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## Consider the following pictures

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**Consider the following pictures**AL-HOCEIMA EARTHQUAKE 24 /02 /2004**Robert Hooke 1635-1703**• Researched and wrote papers on Geometry, Snowflakes, Heat, Astronomy, Fossils, Air Pumps, Light, Watches, Telescopes and Silkworms…..among other things. • Hooke’s Law was one of the few things he was popularly credited with creating. • A rather crabby individual but credited with also inventing the universal joints, the balance wheel and the iris diaphragm**What did Hooke discover about forces ?**• the more force that was put on materials the more they extended • With some materials they also extended in a regular way e.g.- if the force was doubled so did the extension • this was true as long as their elastic limit was not exceeded**Lets try an activity**• Read the instructions on lab sheet provided. • Arrange apparatus and perform experiments. • Analyse and present data**What is the elastic limit?**• The material no longer shows elastic behaviour (i.e. does not return to original size when stretching force is removed) • The material is permanently deformed i.e. is larger or longer than originally • The material is weaker as the above effects are caused by fracture of some atomic bonds**Hence in general, the Forceis proportional**toextension.SoHooke’s 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**k decreases by 2**Combined springs slacken compared with single spring extension doubles Series and parallel springs Parallel Series k increases by 2 Combined springs stiffen compared with single spring extension halves**So how does this help me?**• Understanding Hooke’s law is critical to the construction of any structure eg bridges or buildings. • During earthquakes deformations of buildings occur.**Summary**Hooke’s law explains how a material’s extension/deformation depends on the applied force.Deformation of materials though in practice is looked at in terms of stress ( the force per square metre) and strain (the extension per unit length) are more commonly used instead of simply force and extension