When friends measured the same table using their own handspans, they all got different numbers — because every handspan is a different size.
A measurement always has a number and a unit — like “13 handspans.” 13 is the number, handspan is the unit.
A 15-cm scale for a pencil, a metre tape for a room, a flexible tape for a tree's girth.
Place the scale in direct contact with the object, along its full length.
A tilted eye gives a wrong reading — a parallax error.
Subtract the starting reading from the ending reading to get the true length.
Lay a flexible measuring tape directly along the curve.
Lay a thread along the curve, mark its length, then straighten and measure with a scale.
A sign reading “Delhi 70 km,” then later “Delhi 60 km,” shows you're getting closer to Delhi — the reference point.
An object is in motion if its position changes with respect to a reference point, over time.
A bus passenger is at rest relative to the bus, but in motion relative to a building outside.
Along a straight line — a falling eraser, a pushed box.
Along a circular path — a merry-go-round, a whirled string.
To-and-fro about a fixed position — a swing, a pendulum.
The SI unit of length.
1/100 and 1/1000 of a metre.
A fixed point used to describe position.
Position changes with time relative to a reference point.
Position stays the same relative to a reference point.
Motion along a straight line.
Motion along a circular path.
To-and-fro motion about a fixed position.