A spring is generally a helical metallic wire. When the two free ends of the spring are pulled away or pushed towards each other, the length of the spring is changed. The spring has a tendency to come back to its original length or it develops an opposition to change in its length. This opposing force (F) is the restoring force. The opposing force is directly proportional to the change in length. It is given by F = kx, where k is a constant for a given spring.