What is Kinematics?

Kinematics is the branch of physics that deals with the motion of objects without considering the forces that cause the motion. It describes how objects move in terms of displacement, velocity, and acceleration.

  • It answers the question: “How does an object move?” — not “Why does it move?”

One-Dimensional Motion (Straight-Line Motion)

When an object moves along a straight path, its motion is said to be one-dimensional or linear motion. Examples include:

  • A train moving along a straight track
  • A car on a straight highway
  • A free-falling ball (along a vertical line)

Assumption: The object is treated as a point particle (dimensions are negligible compared to the distance moved).

This simplification allows us to describe its position using only one coordinate (x) along the line.

Distance vs Displacement

  • Distance: Total length of the path covered; a scalar quantity (only magnitude).
  • Displacement: Change in position; a vector quantity (has magnitude and direction).

Example: An object moves 4 m east and then 3 m west.

  • Distance = 4+3=7 m4 + 3 = 7\ \text{m}
  • Displacement = 4−3=1 m4 - 3 = 1\ \text{m} east

Position and Reference Frame

To describe motion:

  • Choose a reference point (origin)
  • Define a coordinate axis (positive and negative directions)
  • Describe motion relative to this reference frame

Types of Motion

  1. Uniform motion: Constant velocity (no acceleration)
  2. Non-uniform motion: Changing velocity (acceleration present)
  3. Accelerated motion: Increasing or decreasing velocity

In this chapter, we deal only with motion along a straight line — often represented using a number line, with direction considered via sign (+ or −).