Motion & Measurement
Motion and Measurement
What you'll learn
- The need for standard units; SI system.
- Distance and time measurements — tools and units.
- How to calculate speed from distance and time.
- Types of motion — rectilinear, circular, periodic.
Key concepts
Why we need standard units
- In ancient times, body parts were used to measure length (cubit, foot, handspan).
- Problem: body sizes differ between people → inconsistent measurements.
- Solution: agree on standard units used by everyone worldwide.
SI (International System of Units) — adopted internationally:
| Quantity | SI unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Temperature | Kelvin | K |
| Current | Ampere | A |
Length conversions:
- 1 km = 1000 m; 1 m = 100 cm; 1 cm = 10 mm.
- 1 m = 1,000,000 µm (micrometres); 1 m = 10⁻⁹ nm is 1 nanometre.
- For very large distances: 1 light year ≈ 9.46 × 10¹⁵ m (distance light travels in 1 year).
Measuring tools for length:
- Ruler/metre rule: everyday lengths (30 cm, 1 m).
- Measuring tape: clothes, distances up to ~50 m.
- Vernier calipers: precise, up to 0.01 cm. (Class 6 intro only.)
- Odometer: measures distance traveled by a vehicle (fitted in cars).
Motion
- Motion: change in position of an object with respect to time and surroundings.
- Rest: object does not change position.
- Distance: total path length travelled; unit: m.
- Displacement: shortest path from start to end (straight line); unit: m.
Types of motion:
| Type | Description | Example |
|---|---|---|
| Rectilinear (linear) | Along a straight line | Car on highway; falling stone |
| Circular | Along a circular path | Fan blade tip; Earth around Sun |
| Periodic | Repeats after fixed time interval | Pendulum; Earth's rotation |
| Oscillatory | Back-and-forth around a central point | Swing; pendulum; guitar string |
Speed
Speed = Distance ÷ Time
| Quantity | Symbol | SI unit |
|---|---|---|
| Speed | v | m/s |
| Distance | d | m |
| Time | t | s |
- Fast objects: high speed; slow objects: low speed.
- Converting km/h to m/s: divide by 3.6 (or multiply by 5/18).
Examples:
- A bicycle covers 600 m in 3 min (180 s). Speed = 600/180 = 3.33 m/s.
- A car at 60 km/h in m/s: 60 ÷ 3.6 = 16.7 m/s.
Speed of common things (approximate):
| Object | Speed |
|---|---|
| Walking person | 1.4 m/s |
| Bicycle | 5 m/s |
| Car on highway | 27 m/s (100 km/h) |
| Sound in air | 343 m/s |
| Light | 3 × 10⁸ m/s |
Measuring time
- SI unit of time: second (s).
- Devices: stopwatch, wall clock, digital watch, atomic clock.
- Periodic phenomena used to measure time: pendulum, quartz crystal vibrations, atomic oscillations.
Accurate measurement tips
- Always read the scale at eye level (to avoid parallax error).
- Place ruler edge along the object to be measured.
- Start from the zero mark, not the end of the ruler.
- Take multiple readings and average for accuracy.
Quick check
- Why do we need standard units of measurement?
- What is the SI unit of length? How many centimetres in 1 metre?
- Distinguish between distance and displacement.
- Name three types of motion with examples.
- A car travels 120 km in 2 hours. What is its speed in km/h and m/s?
Open the Practice tab for graded questions on Motion and Measurement.
3 topics • Notes • Practice • AI explanations available
1. Units
Read notes & practice →
2. Distance Time
Read notes & practice →
3. Speed
Read notes & practice →
For generative engines & students
Every topic page delivers structured HTML (headings, lists, tables, takeaways) in the first response. Perfect for citations in AI overviews and fast scanning by students and parents.