Big Ideas (at a glance)
- We need standard units so everyone gets the same answer for the same length.
- SI unit of length = metre (m); other useful units: km, cm, mm.
- Measure right: correct tool, correct placement, correct eye position (no parallax), and correct notation.
- Reference point tells where something is; motion means position changes with time relative to that reference.
- Types of motion you'll see everywhere: linear, circular, oscillatory; some are periodic.
Why Standard Units?
- Body-part units (handspan, foot, arm length) vary from person to person → results differ.
- Countries agreed on SI units to avoid confusion in trade, travel, and science.
🧠 Memory Hook — "k→m→cm→mm"
Think of a ladder that zooms in by ×10 steps:
1km = 1000m, 1m = 100cm, 1cm = 10mm.
✅ Symbols are lowercase (km, m, cm, mm), no plural s, and no space between number and unit: 12cm, not 12 cm.
Choosing the Right Measuring Tool
Tool Selection Guide
- 15-cm scale → small objects (eraser, coin, leaf)
- Metre scale → classroom objects, furniture, door height
- Flexible tape → curved or round things (tree trunk, chest size)
- Kilometre readings/milestones → long distances (towns, roads)
- Inches may appear on some scales; 1 inch = 2.54cm
🧩 Tool Picker Rule
If it's curved or round, use flexible tape or thread method; if it's straight and small, use a short scale;bigger, use a metre scale or long tape.
Measure Correctly (avoid the classic mistakes)
- 1Place the scale flush along the object's length.
- 2Eye above the mark (avoid parallax): keep your eye perpendicular to the scale at the reading point.
- 3Broken zero? Start from a full mark (say 1.0cm) and subtract the starting reading from the ending reading.
- 4Curved line? Lay a thread along the curve, then straighten it and measure with a scale.
- 5Record properly: value + unit (e.g.,
9.4cm).
🧪 Mini-lab
Measure a page thickness by measuring 100 pages together, then divide by 100 for better accuracy.
Describing Position — the Reference Point Idea
- A reference point is a fixed place you measure from (bus stand, "0km" stone, centre line on a field).
- Everyone using the same reference point will agree on distances and positions.
- Milestones (e.g., "Delhi 70km" → "Delhi 60km") tell how far you are from Delhi—your position changes with time as you move.
💡 Mind Trick — "From where?"
Whenever you hear a distance, ask: From where? That's your reference point.
Rest vs Motion (it depends on reference!)
In Motion
Position changes with time relative to a reference point.
At Rest
Position does not change relative to that point.
Bus example: Passengers are at rest relative to the bus, but in motion relative to trees outside.
🧠 Switch the frame
Change the reference, and rest ↔ motion judgment can change!
Types of Motion (with daily-life examples)
A) Linear Motion
(straight-line path)
- • Falling apple
- • Marching parade
- • Box pushed straight
- • Car on straight road
B) Circular Motion
(around a centre)
- • Merry-go-round
- • Spinning fan blades
- • Stone tied to string
- • Whirled object
C) Oscillatory Motion
(to-and-fro about fixed position)
- • Swing in park
- • Pendulum
- • Bent ruler released
- • Load on spring
D) Periodic Motion
(repeats after equal time intervals)
- • Many circular motions
- • Many oscillatory motions
- • Fan blades
- • Pendulum swings
🧠 Memory Hook — "Li-Ci-Os (LEE-SEE-OSS)"
Linear, Circular, Oscillatory → classify any motion you see.
Conversion & Notation Quick Sheet
Unit Conversions
- km → m: ×1000 (e.g., 1.5km = 1500m)
- m → cm: ×100 (e.g., 2m = 200cm)
- cm → mm: ×10 (e.g., 9.4cm = 94mm)
- in → cm: ×2.54 (e.g., 3in ≈ 7.62cm)
✍️ Write like a pro
5m, 27cm, 3mm
Typical Exam/Practical Traps & Fixes
- Wrong eye angle (parallax): always look straight above the mark.
- Starting from a broken zero: start at 1.0cm (or any clear mark) and subtract.
- Curved lengths with rigid scales: use thread/tape first, then a scale.
- Wrong unit choice: don't measure road distance in metres or page thickness in centimetres—pick sensible units.
HOTS / Practice Prompts
- 1
You and your friend measure the same desk—your answers differ. List three reasons and how to avoid them.
- 2
A string light must go along an arch. Describe a two-step method to get its exact length.
- 3
From a moving train with closed windows, can you tell if it's moving at constant speed? Explain with the reference point idea.
- 4
Classify motions you see in a park: swing, slide, roundabout, running child, rotating sprinkler—with reasons.
- 5
A kilometre stone changes from "City A 120km" to "City A 95km". What changed: position or reference? Explain.
Suggested Visuals for Your Notes
- Unit Ladder Poster: km ⇄ m ⇄ cm ⇄ mm with ×/÷ arrows.
- Correct Reading Diagram: eye position A/B/C—only B is correct (no parallax).
- Broken-Zero Method: start at 1.0cm → end at 10.4cm → length = 9.4cm.
- Curved-Line Method: thread along curve → straighten → measure.
- Motion Gallery: linear vs circular vs oscillatory, and a periodic label for repeating ones.
Quick Recap
- SI length unit: metre (m); conversions among km, m, cm, mm.
- Accurate measuring needs right tool, right placement, right eye, and right notation.
- Reference point defines position; motion = changing position with time relative to it.
- Recognise and classify linear, circular, oscillatory motions; many are periodic.