = to and fro (swing, pendulum)
- 🔄 Periodic = repeats after fixed time (circular + oscillatory)
📌 Exam Tips for all 3 chapters:
- Ch 3: Learn the Vitamins table (disease, symptoms, source) by heart — most exam marks come from here!
- Ch 4: Remember repulsion identifies a magnet and poles always come in pairs
- Ch 5: Know unit conversions and be able to give examples of all 3 types of motion 🎯