CLASS 6 SCIENCE · CHAPTER 5 · STUDY NOTES
Measurement of Length and Motion
Measure carefully, explain your units, and describe motion using a clear reference.
01 / EXPLORE & UNDERSTAND
A measurement needs a number and a unit
Saying a desk is “eight long” leaves out an important detail: eight of what?
1. Compare with a repeated unit
Measuring a desk with handspans compares its length with one handspan. A longer handspan fits fewer times along the same desk. Different results need not mean the desk changed.
2. Choose a shared standard
People’s handspans differ. A standard unit keeps its agreed size so people can compare measurements. The SI unit of length is the metre, symbol m. Traditional units such as angula are historically useful, but a body-based size must be specified to compare results.
Your turn
Question
A smaller hand measures the same desk using more handspans. Has the desk become longer?
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02 / EXPLORE & UNDERSTAND
Change the unit, keep the length
A length can have different numerical values when its unit changes.
1. Think about the size of the unit
A smaller unit fits more times. A ribbon 2 m long is 200 cm long because each metre contains 100 centimetres. It is still the same ribbon.
2. Use the relationship, not a guessed zero
For 350 cm, divide by 100 to get 3.5 m. For 4 cm, multiply by 10 to get 40 mm. Write a space before the unit: 3.5 m. Symbols stay lowercase and do not take a plural s: 5 cm, not 5 cms.
Your turn
Question
A strip is 75 mm long. How many centimetres is that? Explain the operation.
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03 / EXPLORE & UNDERSTAND
Choose a tool that fits the job
A tool must suit the object’s size and shape.
1. Match straight and curved measurements
A short ruler can measure a pencil. A long tape is easier for a room. A flexible tape can go around a tree trunk; a rigid straight ruler cannot follow that curve directly.
2. Read what one small interval means
On a common school ruler, each centimetre has ten equal intervals, so one interval is 1 mm. Do not assume every instrument has the same smallest division. Stretchy material is unsuitable for a reliable ruler because its unit spacing can change.
Your turn
Question
Why is a stretchy rubber strip a poor choice for making a centimetre ruler?
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04 / EXPLORE & UNDERSTAND
Read the ruler carefully
Correct placement matters as much as choosing the right tool.
Look directly above the reading point
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1. Align with the object
Place the ruler close to and along the object’s length. Start from the zero mark, not automatically from the plastic edge. Keep the ruler straight along the length you intend to measure.
2. Look directly above the end
Looking from one side can make the end seem to line up with the wrong mark. Put your eye directly above the reading point. Raised markings allow some learners to read a scale by touch.
Your turn
Question
A ruler has extra plastic before its zero. Why should the pencil not automatically start at the edge?
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05 / EXPLORE & UNDERSTAND
A broken zero does not end the measurement
Use two readings to find the distance between the ends.
Move the strip. Its length stays 6.6 cm.
8.6 cm − 2 cm = 6.6 cm
1. Start at a clear mark
If zero is damaged, place one end at another clear mark. For our example, put it at 2 cm. The other end reaches 8.6 cm.
2. Subtract the starting part
The reading 8.6 cm includes the unused 2 cm before the object. Subtract it: 8.6 − 2 = 6.6 cm. Moving the object along the ruler changes both readings but not its length.
Your turn
Question
An object begins at 3 cm and ends at 9.4 cm. What is its length, and why is it not 9.4 cm?
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06 / EXPLORE & UNDERSTAND
Follow the curve before measuring
The curved route is not the same as the straight gap between its ends.
Let the thread follow the bend
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1. Lay a thread along the line
Fit a non-stretching thread carefully along the curve. Mark where the curve starts and ends on the thread. Do not leave loops or pull it away from the path.
2. Straighten without stretching
Straighten the marked part and measure it with a ruler. A flexible tape can measure many curves directly. What matters is following the route whose length is needed.
Your turn
Question
A straight ruler joins the ends of a curved ribbon. Has it measured the ribbon’s full length?
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07 / EXPLORE & UNDERSTAND
“Near” needs a reference
Two learners can disagree about which place is closer because they start from different homes.
Each home gives a different starting point
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1. Name the starting reference
“The garden is closer” is incomplete. Say closer to your home, the school or the bus stand. Using the same reference allows a fair comparison.
2. Keep the direction or route clear
A road sign giving the distance to a town uses that town as the reference. Two places can be the same distance from a reference in different directions, so distance alone may not identify the exact location.
Your turn
Question
Two houses are each 1 km from a school. Must they be next to each other?
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08 / EXPLORE & UNDERSTAND
Rest and motion depend on the reference
Ask whether position changes with time relative to something named.
1. Compare a passenger with the seat
A seated passenger keeps the same position relative to the bus seat. In that comparison, the passenger is at rest.
2. Now compare with a roadside tree
As the bus travels, the passenger’s position changes relative to the tree. The same passenger is moving in that comparison. These statements do not contradict each other: their reference points differ.
Your turn
Question
A bag rests on the floor of a moving train. Give one reference for rest and one for motion.
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09 / EXPLORE & UNDERSTAND
Describe the path: straight, round or back-and-forth
Choose a particular moving object or point before naming the motion.
Follow the orange marker
Linear
Along a straight path
Circular
Around a circle
Oscillatory
To and fro about the middle
1. Follow one point over time
A toy car on a straight track has linear motion. A seat on a rotating merry-go-round follows a circle. A swing moves back and forth about its middle position: oscillatory motion.
2. Do not force every part into one label
A moving machine can have several motions at once. For example, a wheel turns while its centre moves forward. State which part you describe and which reference you use.
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Question
A swing moves along a curved arc. Why is oscillatory a useful description?
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10 / EXPLORE & UNDERSTAND
Repeating motion needs a time check
A repeated path and a regular repetition are related but not identical ideas.
1. Check the interval
A steadily rotating wheel returns a marked point to the same place after each equal time interval. A regularly swinging pendulum is another useful example.
2. Separate path from timing
Circular tells us the shape of the path. Periodic tells us about repetition in time. If a ride keeps stopping and speeding up irregularly, a circular path alone does not prove equal repetition times.
Your turn
Question
A toy completes rounds in 4 s, then 7 s, then 3 s. Is the repetition periodic in this observation?
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Sign inNew here? Sign up freeCHAPTER RECAP
Take these ideas with you
- A measurement combines a number with a unit.
- 1 km = 1000 m; 1 m = 100 cm; 1 cm = 10 mm.
- Align the ruler and read directly above the endpoint.
- For a non-zero start, subtract start reading from end reading.
- Motion is a change of position with time relative to a reference.
- Path shape and regularity in time describe different features of motion.
Use your understanding in a new situation
Question
A paper strip starts at 4 cm and ends at 11.2 cm. Find its length in centimetres and millimetres.
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A child sits still on a turning merry-go-round. Explain how the child can be at rest and moving.
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Sign inNew here? Sign up freeNCERT · Chapter source PDF, pages 16, 18 · Original Learnijoy explanations and practice.