Answer & solution
Answer: (c) displacement
Correct Answer: Option 3 — displacement
Displacement is defined as the shortest distance between the initial and the final position of a moving object, measured in a definite direction. It is therefore a vector quantity: it has both magnitude and direction. Its SI unit is the metre (m).
Displacement must be carefully distinguished from distance. Distance is the total length of the actual path covered, is a scalar, and can never be negative or zero for a moving body. Displacement, on the other hand, can be positive, negative or even zero. If a boy runs once around a circular track and comes back to the starting point, his distance equals the circumference of the track, but his displacement is zero.
Also, displacement ≤ distance always. The two are equal only when the body moves along a straight line in one direction without turning back.
❌ Why the other options are incorrect
✗ acceleration — the rate of change of velocity, a = (v − u)/t, measured in m s⁻², not a length at all.
✗ velocity — displacement per unit time, measured in m/s; it is a rate, whereas displacement is a length.
✗ speed — distance per unit time, a scalar measured in m/s; again a rate, not the shortest length between two positions.
📌 SSC & Railway Exam Facts
• Distance — scalar, total path length; Displacement — vector, shortest straight-line length between the two positions.
• |Displacement| ≤ Distance; they are equal only for motion in a straight line without reversal.
• Speed = distance/time (scalar); Velocity = displacement/time (vector); both in m/s.
• For one complete circular round, displacement = 0 but distance = 2πr.
• In a displacement–time graph, the slope gives velocity; in a velocity–time graph, the slope gives acceleration and the area gives displacement.