CBSE Class 6 Physics — Motion and Measurement of Distances

Story of Transport

Before modern vehicles, people walked and used animals for transport on land. For water, simple logs with hollow cavities were used, which later evolved into streamlined boats inspired by fish. The invention of the wheel revolutionised transport, leading to carts, bicycles, and eventually engines and aeroplanes.

The Need for Standard Units of Measurement

A measurement consists of two parts: a number and a unit.

Non-Standard Units: In ancient times, people used lengths of their foot, handspan, cubit (elbow to fingertip), or pace to measure distance. Since body parts vary in size from person to person, these led to confusion and inaccuracy.

Standard Units: For the sake of uniformity, scientists all over the world accepted a standard set of measurement units known as the International System of Units (SI Units).

Conversions for Length:

• 1 kilometre (km) = 1000 metres (m)
• 1 metre (m) = 100 centimetres (cm)
• 1 centimetre (cm) = 10 millimetres (mm)

Measuring Length Properly

To measure the length of an object properly using a scale:

1. Keep the scale straight along the length of the object.
2. The eye must be placed exactly vertically above the mark to be read to avoid parallax error.
3. If the zero mark is broken, use any other full mark (like 1.0 cm) and subtract it from the final reading.

Measuring a Curved Line

You cannot measure a curved line directly with a scale. Use a non-stretchable thread or string to trace the curve, then straighten the thread and measure it against a scale.

Types of Motion

An object is in motion if it changes its position concerning time and a stationary reference point.

graph TD
    Motion --> Rectilinear["Rectilinear Motion"]
    Motion --> Circular["Circular Motion"]
    Motion --> Periodic["Periodic Motion"]
    Motion --> Combined["Combined / Complex Motion"]

1. Rectilinear Motion

Motion along a straight line.

• Examples: Vehicles on a straight road, march-past of soldiers, a falling stone.

2. Circular Motion

Motion where an object remains at the same distance from a fixed central point.

• Examples: Hands of a clock, blades of a moving fan, a stone tied to a string being whirled.

3. Periodic Motion

Motion that repeats itself after equal intervals of time.

• Examples: A swinging pendulum, a child on a swing, strings of a guitar being plucked.

4. Combined Motion

An object executing more than one type of motion simultaneously.

• Example: A moving bicycle wheel spins around its axis (circular motion) and at the same time translationally moves forward (rectilinear motion). A sewing machine needle has periodic motion while its wheel has circular motion.