In everyday life, we observe both rest and motion. Birds fly, fish swim, blood flows through veins and arteries, and cars move. Even at the atomic level, molecules and atoms are in constant motion. We perceive an object to be in motion if its position changes with time. Sometimes, motion is inferred indirectly, such as by observing the movement of dust or leaves to infer wind motion. Phenomena like sunrise, sunset, and changing seasons are due to the Earth's motion, although we do not directly perceive this motion.

The perception of motion can differ based on the observer's frame of reference. For instance, a passenger in a moving bus perceives roadside trees as moving backward, while a person standing outside perceives the bus and its passengers as moving. Within the bus, passengers perceive each other as stationary. This indicates that motion is relative.

Most motions are complex. Objects can move in straight lines, circles, rotate, or vibrate, often combining these types of movements. This chapter focuses on describing motion along a straight line using simple equations and graphs, and later discusses circular motion.

To describe an object's location, a reference point is needed. For example, if a school is 2 km north of a railway station, the railway station is the reference point. This point is called the origin.

Motion Along a Straight Line

The simplest motion is along a straight line. Consider an object moving along a straight path starting from point O (reference point). Suppose it moves to point A, then back to points B and C. The total path length covered by the object (distance) is the sum of the individual segments it travels (e.g., OA + AC). Distance is a scalar quantity, meaning it has only magnitude and no direction.

The shortest distance from the initial to the final position of an object is called displacement, which is a vector quantity, meaning it has both magnitude and direction. For example, if an object moves from O to A and then back to B, the total distance covered could be different from the magnitude of the displacement.