Distance and Displacement
Simple Explanation
Distance is the total length of the path travelled, no matter the direction — it's a scalar quantity. Displacement is the straight-line distance from the starting point to the finishing point, in a specific direction — it's a vector quantity.
Why Do We Need It?
The difference matters: someone who walks 5 laps of an oval track has travelled a large distance but ends up with a displacement of zero, since they're back where they started.
Worked Example
Find distance and displacement
A person walks 300 m east, then 100 m west. Find (a) the total distance travelled and (b) the displacement.
Why Does This Work?
Distance simply accumulates every bit of path length regardless of direction, while displacement only measures the net straight-line change — which is why opposite-direction movements partially cancel out for displacement but always add up for distance.
Real-Life Example
A running track vs. a straight sprint
A runner completes one full lap of a 400 m track.
They have run a distance of 400 m, but their displacement is 0 m, since they finish exactly where they started.
Practice
A car travels 50 m north then 20 m south. What is the total distance travelled?
EasyUsing the same journey (50 m north, then 20 m south), what is the displacement? Give the size only, in metres.
MediumCommon mistake
Using the words 'distance' and 'displacement' interchangeably — they can give very different answers whenever an object changes direction.
Quick Review
- Distance = total path length travelled (scalar).
- Displacement = straight-line distance from start to finish, with direction (vector).
- Displacement can be smaller than distance, or even zero, if the path doubles back.