TEACHGYAN • CLASS 6 SCIENCE • CURIOSITY
Chapter 5 – Measurement of Length and Motion
Measurement • Units • Accurate measurement • Curved lengths • Motion • Types of motion • MCQs • Worksheet
Chapter focus: We measure length to describe size and distance accurately. We also describe motion by observing how an object’s position changes with time.
🎯 Learning Goals
- Explain why standard units are needed.
- Use metre, centimetre and millimetre appropriately.
- Measure length carefully using a ruler or scale.
- Understand how to measure curved lengths using a flexible method.
- Distinguish rest and motion using a reference point.
- Recognise translatory, rotational and oscillatory motion.
- Identify repetitive, non-repetitive, periodic and non-periodic motion.
📏 1. Why Do We Measure?
Descriptions such as “long”, “short” or “near” are useful in conversation but may be too vague for scientific work. Measurement assigns a number to a quantity using an agreed unit.
📐 2. Standard Units of Length
The SI base unit of length is the metre (m). Smaller lengths are commonly expressed in centimetres (cm) and millimetres (mm), while larger distances may be expressed in kilometres (km).
- 1 m = 100 cm
- 1 cm = 10 mm
- 1 km = 1000 m
🧰 3. Measuring with a Ruler
- Place the ruler along the object.
- Keep the ruler straight and aligned.
- Start from the zero mark when possible.
- Read the scale at eye level to reduce parallax error.
- Record the number together with its unit.
If the zero edge is damaged, start from another clear mark and subtract the starting reading from the ending reading.
🌀 4. Measuring a Curved Length
A straight ruler cannot directly follow a curved line. A thread, flexible strip or suitable measuring tape can be placed along the curve, marked, and then compared with a ruler.
🚗 5. What Is Motion?
An object is said to be in motion when its position changes with respect to a chosen reference point as time passes. If its position does not change relative to that reference point, it is considered at rest.
Important: Rest and motion are relative to the reference point. A passenger sitting in a moving bus is at rest relative to the seat but moving relative to the road.
🔄 6. Types of Motion
| Type | Meaning | Example |
|---|---|---|
| Translatory | Object changes position from one place to another. | Car moving on a road |
| Rotational | Object turns about an axis. | Fan blades |
| Oscillatory | To-and-fro motion about a position. | Swing moving back and forth |
| Repetitive | Pattern of motion repeats. | Hands of a clock |
⏱️ 7. Periodic and Non-Periodic Motion
A motion that repeats itself after equal intervals of time is periodic. Motion that does not repeat at equal time intervals is non-periodic. A clock hand provides a familiar example of periodic motion, while traffic movement is generally non-periodic.
📋 Worked Examples
Example 1 – Unit Choice
The thickness of a coin is more conveniently expressed in millimetres, while the distance between two cities is more conveniently expressed in kilometres.
Example 2 – Damaged Zero Mark
If one end of a pencil is at 2.0 cm and the other is at 17.0 cm, its length is 17.0 − 2.0 = 15.0 cm.
Example 3 – Relative Motion
A child sitting inside a moving bus is at rest relative to another seated passenger but in motion relative to a tree outside.
🧠 Think Like a Scientist
- Why are standard units important?
- Why should the eye be placed directly above the scale reading?
- Why is a thread useful for measuring a curved line?
- Can the same object be at rest and in motion at the same time? Explain.
- How is rotational motion different from translatory motion?
📝 Important Practice Questions
- What is measurement?
- Name the SI unit of length.
- Convert 3 m into centimetres.
- Why should a ruler be aligned with the object?
- How can a curved length be measured?
- Define motion using a reference point.
- What is rotational motion?
- Give two examples of oscillatory motion.
- What is periodic motion?
- Why are units written with measurements?
🔥 Competency / HOTS
- A ruler begins at 1 cm instead of zero. Explain how you would measure an object’s length.
- Design an activity to measure the curved edge of a table or bottle.
- Explain why a passenger can be at rest relative to the bus but moving relative to the road.
- Classify the motion of a bicycle wheel, a swing and a walking person.
- Give an example of motion that combines more than one type of motion.
🧩 MCQ Practice
- The SI base unit of length is — A. metre B. litre C. kilogram D. second
- 1 metre equals — A. 100 cm B. 10 cm C. 1000 cm D. 1 cm
- A very small length can conveniently be measured in — A. millimetres B. kilometres C. metres only D. tonnes
- A flexible thread is useful for measuring — A. curved length B. temperature C. mass D. time
- Motion is identified by a change in — A. position relative to a reference point B. colour C. mass only D. shape only
- Fan blades show mainly — A. rotational motion B. oscillatory motion C. no motion D. random growth
- A swing shows — A. oscillatory motion B. only rotational motion C. no motion D. straight-line rest
- Motion repeating after equal intervals is — A. periodic B. non-periodic C. stationary D. irregular measurement
Answer Key: 1-A, 2-A, 3-A, 4-A, 5-A, 6-A, 7-A, 8-A
📄 Printable Worksheet
- Write three standard units of length.
- Convert 250 cm into metres and centimetres.
- Explain how to read a ruler correctly.
- Describe how to measure a curved line.
- Define reference point.
- Give two examples each of translatory, rotational and oscillatory motion.
- Classify five motions as periodic or non-periodic.
- Explain relative rest and motion with a bus example.
- Measure three objects in your classroom and record their lengths with units.
- Write two precautions for accurate measurement.
🔁 Quick Revision
| Concept | Remember |
|---|---|
| Metre | SI base unit of length. |
| Measurement | Comparison with an agreed unit. |
| Motion | Change in position relative to a reference point. |
| Rotational | Turning about an axis. |
| Oscillatory | To-and-fro motion. |
| Periodic | Repeats after equal intervals of time. |
✅ Mastery Checklist
- ☐ I can choose an appropriate unit.
- ☐ I can measure length accurately.
- ☐ I can measure a curved length.
- ☐ I can explain motion using a reference point.
- ☐ I can identify different types of motion.
- ☐ I can distinguish periodic and non-periodic motion.