An experiment is set up to investigate Hooke's Law using a helical steel spring suspended from a rigid support. A student measures the length of the spring before and after suspending a heavy mass from its lower end.

The length of the spring is measured using a standard metre ruler:

• The initial length is recorded as $15.5cm$.
• The final length is recorded as $23.5cm$.
• The metre ruler used is accurate to the nearest $0.1cm$ (giving an uncertainty of $\pm 0.1cm$ for each individual reading).