Revision sheet · night-before
Measurement of Length and Motion
Read top to bottom — it follows the order of the chapter. Tick each section off as you finish it.
1 · Why standard units?
- Body-part units — handspan (balisht), arm, stride, foot — differ from person to person (Table 5.1: 13 to 14 handspans for the same table).
- Length = number + unit.
- Ancient Indian units: angula (finger width), dhanusa, yojana; angula still used by carpenters and tailors. Scales found at Harappan sites.
- Travel caused confusion → countries adopted SI units. SI unit of length = metre (m).
2 · Conversions
Numbers to memoriseEvery conversion question comes from this line.
| Bigger | = | Smaller |
|---|---|---|
| 1 km | = | 1000 m |
| 1 m | = | 100 cm |
| 1 cm | = | 10 mm |
| 1 mm | = | 0.1 cm |
| 1 inch | = | 2.54 cm |
Big → small: multiply (1.5 km × 1000 = 1500 m). Small → big: divide (250 cm ÷ 100 = 2.5 m).
3 · Measuring correctly — four rules
- Right tool: 15-cm scale (pencil) · metre scale/tape (room) · flexible tape (tree girth, chest).
- Scale in contact with the object, along its length.
- Eye directly above the reading point (position B).
- Broken scale: start at a full mark and subtract (10.4 − 1.0 = 9.4 cm).
Curved line → thread along the curve, straighten, measure. Smallest reading on a 15-cm scale = 1 mm.
4 · Writing units
- Lowercase: metre, km, m, cm, mm.
- No plural 's': 5 cm, never "5 cms".
- No full stop after a symbol (except at the end of a sentence).
- A space between number and unit: 15 cm.
5 · Reference point, motion and rest
- Reference point = fixed object/point from which distance is stated (bus stand; Delhi on kilometre stones).
- Motion = position changes w.r.t. reference point with time. Rest = it doesn't.
- Bus passengers: at rest w.r.t. the bus; in motion w.r.t. a building outside.
6 · Types of motion
| Type | Path | Examples |
|---|---|---|
| Linear | Straight line | Falling eraser/orange, march-past, pushed box, car on straight road |
| Circular | Circular path | Whirled eraser, merry-go-round, fan blades |
| Oscillatory | To and fro about a fixed position | Swing, hanging eraser, vibrating metal strip |
PeriodicRepeats after a fixed interval — circular and oscillatory motions are periodic.
7 · 10 facts most likely to be asked
- SI unit of length: metre (m).
- 1 km = 1000 m; 1 m = 100 cm; 1 cm = 10 mm.
- Handspan is not a standard unit.
- Eye directly above the reading point.
- Broken scale: subtract the starting mark.
- Curved line: thread or flexible tape.
- Reference point = fixed point from which distance is stated.
- Motion = change of position w.r.t. reference point with time.
- Linear / circular / oscillatory — straight / circle / to and fro.
- Circular and oscillatory motion are periodic.
Common mistakes to avoid
Don't lose marks here
- Writing 1 km = 100 cm — it is 1000 m = 1,00,000 cm.
- Writing units as "cms", "Cm" or "15cm" — correct is 15 cm.
- Reading the scale from the side — the eye must be directly above.
- Using a broken scale from its broken end and forgetting to subtract the starting mark.
- Saying an object is "simply at rest" without a reference point — rest and motion depend on the reference point.
- Calling a swing's motion circular — it is oscillatory (to and fro).
- Forgetting the unit — a length is always number + unit.