Measurement of Length and Motion
How long is it, and is it moving? This Class 6 Curiosity chapter explains why we need standard units of measurement, how to measure length using the metre and its smaller and larger units, the right way to read a scale, and the different kinds of motion objects can have.
Learning objectives
- Explain the need for standard units.
- Measure length using SI units.
- Measure correctly using a scale.
- Identify types of motion.
Key concepts
The need for standard units
Long ago people measured length using body parts like the handspan or foot, but these differ from person to person, giving different answers. To avoid confusion, the world uses standard units that are the same everywhere, so a measurement means the same thing to everyone.
Units of length
The standard (SI) unit of length is the metre (m). Smaller lengths are measured in centimetres (cm) and millimetres (mm), and larger distances in kilometres (km). Useful relations are 1 m = 100 cm, 1 cm = 10 mm, and 1 km = 1000 m.
Measuring length correctly
To measure length with a scale, place the object along the scale and start from the zero mark (or a clear mark if the end is worn). Keep your eye directly above the reading to avoid errors, and measure along a straight line. For curved paths, a thread can be laid along the curve and then measured straight.
Types of motion
Motion is a change in position with time. In rectilinear motion an object moves along a straight line, like a car on a straight road. In circular motion it moves along a circle, like a fan blade. In periodic motion it repeats after fixed times, like a swinging pendulum. An object not changing position is at rest.
Key definitions
- Measurement
- Comparing an unknown quantity with a known standard unit.
- Standard unit
- A fixed unit, the same everywhere, used for measuring.
- Motion
- A change in the position of an object with time.
- Rectilinear motion
- Motion along a straight line.
Solved examples
Q1. How many centimetres are in 1 metre?
Solution: 1 metre = 100 centimetres.
Q2. What kind of motion does a swinging pendulum show?
Solution: Periodic motion — it repeats after fixed intervals.
Q3. Why is the handspan not a good unit for measuring?
Solution: It varies from person to person, so it does not give the same answer.
Common mistakes to avoid
- Measuring from the edge of the scale instead of the zero mark.
- Viewing the scale from an angle, causing a reading error.
- Confusing rectilinear motion (straight) with circular motion.
- Forgetting that 1 cm = 10 mm and 1 m = 100 cm.
Measurement of Length and Motion — MCQ Quiz
10 questions with instant feedback. Use number keys 1–4 to answer.
The SI unit of length is the:
Practice questions
Short answer
What is the SI unit of length?
The metre (m).
Why are body parts not used as standard units?
Because they differ from person to person, giving different answers.
What is motion?
A change in the position of an object with time.
Long answer
Why do we need standard units of measurement, and what units are used for length?
In the past, people measured length using body parts such as the handspan, cubit or foot, but these vary from one person to another, so the same object could give different measurements. To remove this confusion, the world agreed on standard units that are fixed and the same everywhere. For length, the standard (SI) unit is the metre (m). Shorter lengths are measured in centimetres (cm) and millimetres (mm), and longer distances in kilometres (km), with 1 m = 100 cm, 1 cm = 10 mm and 1 km = 1000 m.
Describe the main types of motion with an example of each.
Motion is a change in an object's position with time, and it comes in different kinds. Rectilinear motion is movement along a straight line, such as a car going down a straight road. Circular motion is movement along a circular path, such as the tip of a fan blade. Periodic motion is movement that repeats after fixed intervals of time, such as the to-and-fro swing of a pendulum or a child on a swing. An object that does not change its position is said to be at rest.
HOTS (Higher Order Thinking)
Two people measure the same table using their handspans and get 6 and 8 spans. Why, and how is this fixed?
Their handspans are different sizes, so they get different counts; using a standard unit like the centimetre gives both the same answer.
A moving merry-go-round carries a child round and round. Name the type of motion and explain.
Circular motion, because the child keeps moving along a circular path around the centre.
Quick revision
Revision notes
- Standard units are fixed and the same everywhere (body parts are not).
- SI unit of length = metre; 1 m = 100 cm, 1 cm = 10 mm, 1 km = 1000 m.
- Measure from the zero mark, eye directly above the scale.
- Motion types: rectilinear (straight), circular (circle), periodic (repeats).
Key takeaways
- Standard units make measurements mean the same to everyone.
- The metre is the SI unit of length.
- Motion can be rectilinear, circular or periodic.
Frequently asked questions
What is the SI unit of length?
The metre (m).
How many millimetres are in a centimetre?
Ten — 1 cm = 10 mm.
What is periodic motion?
Motion that repeats after fixed intervals of time, like a pendulum's swing.