Measuring a curved line

For a celebration at home, Anish and his parents hang electric string lights along the arches of their verandah. The arches are curved. How long must the string of lights be?

A metre scale is straight and stiff. It cannot bend to lie along a curve, so it cannot measure a curved line directly.

Your book gives two ways:

• Use a flexible measuring tape, like a tailor's tape, that bends along the curve.
• Lay a thread carefully along the whole curved line. Mark where it ends, straighten the thread, and measure it with a metre scale.

The length of the straightened thread is the length of the curved line. We straighten it because a straight thread can be measured against the straight scale.