What this chapter is about
This chapter teaches you how to measure length correctly and understand different types of motion. Measuring length is one of the most basic skills in science. From buying cloth at the market to building a house, everyone needs to measure lengths accurately.
You will learn why we need standard units like the metre, and how to use different tools to measure short and long lengths. The chapter also introduces motion — what it means when something moves, and how we can describe that movement. By the end, you should be able to measure lengths using a ruler or tape, choose the right unit for different situations, and identify different types of motion around you.
Key ideas
- Length is the distance between two points. We measure it using standard units so everyone gets the same answer.
- The metre (m) is the standard unit of length. For smaller lengths, we use the centimetre (cm) and millimetre (mm). For longer distances, we use the kilometre (km).
- One metre equals 100 centimetres. One centimetre equals 10 millimetres. One kilometre equals 1000 metres.
- When measuring with a ruler, always place your eye directly above the mark to avoid errors. This prevents parallax error, which happens when you look at an angle.
- Motion means a change in position of an object with time. If an object stays in the same place, it is at rest.
- Objects can move in different ways: straight-line motion (like a car on a highway), circular motion (like a fan blade), and back-and-forth motion (like a swing).
- A reference point helps us decide if something is moving. A tree outside your train window seems to move backward because you are moving forward.
Formulas and facts to remember
- 1 metre (m) = 100 centimetres (cm)
- 1 centimetre (cm) = 10 millimetres (mm)
- 1 kilometre (km) = 1000 metres (m)
- A ruler or scale is used to measure short lengths like the length of your pencil.
- A measuring tape is used for longer lengths like the length of a room or your height.
- Motion is described by comparing the position of an object to a reference point.
- The end of a ruler marked zero should be placed exactly at the starting point of what you measure.
- Avoid using a broken or worn-out edge of a ruler to start your measurement.
Worked examples
Example 1: Converting centimetres to metres
Riya measured the length of her classroom as 850 cm. What is this length in metres?
Step 1: We know that 100 cm = 1 m. Step 2: To convert cm to m, divide by 100. Step 3: 850 ÷ 100 = 8.5 m. Answer: The classroom is 8.5 metres long.
Example 2: Measuring with a ruler
Arjun wants to measure his eraser. He places the zero mark of his ruler at one end of the eraser. The other end is at the 4 cm and 5 mm mark.
Step 1: Read the whole centimetre first: 4 cm. Step 2: Read the extra millimetres: 5 mm. Step 3: The length is 4 cm 5 mm, or 4.5 cm, or 45 mm.
Example 3: Identifying types of motion
Neha is on a swing in the park. What type of motion is she making?
Step 1: Observe how the swing moves. It goes forward, then backward, then forward again. Step 2: This repeated to-and-fro movement is called back-and-forth motion (or oscillatory motion). Answer: Neha is making back-and-forth motion.
Common mistakes
Wrong: Starting measurement from the edge of the ruler instead of the zero mark → Fix: Always align the object with the zero mark, not the ruler's edge.
Wrong: Thinking 1 km = 100 m → Fix: Remember 1 km = 1000 m, not 100.
Wrong: Looking at the ruler from the side when reading → Fix: Keep your eye directly above the mark to read correctly.
Wrong: Believing a sleeping person in a moving train is not in motion → Fix: Motion depends on the reference point; compared to the ground, the person is moving.
Wrong: Mixing up circular motion and back-and-forth motion → Fix: Circular means moving in a circle (fan blade); back-and-forth means swinging (pendulum).
Quick revision
- The standard unit of length is the metre (m).
- 1 m = 100 cm, 1 cm = 10 mm, 1 km = 1000 m.
- Use a ruler for small lengths and a measuring tape for bigger lengths.
- Motion means change in position; rest means no change.
- Types of motion: straight-line, circular, back-and-forth.
- Always measure from the zero mark and look straight down at the scale.