Hookes Law Robert Hooke 1635 1703 Researched and

Hookes Law .

Robert Hooke 1635 -1703 • Researched and wrote papers on Geometry, Snowflakes, Heat, Astronomy, Fossils, Air Pumps, Light, Watches, Telescopes and Silkworms…. . among other things • Continually squabbled with Newton with whom he was miffed because Newton seemed to get all the attention and credit. . Hookes Law was one of the few things he was popularly credited with • A rather crabby individual but credited with the invention of universal joints, the balance wheel and the iris diaphragm

What did Hooke discover ? • the more force that was put on materials the more they extended • With some materials they also extended in a regular way eg if the force was doubled so did the extension • this was true as long as their elastic limit was not exceeded

What does this mean?

What the graph shows

What is the elastic limit? • The material no longer shows elastic behaviour (ie does not return to original size when stretching force is removed) • The material is permanently deformed ie is larger or longer than originally • The material is weaker as the above effects are caused by fracture of some atomic bonds

Since Force is proportional to extension Hookes Law could be put as Where F is the applied force in Newtons x is the extension in metres Or if k is the proportionality constant F=kx

What does k mean in F=kx? • k is called the spring constant and is a measure of the stiffness of the spring or material • It has units of Nm-1 (newtons per metre) • The higher the k the stiffer the spring • Materials with a high k need a large force to for a given extension • adding springs in series or parallel changes k

Series and parallel springs Series Parallel k decreases by 2 Combined springs slacken compared with single spring extension doubles k increases by 2 Combined springs stiffen compared with single spring extension halves

So what? • Understanding Hookes law is critical to the construction of any structure eg bridges buildings • Deformation of materials is also critical in vehicle design though in practice the terms stress ( the force per square metre) and strain (the extension per unit length) are more commonly used instead of simply force and extension

Summary: A material is said to obey Hookes Law if its extension is directly proportional to the applied force More info on Hooke at http: //www. roberthooke. org. uk/