There is a point, however, beyond which Hooke’s law is no longer obeyed. This is called the limit of proportionality. If the substance is stretched further than this point, it reaches its elastic limit. The substance stops being elastic and remains distorted even when the stretching force is removed.
Provided a material’s elastic limit is not exceeded, the principle of Hooke’s law can be used in calculations to determine an unknown force or extension.
For example, if a force of 10N stretches a spring by 60mm, the force which would produce an extension of 42mm is calculated as follows:
60mm extension produced by 10N
1mm extension is produced by 10/60
Therefore, the force which would produce a 42mm extension is calculated as follows: 10X42/60 = 7N