It's a Great Day For Physics: Elasticity and Hooke's Law

Elasticity, the property of a substance that enables it to recover its original shape and size after it has been stretched, squeezed, or bent. All substances are elastic in one way or another.

Gases and liquids have elasticity of volume. They have the ability to expand to their original volume after a compressive force has been removed. They also expand or contract to their original volume after being heated or cooled.

Solids have elasticity of form. They tend to resume their original shapes after being deformed by bending, twisting, pulling, or pressure. Some solids, such as putty and modeling clay, are plastic, or relatively inelastic. Others, such as rubber and steel, are very elastic. All solids can be deformed beyond their elastic limit—the point at which they will no longer resume their original form, even if the deforming force is removed.

Hooke's Law, formulated by the 17th-century English scientist Robert Hooke, is the basic law of elasticity. It states that the strain (tendency to deform) of a body is proportional to the stress (deforming force) applied to the body.

Hooke's Law states that "the extension of a helical spring is directly proportional to the weight applied, provided the elastic limit of the spring is not exceeded."

You may also see it written as: "Hooke's Law states that in an elastic material strain is proportional to stress and the point at which a material ceases to obey Hooke's Law is known as its elastic limit."

What this means is if you have a helical spring or an elastic material and apply a weight to it will stretch by a certain amount. If you remove the weight and apply another which is twice as heavy then the spring will stretch twice as far as it did the first time. If you remove that weight and apply a weight that is three times heavier than the first one then the spring will stretch three times further than it did the first time. This is where the "directly proportional to the weight applied" or "strain is proportional to stress" bit is relevant and importantly each time you remove a weight the spring returns to its unstressed size. However this cannot go on forever. There will come a point when the spring is stretched too far and it cannot return to its original state or in fact snaps. At this point the "elastic limit" has been exceeded and Hooke's Law no longer applies.

Who is Robert Hooke? He was a brilliant scientist, yet for some reason is relatively unknown.

Robert Hooke was born on the 18th July 1635, in Freshwater, on the Isle of Wight. The son of John Hooke, who taught him at home in his early years. Robert soon showed a keen mind, being a quick learner and showing great manual dexterity, making mechanical toys when he was a boy. He went to Westminster School when he was thirteen, and from there on to Oxford where he would meet many of the great scientists of the day. There he impressed with his abilities in constructing equipment and designing experiments and in 1658 he became assistant to Robert Boyle. He was friends with, worked with and sometimes argued with many scientists of note such as Christian Huygens, Christopher Wren, Robert Boyle, Antony van Leeuwenhoek and Isaac Newton. Regarding arguing Hooke and Newton had a somewhat tempestuous working relationship culminating in a bust up regarding their views on gravity. In 1662 Hooke was named Curator of Experiments of the Royal Society of London. He died in London on March 3, 1703.