Motion in a Straight Line

Syllabus Unit: Mechanics

Why it matters

Chapter overview

One-dimensional kinematics is where NEET builds the vocabulary — position, distance, displacement, speed, velocity, acceleration — and the three equations of motion that describe any straight-line trip under constant acceleration, before layering in how to read that same motion off a graph and compare two moving objects' relative velocity.

Physics Syllabus Mechanics 30%–35% weightage

Live Animation1 / 4

Distance Travelled (7 m)

Displacement Magnitude (5 m)

1Position, Distance & Displacement

Distance is the total path length walked; displacement is the straight-line shortcut from start to finish — NEET tests whether you can tell them apart in a specific scenario.

Learning Objectives

Define position relative to a chosen origin and reference direction on a number line.
Distinguish distance (scalar, path length) from displacement (vector, shortest straight-line change in position).
Explain why distance ≥ |displacement| always, with equality only for one-directional straight-line motion.
Compute displacement for multi-leg 1-D motion problems.

Key Terms

PositionThe location of an object at a given instant, specified relative to a chosen origin and coordinate axis.

DistanceThe total length of the actual path travelled — a scalar quantity that never decreases.

DisplacementThe vector from initial to final position — the shortest straight-line change, which can be zero even if distance is huge.

Reference Point (Origin)The fixed point relative to which position, and therefore displacement, is measured.

Before motion can be described mathematically, you need a fixed reference — an origin and a positive direction. Only relative to that reference does 'position' have a numerical meaning.

Position needs a reference

A point on a line only has meaning once you fix an origin O and a positive direction; position x can be positive or negative relative to O.

Distance vs displacement

Distance — scalar, total path length covered, always increases or stays the same with time.
Displacement — vector, straight line from initial to final position, can increase, decrease, or return to zero.

Key PointIf a body returns to its starting point, displacement = 0 even though distance travelled is large — a classic NEET trap.

Distance ≥ |Displacement|, with equality only when motion is along a straight line without reversing direction.

Worked example

A person walks 4 m east then 3 m north. Distance = 4+3 = 7 m. Displacement = √(4²+3²) = 5 m, directed north-east of the start.

Common MistakeDisplacement magnitude can never exceed distance travelled — if your answer suggests otherwise, recheck your vector addition.