Forces are responsible for changing the motion of objects. If more than one force is present, the shape of an object can also be changed.

Extension happens when an object increases in length and compression happens when it decreases in length. The extension of an elastic object, such as a spring, is described by Hooke's law:

force = spring constant × extension

This is when:

- force (
*F*) is measured in newtons (N) - spring constant (
*k*) is measured in newtons per metre (N/m) - extension, or increase in length (
*x*), is measured in metres (m)

A force of 3 N is applied to a spring. The spring stretches reversibly by 0.15 m - the fact that the string stretches reversibly means that it will go back to its normal shape after the force has been removed. Calculate the spring constant.

First rearrange
to find *k*:

Then calculate using the values in the question:

*k* = 3 ÷ 0.15

*k* = 20 N/m

**Spring constant** is a measure of the stiffness of a spring up to its limit of proportionality or elastic limit. The limit of proportionality refers to the point beyond which Hooke's law is no longer true when stretching a material. The **elastic limit** of a material is the furthest amount it can be stretched or distorted without being able to return to its previous shape. Once a material has gone past its elastic limit, its distortion is said to be **inelastic**.

The higher the spring constant, the stiffer the spring. The spring constant is different for different elastic objects. For a given spring and other elastic objects, the extension is directly proportional to the force applied. For example, if the force is doubled, the extension doubles. This works until the limit of proportionality is exceeded.

When an elastic object is stretched beyond its limit of proportionality, the object does not return to its original length when the force is removed. In this instance, the relationship between force and extension changes from being linear, or directly proportional, to being non-linear.

Non-linear extension occurs more in some materials than others. Materials like clay or putty usually show non-linear extension.

Linear extension and elastic distortion can be seen below the limit of proportionality.

Non-linear extension and inelastic distortion can be seen above the limit of proportionality. The limit of proportionality is also described as the elastic limit. The gradient of a force-extension graph before the limit of proportionality is equal to the spring constant.