Lesson 1 of 9 · 6 min
Background: position, displacement and distance (prerequisite recap)
NCERT § "Introduction"
Meera's odometer says 1.2 km. Her mother asks how far from home she is. The answer is not 1.2 km.
The story this chapter follows: Meera's ride to coaching
The lesson in notes
In short
Background recap: the ideas in this block (displacement, path length, average velocity and average speed) are prerequisites the rest of the chapter builds on, summarised here so the later sections read clearly.
Motion along a straight line is described by one coordinate x measured from a chosen origin, with one direction taken as positive; the choice is free, but it must be kept for the whole problem.
A body can be handled as a single point (a point object) whenever its own size is negligible next to how far it travels during the motion being studied.
Displacement is the change in position, Δx = x₂ − x₁. It carries a sign and depends only on the start and end points.
Path length (distance) is the total length actually covered. It is never negative and is never less than the magnitude of the displacement.
For a round trip ending at the start point the displacement is zero even though the distance covered is not.
Average velocity is displacement divided by the time taken, while average speed is path length divided by the time taken; the two are equal in size only if the body never reverses direction.