Where it is
A bus waits at a level crossing. There a road crosses a railway. Where is the bus? First you must say where you measure from.
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Step 1 of 4
- A bus stands at a crossing on a straight road.
- Zero at the depot: the bus reads +500 m.
- Zero at the crossing: the bus reads 0 m.
- Zero at the turning: the bus reads −700 m.
The bus is in one place. Its position, as a number, depends on the reference point you choose.
The rule
Position is distance and direction from a reference point. Right of the point is positive. Left is negative.
A reference point is the place you measure from. An object is in motion if its position changes with time. The object is at rest if its position does not change.
A cyclist starts at point S and rides right. Point P is 30 m from S. Point Q is 90 m from S.
| Reference point | Position of P | Position of Q |
|---|---|---|
| S | +30 m | +90 m |
| Q | −60 m | 0 m |
Take Q as the reference point. P is 90 − 30 = 60 m left of Q. So P is −60 m.