Motion and Measurement of Distances - Moving Things

Measurement is the comparison of an unknown quantity with a known fixed quantity of the same kind. This known fixed quantity is called a unit.

Ancient units of measurement included the length of a foot, the width of a finger, and the distance of a step (pace). These were unreliable as they varied from person to person.

The International System of Units ($SI$ units) was adopted globally for uniformity. The $SI$ unit of length is the metre ($m$).

Standard conversions: $1 \text{ kilometre (km)}$ is used for large distances, while $1 \text{ centimetre (cm)}$ and $1 \text{ millimetre (mm)}$ are used for smaller lengths.

For accurate measurement, the eye must be placed exactly in front of the point where the measurement is to be taken to avoid parallax error.

Motion is a change in the position of an object over time with respect to a stationary object.

Rectilinear Motion: When an object moves along a straight line (e.g., a march-past of soldiers or a falling stone).

Circular Motion: When an object moves such that its distance from a fixed point remains the same (e.g., the tip of the hands of a clock or a point marked on the blades of an electric fan).

Periodic Motion: Motion that repeats itself after a fixed interval of time (e.g., a swinging pendulum, a branch of a tree moving to and fro, or the string of a guitar).

Rotational Motion: When an object turns around an internal axis (e.g., a spinning top or the Earth rotating on its axis).