Measurement of Length and MotionIntroduction
Deepa, a curious eleven-year-old girl, lives in Andhra Pradesh. The new school year has started, and Deepa needs a new uniform — she has grown taller! Her mother asks the shopkeeper for a two-metre cloth piece, and the tailor measures her using a flexible measuring tape.
How did the shopkeeper and the tailor know how much cloth to use? They measured. Measurement is everywhere: the length of your desk, the distance to school, the time a train takes — and even whether an object is moving at all is a question of measurement.
In this chapter you will learn how to measure length correctly, why we need standard units, and how to describe the position and motion of objects.
How do we Measure? — Standard Units
Long ago, people measured using body parts: a handspan, a cubit, a footstep. But there is a problem — try it with a friend:
Find out
Measure the width of your classroom door using your handspan, and ask a friend to do the same. Do you get the same number of handspans?
Everyone's handspan is different — so handspans, footsteps and cubits are not reliable. To avoid confusion, scientists use standard units: the International System of Units (SI units).
Length units
| Unit | Symbol | Value |
|---|---|---|
| 1 metre (base) | m | — |
| 1 centimetre | cm | 0.01 m |
| 1 millimetre | mm | 0.001 m |
| 1 kilometre | km | 1000 m |
The metre is the standard unit of length. Your height is about 1.3
To convert, remember:
The Correct Way of Measuring
A small mistake in measuring can waste a lot of cloth — or worse! Place the pencil below correctly on the scale. Watch out for broken rulers and wrong eye positions:
Measured length: — cm
Three rules for measuring correctly:
- Place the scale alongside the object, touching it.
- If the scale is broken at its zero mark, start from any other full mark and
the starting reading at the end. - Your eye must be directly above the mark being read — looking from an angle gives a wrong value.
Correct (A) and incorrect (B, C) eye positions
You measured
Measuring a Curved Line
How would you measure the length of a curved line — say, the border of a leaf? You cannot place a straight ruler along it! Here is the trick — move the slider to lay a thread along the curve and then straighten it:
Instead:
- Place a thread along the curve, matching it exactly.
- Mark the thread where the curve ends.
- Straighten the thread and measure it against a scale.
The straightened thread measures about
Describing Position
When we say the school is 2 km north of your home, we are describing its position — and a position always needs a reference point (the home, in this case).
An object is in motion if its position changes with time relative to the reference point. If the position does not change, the object is at
Here is a girl walking along a straight path. Move the slider and watch her position (distance from the starting tree) change with time:
Position: 0 m · Time: 0 s
Notice the position increases steadily —
But wait — is the girl in motion for her friend walking beside her? For the friend, her position is not changing, so she appears at
Types of Motion
Objects can move in many different ways. Drag each one into the right bin:
Sort the motions
Sorted: 0 of 7
A cycle wheel shows two motions at once: the wheel rotates (circular) while the cycle moves forward (linear). Such a combination is called rotational motion — or in this case, linear + rotational!
Quick quiz
- The SI unit of length is the
. 1 km =m. - If a scale is broken at the zero end, start from another mark and
the starting reading. - A merry-go-round shows
motion. - A pendulum shows
motion.
Chapter Summary
- Measurement means comparing an unknown quantity with a known standard unit. Body-based units (handspan, cubit) vary from person to person, so we use SI units.
- The SI unit of length is the metre (m);
1 m = 100 cm ,1 cm = 10 mm ,1 km = 1000 m . - Measuring correctly: place the scale along the object; if the zero mark is broken, subtract the starting reading; keep the eye directly above the mark.
- The length of a curved line is measured with a thread, which is then straightened and measured.
- An object's position needs a reference point. An object is in motion if its position changes with time; otherwise it is at rest.
- Types of motion: linear (straight path), circular (around a fixed centre), periodic (repeats after equal intervals) — and combinations like rotational motion.
Linear, circular and periodic motion are all around you — a moving bus, a spinning fan, a swinging swing.