Everything around us is in motion. The Earth moves around the Sun, birds fly in the sky, cars move on roads, and even the tiny dust particles move in the air. When an object changes its position with time, it is said to be in motion. In this chapter, we learn about motion and how to measure distances.
Measuring distance correctly requires a standard unit. In ancient times, people used body parts like the hand span, cubit, and foot to measure length. But these units were not reliable because they differ from person to person. So a standard system of units was developed, and the metre became the standard unit of length.
In this chapter, we study different types of motion, how to measure the length of objects using a scale, and how to measure very large distances like the distance between cities. We also learn how to measure the size of small objects. This chapter helps us understand the world in a scientific and accurate way.
An object is said to be in motion when its position changes with respect to time and a fixed point in its surroundings. If an object's position does not change with time, it is said to be at rest.
For example, a moving bus changes its position as time passes, so it is in motion. The trees on the roadside are at rest because their position does not change. But the idea of motion and rest is relative. A person sitting in a moving bus is at rest with respect to the bus but in motion with respect to the trees outside.
To understand motion, we need a reference point, which is a fixed object in the surroundings. We observe whether the position of an object changes with respect to this reference point over time.
Motion can be classified into different types based on how an object moves.
An object can show more than one type of motion at the same time. For example, the wheels of a moving car rotate (circular motion) while the car itself moves forward (linear motion).
In ancient times, people used their body parts to measure length. Some of these early units were: - Hand span: The distance between the tip of the thumb and the tip of the little finger when the fingers are stretched. - Cubit: The distance from the elbow to the tip of the middle finger. - Foot: The length of a person's foot. - Finger (digit): The width of a finger.
These units were not standard because the size of body parts differs from person to person. The hand span of a tall person is longer than that of a small child. This caused problems in trade and construction. So the need for a standard unit arose.
To avoid confusion, scientists developed a standard system of measurement called the International System of Units (SI). In this system, the metre is the standard unit of length.
The symbols for units are standard: m for metre, cm for centimetre, mm for millimetre, and km for kilometre. When writing the symbol, no full stop is used after it, and it is not written in plural form. For example, we write 5 m, not 5 m. or 5 ms.
To measure the length of an object, we use a scale or a measuring tape. The correct way to measure length is: 1. Place the scale along the object whose length is to be measured. 2. Keep the object and the scale on a flat surface. 3. Align one end of the object with the zero mark of the scale. 4. Read the number at the other end of the object. 5. The reading is the length of the object.
While measuring, the eye must be placed vertically above the reading point to avoid the error of parallax. If we look at the scale from an angle, the reading will be wrong. This is called the parallax error.
When measuring the length of a curved line or a curved object, we use a thread. The thread is placed along the curved line, and then the length of the thread is measured with a scale.
Very small distances: To measure the thickness of a coin or the diameter of a wire, we cannot use a simple scale easily. We use the method of stacking or rolling: - To find the thickness of a coin or a sheet, we stack many of them, measure the total thickness, and divide by the number of objects. - To find the diameter of a wire, we wind the wire around a pencil many times, measure the total width, and divide by the number of turns.
Very large distances: The distance between two cities is measured in kilometres (km). We can use the odometer of a vehicle to measure the distance travelled. The distance between the Earth and the Moon or the stars is measured in even larger units.
Different instruments are used to measure different lengths: - Measuring tape: Used by tailors and for measuring distances up to a few metres. - Metre scale: Used to measure lengths up to one metre. - Odometer: Fitted in vehicles, it measures the distance travelled by the vehicle. - Screw gauge and vernier callipers: Used to measure very small lengths precisely, like the diameter of a wire.
Each instrument is chosen according to the size of the object and the accuracy required.
Measurement of distances is used everywhere in daily life: - A tailor measures the cloth with a measuring tape. - We measure the height of a child with a wall scale. - The distance between two cities is measured in kilometres. - Athletes run fixed distances like 100 m, 200 m, and 400 m. - Astronomers measure the distance between planets.
Motion and measurement are also used to compare objects. For example, we can compare the length of a pencil and a pen only by measuring them in the same unit. This is why standard units are so important.
| Unit | Value |
|---|---|
| 1 metre (m) | 100 cm |
| 1 centimetre (cm) | 10 mm |
| 1 kilometre (km) | 1000 m |
| Type of Motion | Description | Example |
|---|---|---|
| Linear | Motion in a straight line | Car on a straight road |
| Circular | Motion along a circular path | Hands of a clock |
| Periodic | Repeats after a fixed time | Swing of a pendulum |
| Oscillatory | To and fro motion | Swing moving to and fro |
| Random | No specific direction | Dust particles in air |
In this chapter, we learned that motion is a change in position with time, and that measuring distances accurately requires standard units. We studied the early methods of measurement like the hand span and cubit, which were unreliable because they differed from person to person. The metre became the standard unit of length in the SI system, with conversions of 100 cm in a metre and 1000 m in a kilometre. We explored different types of motion, including linear, circular, periodic, oscillatory, and random, and learned how to measure length correctly while avoiding the parallax error. Measurement and motion are not just classroom topics; they are essential for science, trade, travel, and everyday life.