Transcript Hookes Law
Consider the following pictures AL-HOCEIMA EARTHQUAKE 24 /02 /2004 Consider the following pictures Consider the following pictures Hooke’s 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. • 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 did the experiment show? What the graph shows Extension (m) Breakage Extension proportional to Elastic Force. Straight line graph. Limit Object will return to original shape. (Elastic) Ext’ not proportional to Force. Object will not return to original shape. (Not Elastic). Force (Newtons) 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 Explaining plasticity Polythene ‘neck’ polythene strip 10 mm 100 mm crystalline amorphous thin crystalline strip ‘pulled out of’ wider region new crystalline region Polythene is semi-crystalline. Think of polythene When stretched plastically, the chains slip past as like cooked spaghetti. In amorphous regions each other. More of the material has lined-up the chains fold randomly. In crystalline regions chains. More of it is crystalline. the chains line up. Plastic extensibility > 100% Hence in general, the Force is proportional to extension. So Hooke’s Law could be put as Fx 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 k decreases by 2 Combined springs slacken compared with single spring extension doubles Parallel 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