How Do We Measure? — Body Parts as Units
Before standard measuring instruments were invented, people often used body parts to measure length.
Common body-part units
- Finger width – The width of one finger was used to measure small lengths.
- Handspan – The distance between the tip of the thumb and the tip of the little finger when the hand is stretched.
- Cubit – The distance from the elbow to the tip of the middle finger.
- Foot – The length of a person’s foot.
- Pace/Step – The distance covered by one normal step.
- Stride: Distance covered in one complete step cycle, usually from one foot to the same foot again.
Ancient Indian Units of Measurement:-
Ancient Indians used different units to measure length and distance. Some were based on body parts, while others were standardized for trade, construction, and travel.
Dhanusa: A larger unit of length associated with the length of a bow.
Yojana: A large unit used to measure long distances, especially for journeys.
Harappan Scales: The people of the Indus Valley/Harappan Civilization used carefully marked measuring scales, showing that they had an organized system of measurement.
Angula: An ancient Indian unit of length based on the width of a finger.
Standard Units of Length:
Standard units are fixed units of measurement that are accepted and used everywhere. They help us get the same measurement regardless of who measures.
Common Units of Length: Millimetre (mm), Centimetre (cm), meter (m) , Kilometre (km) etc.
Conversions
1 cm = 10 mm
1 m = 100 cm = 1,000 mm
1 km = 1,000 m
Steps for Correct Measurement of length :-
1. Choose the correct measuring instrument.
2. Place the scale correctly.
3. Start from the zero mark.
4. Keep your eye vertically above the marking.
5. Read the measurement carefully.
Measuring Length of a Curved Line:-
1. Take a thin, non-stretchable thread.
2. Place one end of the thread at the starting point of the curved line.
3. Carefully lay the thread along the entire curve.
4. Mark the thread at the end point of the curved line.
5. Straighten the thread.
6. Place it along a ruler and measure the distance between the two marked points.
Describing Position – Reference Point
To describe the position of an object, we need to tell where the object is with respect to a fixed point. This fixed point is called a reference point.
What is a Reference Point?
A reference point is a fixed or known point from which we describe the position of an object.
Example:
If a school is 2 km east of a park, we can say:
“The school is 2 km east of the park.”
Here, the park is the reference point.
Moving Things :---
Motion- An object is said to be in motion if its position changes with time with respect to a reference point.
Examples:
A moving car is in motion with respect to a roadside tree.
A walking person is in motion with respect to a building.
Rest- An object is said to be at rest if its position does not change with time with respect to a reference point.
Examples:
A book lying on a table is at rest with respect to the table.
A parked car is at rest with respect to the road.
Motion and Rest are Relative:-
An object can be at rest for one observer but in motion for another.
Example:
Suppose you are sitting inside a moving bus.
You are at rest with respect to another passenger sitting beside you.
You are in motion with respect to a person standing on the roadside.
Types of Motion
Motion can be classified into different types based on the path followed by an object and the way it moves.
1. Linear Motion
When an object moves along a straight line, it is called linear motion.
Examples:
1. A car moving on a straight road
2. A person walking along a straight path
2. Circular Motion
When an object moves along a circular path around a fixed centre, it is called circular motion.
Examples:
1. A stone tied to a string and whirled around.
2. The hands of a clock.
3. A point on the edge of a rotating wheel.
3. Rotational Motion
When an object spins around its own axis, it is called rotational motion.
Examples:
1. A spinning top
2. The Earth rotating about its axis
4. Oscillatory or To-and-Fro Motion
When an object moves repeatedly to and fro about a fixed or mean position, it is called oscillatory motion.
Examples:
A swinging pendulum
A child moving on a swing
4.Oscillatory Motion
Oscillatory motion is a type of motion in which an object moves to and fro repeatedly about a fixed mean (central) position.
Examples
A simple pendulum moving to and fro.
A swing moving back and forth.
5. Periodic Motion
Periodic motion is a type of motion in which an object repeats its motion after equal intervals of time.
Examples
1. Earth revolving around the Sun — repeats approximately every 1 year.
2. Clock hands — repeat their motion after fixed intervals.
3. A pendulum — repeats its to-and-fro motion after a fixed time
Uniform Motion:-
Uniform motion is a type of motion in which an object covers equal distances in equal intervals of time, however small the time intervals may be.
Example
Suppose a car moves:
In 1 second → 10 m
In 2 seconds → 20 m
In 3 seconds → 30 m
In 4 seconds → 40 m
Non-Uniform Motion
Non-uniform motion is a type of motion in which an object does not cover equal distances in equal intervals of time.
Example
Suppose a car travels:
1st second → 5 m
2nd second → 8 m
3rd second → 12 m
4th second → 20 m
The distances are different in equal time intervals, so the car is in non-uniform motion.
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